chapter
9
matter, light, and
glass examination
Learning Objectives
L
I
D
D
E
L
L
,
After studying this chapter you should be able to:
T
t Define and distinguish the physical and chemical
properties of
I
matter
F
t Define and distinguish elements and compounds
F and gas
t Contrast the differences among solid, liquid,
A
t Understand how to use the basic units of the metric system
N
t Define and understand the properties of density and
Y
refractive index
t Understand and explain the dispersion of light through a prism
1
t Explain the relationship between color and the selective
5
absorption of light by molecules
6
t Understand the differences between the wave and particle
8
theories of light
T
S
t List and explain forensic methods for comparing
glass
t Describe the electromagnetic spectrum
ISBN: 978-1-323-16745-8
fragments
t Understand how to examine glass fractures to determine
the direction of impact for a projectile
t Describe the proper collection of glass evidence
KEY TERMS
amorphous solid
atom
Becke line
birefringence
Celsius scale
chemical property
compound
concentric fracture
crystalline solid
density
dispersion
electromagnetic spectrum
element
Fahrenheit scale
frequency
gas (vapor)
intensive property
laminated glass
laser
liquid
mass
matter
periodic table
phase
photon
physical property
physical state
radial fracture
refraction
refractive index
solid
sublimation
tempered glass
visible light
wavelength
weight
X-ray
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
204
CHAPTER 9
physical property
The behavior of a substance
without alteration of the
substance’s composition through
a chemical reaction.
chemical property
The behavior of a substance when
it reacts or combines with another
substance.
The forensic scientist must constantly determine the properties that impart distinguishing characteristics to matter, giving it a unique identity. The continuing search for distinctive properties
ends only when the scientist has completely individualized a substance to one correct source.
Properties are the identifying characteristics of substances. In this chapter we will examine properties that are most useful for characterizing glass and other physical evidence. However, before
we begin, we can simplify our understanding of the nature of properties by classifying them into
two broad categories: physical and chemical.
Physical properties describe a substance without reference to any other substance. For
example, weight, volume, color, boiling point, and melting point are typical physical properties that can be measured for a particular substance without altering the material’s composition through a chemical reaction; they are associated only with the physical existence of that
substance. A chemical property describes the behavior of a substance when it reacts or
combines with another substance. For example, when wood burns, it chemically combines
with oxygen in the air to form new
L substances; this transformation describes a chemical property
of wood. In the crime laboratory, a routine procedure for determining the presence of heroin in
a suspect specimen is to react itI with a chemical reagent known as the Marquis reagent, which
turns purple in the presence of heroin.
D This color transformation becomes a chemical property of
heroin and provides a convenient test for its identification.
D
E
The Nature of LMatter
Before we can apply physical properties,
as well as chemical properties, to the identification and
L
comparison of evidence, we need to gain an insight into the composition of matter. Beginning
, building block of all substances—the element—we will exwith knowledge of the fundamental
tend our discussion to compounds.
T
Elements and Compounds
matter
All things of substance; matter is
composed of atoms or molecules.
element
A fundamental particle of matter; an element cannot be broken
down into simpler substances by
chemical means.
periodic table
A chart of elements arranged in a
systematic fashion; vertical rows
are called groups or families, and
horizontal rows are called series; elements in a given row have similar
properties.
A pure substance composed of two
or more elements.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
compound
Matter is anything that has mass
I and occupies space. As we examine the world that surrounds
us and consider the countless variety of materials that we encounter, we must consider one of
humankind’s most remarkable F
accomplishments: the discovery of the concept of the atom to
explain the composition of all matter.
F This search had its earliest contribution from the ancient
Greek philosophers, who suggested air, water, fire, and earth as matter’s fundamental building
blocks. It culminated with the A
development of the atomic theory and the discovery of matter’s
simplest identity, the element. N
An element is the simplest substance known and provides the building block from which
Y
all matter is composed. At present, 118 elements have been identified (see Table 9–1); of
these, 89 occur naturally on the earth, and the remainder have been created in the laboratory. In
Figure 9–1, all of the elements are listed by name and symbol in a form that has become known
1
as the periodic table. This table is most useful to chemists because it systematically arranges
elements with similar chemical5properties in the same vertical row or group.
For convenience, chemists have chosen letter symbols to represent the elements. Many of
6
these symbols come from the first letter of the element’s English name—for example, carbon (C),
hydrogen (H), and oxygen (O).8Others are two-letter abbreviations of the English name—for
example, calcium (Ca) and zinc (Zn). Some symbols are derived from the first letters of Latin or
T
Greek names. Thus, the symbol for silver, Ag, comes from the Latin name argentum; copper, Cu,
S He, from the Greek name helios.
from the Latin cuprum; and helium,
The smallest particle of an element that can exist and still retain its identity as that
element is the atom. When we write the symbol C we mean one atom of carbon; the chemical symbol for carbon dioxide, CO2, signifies one atom of carbon combined with two atoms of
oxygen. When two or more elements are combined to form a substance, as with carbon dioxide,
a new substance is created, different in its physical and chemical properties from its elemental
components. This new material is called a compound. Compounds contain at least two elements.
Considering that there are 89 natural elements, it is easy to imagine the large number of possible elemental combinations that may form compounds. Not surprisingly, more than 16 million
known compounds have already been identified.
MATTER, LIGHT, AND GLASS EXAMINATION
205
TABLE 9–1
List of Elements with Their Symbols and Atomic Masses
ISBN: 978-1-323-16745-8
Element
Actinum
Aluminum
Americium
Antimony
Argon
Arsenic
Astatine
Barium
Berkelium
Beryllium
Bismuth
Bohrium
Boron
Bromine
Cadmium
Calcium
Californium
Carbon
Cerium
Cesium
Chlorine
Chromium
Cobalt
Copernicium
Copper
Curium
Darmstadtium
Dubnium
Dysprosium
Einsteinium
Erbium
Europium
Fermium
Flerovium
Fluorine
Francium
Gadolinium
Gallium
Germanium
Gold
Hafnium
Hassium
Helium
Holmium
Hydrogen
Indium
Iodine
Iridium
Iron
Krypton
Lanthanum
Lawrencium
Symbol
Ac
Al
Am
Sb
Ar
As
At
Ba
Bk
Be
Bi
Bh
B
Br
Cd
Ca
Cf
C
Ce
Cs
Cl
Cr
Co
Cn
Cu
Cm
Ds
Db
Dy
Es
Er
Eu
Fm
FL
F
Fr
Gd
Ga
Ge
Au
Hf
Hs
He
Ho
H
In
I
Ir
Fe
Kr
La
Lr
Atomic Massa (amu)
(227)
26.9815
(243)
121.75
39.948
74.9216
(210)
137.34
(247)
9.01218
208.9806
(270)
10.81
79.904
112.40
40.08
(251)
12.011
140.12
132.9055
35.453
51.996
58.9332
(285)
63.546
(247)
(81)
(268)
162.50
(254)
167.26
151.96
(253)
(289)
18.998
(223)
157.25
69.72
72.59
196.9665
178.49
(277)
4.00260
164.9303
1.0080
114.82
126.9045
192.22
55.847
83.80
138.9055
(262)
Element
L
I
D
D
E
L
L
,
T
I
F
F
A
N
Y
1
5
6
8
T
S
Lead
Lithium
Livermorium
Lutetium
Magnesium
Manganese
Meitnerium
Mendelevium
Mercury
Molybdenum
Neodymium
Neon
Neptunium
Nickel
Niobium
Nitrogen
Nobelium
Osmium
Oxygen
Palladium
Phosphorus
Platinum
Plutonium
Polonium
Potassium
Praseodymium
Promethium
Protactinium
Radium
Radon
Rhenium
Rhodium
Roentgenium
Rubidium
Ruthenium
Rutherfordium
Samarium
Scandium
Seaborgium
Selenium
Silicon
Silver
Sodium
Strontium
Sulfur
Tantalum
Technetium
Tellurium
Terbium
Thallium
Thorium
Thulium
Symbol
Pb
Li
Lv
Lu
Mg
Mn
Mt
Md
Hg
Mo
Nd
Ne
Np
Ni
Nb
N
No
Os
O
Pd
P
Pt
Pu
Po
K
Pr
Pm
Pa
Ra
Rn
Re
Rh
Rg
Rb
Ru
Rf
Sm
Sc
Sg
Se
Si
Ag
Na
Sr
S
Ta
Tc
Te
Tb
Tl
Th
Tm
Atomic Massa (amu)
207.2
6.941
(293)
174.97
24.305
54.9380
(278)
(256)
200.59
95.94
144.24
20.179
237.0482
58.71
92.9064
14.0067
(254)
190.2
15.9994
106.4
30.9738
195.09
(244)
(209)
39.102
140.9077
(145)
231.0359
226.0254
(222)
186.2
102.9055
(280)
85.4678
101.07
(265)
105.4
44.9559
(271)
78.96
28.086
107.868
22.9898
87.62
32.06
180.9479
98.9062
127.60
158.9254
204.37
232.0381
168.9342
(continued)
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
206
CHAPTER 9
TABLE 9–1
List of Elements with Their Symbols and Atomic Masses (continued)
Element
Tin
Titanium
Tungsten
Ununoctium
Ununpentium
Ununseptium
Ununtrium
Atomic Massa (amu)
Symbol
Sn
Ti
W
Uuo
Uup
Uus
Uut
118.69
47.90
183.85
(294)
(288)
(?)
(284)
Element
Symbol
Uranium
Vanadium
Xenon
Ytterbium
Yttrium
Zinc
Zirconium
U
V
Xe
Yb
Y
Zn
Zr
Atomic Massa (amu)
238.029
50.9414
131.3
173.04
88.9059
65.57
91.22
Based on the assigned relative atomic mass of C 5 exactly 12; parentheses denote the mass number of the isotope with the longest half-life.
a
Group
Period
IA
IIA
IIIB
IVB
VB
L
I
D
VIB VIIB
D
E
L
L
,
VIII
IB
IIB
IIIA
IVA
VA
VIA
VIIA
O
2
He
1
1
H
2
3
Li
4
Be
3
11
Na
12
Mg
4
19
K
20
Ca
21
Sc
22
Ti
23
V
24 25
CrT Mn
26
Fe
27
Co
28
Ni
29
Cu
5
37
Rb
38
Sr
39
Y
40
Zr
41
Nb
42I
Mo
43
Tc
44
Ru
45
Rh
46
Pd
6
55
Cs
56
Ba
57 72
La a Hf
73
Ta
74 75
F
W Re
76
Os
77
Ir
78
Pt
7
87
Fr
88
Ra
89 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
Ac b Rf Db SgN Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
F
5
B
6
C
7
N
8
O
9
F
10
Ne
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
30
Zn
31
Ga
32
Ge
33
As
34
Se
35
Br
36
Kr
47
Ag
48
Cd
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
79
Au
80
Hg
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
A
Y
aLanthanide
series
bActinide
series
69
Tm
70
Yb
71
Lu
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
Eu
64
Gd
65
Tb
66
Dy
67
Ho
68
Er
90
Th
91
Pa
921 93
U5 Np
94
Pu
95
Am
96
Cm
97
Bk
98
Cf
99
Es
100 101 102 103
Fm Md No Lr
6
8
T
Just as the atom is the basic
S particle of an element, the molecule is the smallest unit of a
FIGURE 9–1
The periodic table.
compound. Thus, a molecule of carbon dioxide is represented by the symbol CO2, and a molecule
of table salt is symbolized by NaCl, representing the combination of one atom of the element
sodium (Na) with one atom of the element chlorine (Cl).
As we look around us and view the materials that make up the earth, it becomes an awesome task
even to attempt to estimate the number of different kinds of matter that exist. A much more logical approach is to classify matter according to the physical form it takes. These forms are called
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
States of Matter
MATTER, LIGHT, AND GLASS EXAMINATION
207
physical states. There are three such states: solid, liquid, and gas (vapor). A solid is rigid and
therefore has a definite shape and volume. A liquid also occupies a specific volume, but its fluidity causes it to take the shape of the container in which it is residing. A gas has neither a definite
shape nor volume, and it will completely fill any container into which it is placed.
physical state
CHANGES OF STATE Substances can change from one state to another. For example, as water
A state of matter in which the molecules are held closely together in
a rigid state.
is heated, it is converted from a liquid form into a vapor. At a high enough temperature (100°C),
water boils and rapidly changes into steam. Similarly, at 0°C, water solidifies or freezes into ice.
Under certain conditions, some solids can be converted directly into a gaseous state. For instance,
a piece of dry ice (solid carbon dioxide) left standing at room temperature quickly forms carbon
dioxide vapor and disappears. This change of state from a solid to a gas is called sublimation.
In each of these examples, no new chemical species are formed; matter is simply being
changed from one physical state to another. Water, whether in the form of liquid, ice, or steam,
remains chemically H2O. Simply, what has been altered are the attractive forces between the
water molecules. In a solid, these forces are very strong, and theLmolecules are held closely together in a rigid state. In a liquid, the attractive forces are not as Istrong, and the molecules have
more mobility. Finally, in the vapor state, appreciable attractive forces no longer exist among the
D
molecules; thus, they may move in any direction at will.
D
the solid, liquid, or gaseous states, hoping to create new and E
useful products. Our everyday
observations should make it apparent that not all attempts at mixing matter can be productive.
For instance, oil spills demonstrate that oil and water do not mix.LWhenever substances can be
distinguished by a visible boundary, different phases are saidLto exist. Thus, oil floating on
water is an example of a two-phase system. The oil and water each constitute a separate liquid
phase, clearly distinct from each other. Similarly, when sugar is ,first added to water, it does not
PHASES Chemists are forever combining different substances, no matter whether they are in
dissolve, and two distinctly different phases exist: the solid sugar and the liquid water. However,
after stirring, all the sugar dissolves, leaving just one liquid phase.
T
Physical Properties of Matter
I
All materials possess a range of physical properties whose measurement is critical to the work of
F
the forensic scientist. Several of the most important of these for the forensic characterization of
F
glass is density and refractive index.
Which physical and chemical properties the forensic scientist ultimately chooses to observe
A
and measure depends on the type of material that is being examined. Logic requires, however,
N to a standard system of meathat if the property can be assigned a numerical value, it must relate
surement accepted throughout the scientific community.
Y
A condition or stage in the form of
matter; a solid, liquid, or gas.
solid
liquid
A state of matter in which molecules are in contact with one
another but are not rigidly held in
place.
gas (vapor)
A state of matter in which
the attractive forces between
molecules are small enough
to permit them to move with
complete freedom.
sublimation
A physical change from the solid
state directly into the gaseous
state.
phase
A uniform body of matter; different
phases are separated by definite
visible boundaries.
Basic Units of Measurement
ISBN: 978-1-323-16745-8
The metric system has basic units of measurement for length, mass,
1 and volume: the meter, gram,
and liter, respectively. These three basic units can be converted into subunits that are decimal
multiples of the basic unit by simply attaching a prefix to the unit5name. The following are common prefixes and their equivalent decimal value:
6
8
T
S
Prefix
Equivalent Value
decicentimillimicronanokilomega-
1/10 or 0.1
1/100 or 0.01
1/1000 or 0.001
1/1,000,000 or 0.000001
1/1,000,000,000 or 0.000000001
1,000
1,000,000
Hence, 1/10 or 0.1 gram (g) is the same as a decigram (dg), 1/100 or 0.01 meter is equal to a
centimeter (cm), and 1/1,000 liter is a milliliter (mL). A metric conversion is carried out simply
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
208
CHAPTER 9
inside the science
The Metric System
Although scientists, including forensic scientists,
throughout the world have been using the metric
system of measurement for more than a century, the
United States still uses the cumbersome “English
system” to express length in inches, feet, or yards;
weight in ounces or pounds; and volume in pints or
quarts. The inherent difficulty of this system is that
no simple numerical relationship exists between the
various units of measurement. For example, to convert inches to feet one must know that 1 foot equals
12 inches; conversion of ounces to pounds requires
the knowledge that 16 ounces equals 1 pound. In
1791, the French Academy of Science devised the
simple system of measurement known as the metric
system. This system uses a simple decimal relationship
so that a unit of length, volume, or mass can be converted into a subunit by simply multiplying or dividing
Lby a multiple of 10—for example, 10, 100, or 1,000.
Even though the United States has not yet adI opted the metric system, its system of currency is
Ddecimal and, hence, is analogous to the metric system. The basic unit of currency is the dollar. A dollar
Dis divided into 10 equal units called dimes, and each
Edime is further divided into 10 equal units of cents.
L
L
,
T
I
10 cm
F
F
A
10 cm
N
1,000 cm3
1 liter (1 L) = Y
1,000 mL
10 cm
1 cm
1 cm
1 cm
1 cm3 = 1mL
Volume equivalencies in the metric system.
1
5
6
8
T measurement; 2.54 centimeters 5 1 inch.
Comparison of the metric and English systems of length
S
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
by moving the decimal point to the right or left and inserting the proper prefix to show the direction and number of places that the decimal point has been moved. For example, if the weight of
a powder is 0.0165 gram, it may be more convenient to multiply this value by 100 and express it
as 1.65 centigrams or by 1,000 to show it as its equivalent value of 16.5 milligrams. Similarly, an
object that weighs 264,450 grams may be expressed as 264.45 kilograms simply by dividing it by
1,000. It is important to remember that in any of these conversions, the value of the measurement
MATTER, LIGHT, AND GLASS EXAMINATION
209
has not changed; 0.0165 gram is still equivalent to 1.65 centigrams, just as one dollar is still equal
to 100 cents. We have simply adjusted the position of the decimal and shown the extent of the
adjustment with a prefix.
One interesting aspect of the metric system is that volume can be defined in terms of length.
A liter by definition is the volume of a cube with sides of length 10 centimeters. One liter is therefore equivalent to a volume of 10 cm 3 10 cm 3 10 cm, or 1,000 cubic centimeters (cc). Thus,
1/1,000 liter or 1 milliliter (mL) is equal to 1 cubic centimeter (cc). Scientists commonly use the
subunits mL and cc interchangeably to express volume.
Metric Conversion
At times, it may be necessary to convert units from the metric system into the English system, or
vice versa. To accomplish this, we must consult references that list English units and their metric
equivalents. Some of the more useful equivalents follow:
L
I
D
D
The general mathematical procedures for converting from one
E system to another can be illustrated by converting 12 inches into centimeters. To change inches into centimeters, we need to
L 12 inches by 2.54 centimeters
know that there are 2.54 centimeters per inch. Hence, if we multiply
per inch (12 in. 3 2.54 cm/in.), the unit of inches will cancel out,Lleaving the product 30.48 cm.
Similarly, applying the conversion of grams to pounds, 227 grams is equivalent to 227 g 3
,
1 lb/453.6 g or 0.5 lb.
1 inch 5 2.54 centimeters
1 meter 5 39.37 inches
1 pound 5 453.6 grams
1 liter 5 1.06 quarts
1 kilogram 5 2.2 pounds
Density
T
An important physical property of matter with respect to the analysis of certain kinds of physical
evidence is density. Density is defined as mass per unit volumeI [see Equation (9–1)].
F
(9–1)
F
A
Density is an intensive property of matter—that is, it is the same regardless of the size
Nand can be used as an aid in
of a substance; thus, it is a characteristic property of a substance
identification. Solids tend to be more dense than liquids, and liquids
Y more dense than gases. The
ISBN: 978-1-323-16745-8
mass
Density =
volume
densities of some common substances are shown in Table 9–2.
A simple procedure for determining the density of a solid is illustrated in Figure 9–2. First,
the solid is weighed on a balance against known standard gram1
weights to determine its mass.
The solid’s volume is then determined from the volume of water it displaces. This is easily mea5 the object, and measuring
sured by filling a cylinder with a known volume of water (V1), adding
the new water level (V2). The difference V2—V1 in milliliters is equal
6 to the volume of the solid.
Density can now be calculated from Equation (9–1) in grams per milliliter.
8
The volumes of gases and liquids vary considerably with temperature;
hence, when determining density, it is important to control and record the temperature
T at which the measurements
are made. For example, 1 gram of water occupies a volume of 1 milliliter at 4°C and thus has
a density of 1.0 g/mL. However, as the temperature of water S
increases, its volume expands.
Therefore, at 20°C (room temperature) 1 gram of water occupies a volume of 1.002 mL and has
a density of 0.998 g/mL.
The observation that a solid object either sinks, floats, or remains suspended when immersed
in a liquid can be accounted for by the property of density. For instance, if the density of a solid
is greater than that of the liquid in which it is immersed, the object sinks; if the solid’s density is
less than that of the liquid, it floats; and when the solid and liquid have equal densities, the solid
remains suspended in the liquid. As we will shortly see, these observations provide a convenient
technique for comparing the densities of solid objects.
density
A physical property of matter that
is equivalent to the mass per unit
volume of a substance.
intensive property
A property that is not dependent
on the size of an object.
Fahrenheit scale
The temperature scale using the
melting point of ice as 32° and
the boiling point of water as 212°,
with 180 equal divisions or degrees
between.
Celsius scale
The temperature scale using the
melting point of ice as 0° and the
boiling point of water as 100°, with
100 equal divisions or degrees
between.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
210
CHAPTER 9
inside the science
Temperature
Determining the physical properties of any material often requires measuring its temperature. For
instance, the temperatures at which a substance
melts or boils are readily determinable characteristics that will help identify it. Temperature is a measure of heat intensity, or the amount of heat in a
substance.
Temperature is usually measured by causing a
thermometer to come into contact with a substance.
The familiar mercury-in-glass thermometer functions
because mercury expands more than glass when
heated and contracts more than glass when cooled.
Thus, the length of the mercury column in the glass
tube provides a measure of the surrounding environment’s temperature.
The construction of a temperature scale requires
two reference points and a choice of units. The reference points most conveniently chosen are the freezing point and boiling point of water. The two most
common temperature scales used are the Fahrenheit
and Celsius (formerly called centigrade) scales.
The Fahrenheit scale is based on assigning a
value of 32°F to the freezing point of water and a
value of 212°F to its boiling point. The difference
between the two points is evenly divided into 180
units. Thus, a degree Fahrenheit is 1/180 of the
temperature change between the freezing point
and boiling point of water. The Celsius scale is
derived by assigning the freezing point of water a
value of 0°C and its boiling point a value of 100°C.
A degree Celsius is thus 1/100 of the temperature
change between the two reference points. Scientists in most countries use the Celsius scale to
inside the science
Weight and Mass
100º
L
I
D
D
E
L
L
,
T
I
F
F
A
N
Y
1
5
6
8
T
S
90º
220º
212º
200º
80º
180º
70º
160º
60º
140º
50º
120º
40º
100º
30º
80º
Normal
room
temperature
40º
32º
20º
Freezing
point
of water
10º
- 10º
- 20º
- 30º
Celsius
Normal
body
temperature
60º
20º
0º
Boiling
point
of water
0º
- 20º
Fahrenheit
Comparison of the Celsius and Fahrenheit temperature scales.
Mass differs from weight because it refers to the
amount of matter an object contains and is independent
of its location on earth or any other place in the universe.
The mathematical relationship between weight (w) and
mass (m) is shown in Equation (9–2), where g is the acceleration imparted to a body by the force of gravity.
W ⫽ mg
(9–2)
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
The force with which gravity attracts a body is called
weight. If your weight is 180 pounds, this means that
the earth’s gravity is pulling you down with a force of
180 pounds; on the moon, where the force of gravity
is one-sixth that of the earth, your weight would be
30 pounds.
measure temperature. A comparison of the two
scales is shown below.
Unknown
masses
Known
masses
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D The measurement of mass.
E
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Courtesy Sirchie Fingerprint Laboratories, Youngsville, NC,
www.sirchie.com
The weight of a body is directly proportional to its
mass; hence, a large mass weighs more than a small
mass.
In the metric system, the mass of an object is
always specified, rather than its weight. The basic
unit of mass is the gram. An object that has a mass
of 40 grams on earth will have a mass of 40 grams
anywhere else in the universe. Normally, however, the
terms mass and weight are used interchangeably, and
we often speak of the weight of an object when we
really mean its mass.
The mass of an object is determined by comparing it against the known mass of standard objects.
The comparison is confusingly called weighing, and
the standard objects are called weights (masses
would be a more correct term). The comparison is
performed on a balance. The simplest type of balance for weighing is the equal-arm balance shown
in the figure. The object to be weighed is placed on
the left pan, and the standard weights are placed
on the right pan; when the pointer between the two
pans is at the center mark, the total mass on the
right pan is equal to the mass of the object on the
left pan.
The modern laboratory has progressed beyond the simple equal-arm balance, and either
the top-loading balance or the single-pan analytical balance as shown in the figures is now likely to
be used. The choice depends on the accuracy required and the amount of material being weighed.
Each works on the same counterbalancing principle
as the simple equal-arm balance. Earlier versions
of the single-pan balance had a second pan, the
one on which the standard weights were placed.
This pan was hidden from view within the balance’s
housing. Once the object whose weight was to be
determined was placed on the visible pan, the operator selected the proper standard weights (also
contained within the housing) by manually turning
a set of knobs located on the front side of the balance. At the point of balance, the weights selected
were automatically recorded on optical readout
scales. Modern single-pan balances may employ an
electromagnetic field to generate a current to balance the force pressing down on the pan from the
sample being weighed. When the scale is properly
calibrated, the amount of current needed to keep
the pan balanced is used to determine the weight
of the sample. The strength of the current is converted to a digitized signal for a readout. Another
approach is to employ a bridge circuit incorporating a strain gauge resistor that changes in response
to the force applied to it. The top-loading balance
can accurately weigh an object to the nearest 1 milligram or 0.001 gram; the analytical balance is even
more accurate, weighing to the nearest tenth of a
milligram or 0.0001 gram.
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6
8
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211
Courtesy Sirchie Fingerprint Laboratories, Youngsville, NC, www.sirchie.com
ISBN: 978-1-323-16745-8
MATTER, LIGHT, AND GLASS EXAMINATION
(b)
(a) Top-loading balance. (b) Single-pan analytical balance.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
212
CHAPTER 9
mass
A constant property of matter that
reflects the amount of material
present.
weight
A property of matter that depends
on both the mass of a substance
and the effects of gravity on that
mass.
TABLE 9–2
Densities of Select Materials (at 20°C Unless Otherwise Stated)
Substance
Density (g/mL)
Solids
Silver
Lead
Iron
Aluminum
Window glass
Ice (0°C)
10.5
11.5
7.8
2.7
2.47–2.54
0.92
Liquids
Mercury
Benzene
Ethyl alcohol
Gasoline
Water at 4°C
Water
13.6
0.88
0.79
0.69
1.00
0.998
L
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D
Gases
E
Air (0°C)
L
Chlorine (0°C)
Oxygen (0°C)
L
Carbon dioxide (0°C)
,
FIGURE 9–2
A simple procedure
for determining the
density of a solid is
first to measure its
mass on a scale and
then to measure its
volume by noting
the volume of water
it displaces.
0.0013
0.0032
0.0014
0.0020
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1
5
Density
6 ⫽
8
Density ⫽
T
S
Density ⫽
Mass = 20 g
70
70
60
60
50
50
40
40
30
30
20
20
10
10
Mass
Volume (v2 ⫺ v1)
75g
(50ml ⫺ 40ml)
75g
10ml
Volume
⫽ 7.5g/ml
refraction
The bending of a light wave as
it passes from one medium to
another.
Light, as we will learn in the next section, can have the property of a wave. Light waves travel in
air at a constant velocity of nearly 300 million meters per second until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend.
The bending of a light wave because of a change in velocity is called refraction.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
Refractive Index
MATTER, LIGHT, AND GLASS EXAMINATION
213
FIGURE 9–3
Light is refracted when it travels
obliquely from one medium to
another.
Apparent position
of ball
Air
Water
Ball
L
I
The phenomenon of refraction is apparent when we view an
D object that is immersed in a
transparent medium; because we are accustomed to thinking that light travels in a straight line,
D a ball is observed at the
we often forget to take refraction into account. For instance, suppose
bottom of a pool of water; the light rays reflected from the ball travel
E through the water and into
the air to reach the eye. As the rays leave the water and enter the air, their velocity suddenly inL
creases, causing them to be refracted. However, because of our assumption
that light travels in a
straight line, our eyes deceive us and make us think we see an object
lying
at
a higher point than
L
is actually the case. This phenomenon is illustrated in Figure 9–3.
, determines the refractive
The ratio of the velocity of light in a vacuum to that in any medium
index of that medium and is expressed as follows:
Refractive index 5
T
velocity of light in vacuum
velocity of light in medium
I
refractive index
The ratio of the speed of light in
a vacuum to its speed in a given
substance.
F
For example, at 25°C the refractive index of water is 1.333. This means that light travels
1.333 times as fast in a vacuum as it does in water at this temperature.
F
Like density, the refractive index is an intensive physical property of matter and characterA
izes a substance. However, any procedure used to determine a substance’s refractive index must
be performed under carefully controlled temperature and lightingNconditions because the refractive index of a substance varies with its temperature and the wavelength of light passing through
Y
it. Nearly all tabulated refractive indices are determined at a standard wavelength, usually 589.3
nanometers; this is the predominant wavelength emitted by sodium light and is commonly known
as the sodium D light.
1
5
similar refractive index, light is not refracted as it passes from the liquid into the solid. For this
6 the solid seems to disappear
reason, the eye cannot distinguish the liquid–solid boundary, and
from view. This observation, as we will see, offers the forensic8scientist a simple method for
comparing the refractive indices of transparent solids.
T index value for each waveNormally, we expect a solid or a liquid to exhibit only one refractive
length of light; however, many crystalline solids have two refractiveSindices whose values depend in
ISBN: 978-1-323-16745-8
COMPARING REFRACTIVE INDICES When a transparent solid is immersed in a liquid with a
part on the direction in which the light enters the crystal with respect to the crystal axis. Crystalline
solids have definite geometric forms because of the orderly arrangement of the fundamental
particle of a solid, the atom. In any type of crystal, the relative locations and distances between
its atoms are repetitive throughout the solid. Figure 9–4 shows the crystalline structure of sodium
chloride, or ordinary table salt. Sodium chloride is an example of a cubic crystal in which each sodium atom is surrounded by six chloride atoms and each chloride atom by six sodium atoms, except
at the crystal surface. Not all solids are crystalline in nature; some, such as glass, have their atoms
arranged randomly throughout the solid; these materials are known as amorphous solids.
Most crystals, excluding those that have cubic configurations, refract a beam of light into
two different light-ray components. This phenomenon, known as double refraction, can be observed by studying the behavior of the crystal calcite. When the calcite is laid on a printed page,
crystalline solid
A solid in which the constituent
atoms have a regular arrangement.
atom
The smallest unit of an element,
which is not divisible by ordinary
chemical means; atoms are made
up of electrons, protons, and
neutrons plus other subatomic
particles.
amorphous solid
A solid in which the constituent
atoms or molecules are arranged
in random or disordered positions;
there is no regular order in amorphous solids.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
214
CHAPTER 9
FIGURE 9–4
Diagram of a sodium chloride crystal. Sodium
is represented by the darker spheres,
chlorine by the lighter spheres.
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WhiteE
light
L
L
,
Slit
Prism
Screen
Red
Violet
Red
Orange
Yellow
Green
Blue
Violet
FIGURE 9–5
T
Representation of the dispersion of light by a glass prism.
birefringence
A difference in the two indices
of refraction exhibited by most
crystalline materials.
dispersion
The separation of light into its
component wavelengths.
Colored light ranging from red
to violet in the electromagnetic
spectrum.
transform light into the colors of the rainbow. This observation demonstrates that visible “white
light” is not homogeneous but is actually composed of many different colors. The process
1
of separating light into its component
colors is called dispersion. The ability of a prism to
disperse light into its component
5 colors is explained by the property of refraction. Each color
component of light, on passing through the glass, is slowed to a speed slightly different from
6 component to bend at a different angle as it emerges from the
those of the others, causing each
prism. As shown in Figure 9–5,8the component colors of visible light extend from red to violet.
Dispersion thus separates light into its component wavelengths and demonstrates that glass has a
T
slightly different index of refraction for each wavelength of light passing through it.
We have already seen thatSwhen white light passes through a glass prism, it is dispersed
into a continuous spectrum of colors. This phenomenon demonstrates that white light is not homogeneous but is actually composed of a range of colors that extends from red through violet.
Similarly, the observation that a substance has a color is also consistent with this description of
white light. For example, when light passes through a red glass, the glass absorbs all the component colors of light except red, which passes through or is transmitted by the glass. Likewise,
one can determine the color of an opaque object by observing its ability to absorb some of the
component colors of light while reflecting others back to the eye. Color is thus a visual indication
that objects absorb certain portions of visible light and transmit or reflect others. Scientists have
long recognized this phenomenon and have learned to characterize different chemical substances
by the type and quantity of light they absorb.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
visible light
I
F
the observer sees not one but two images of each word covered. The two light rays that give rise
F at different angles, and each has a different refractive index
to the double image are refracted
value. The indices of refractionA
for calcite are 1.486 and 1.658, and subtracting the two values
yields a difference of 0.172; this difference is known as birefringence. Thus, the optical properties of crystals provide points ofNidentification that help characterize them.
Y held a glass prism up toward the sunlight and watched it
DISPERSION Many of us have
MATTER, LIGHT, AND GLASS EXAMINATION
215
λ
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λ
D
FIGURE 9–6
E
The frequency of the lower wave is twice that of the upper wave.
L
L
Theory of Light
To understand why materials absorb light, one must first comprehend
,
the nature of light.
Two simple models explain light’s behavior. The first model describes light as a continuous wave;
the second depicts it as a stream of discrete energy particles. Together, these two very different
descriptions explain all of the observed properties of light, but by T
itself, no one model can explain
all the facets of the behavior of light.
I
F
continuous wave, as shown in Figure 9–6. Several terms are used to describe such a wave.
The distance between two consecutive crests (or one trough toFthe next trough) is called the
wavelength; the Greek letter lambda (λ) is used as its symbol, and the unit of nanometers is
A
frequently used to express its value. The number of crests (or troughs) passing any one given
N
point in a unit of time is defined as the frequency of the wave. Frequency
is normally designated
by the letter f and is expressed in cycles per second (cps). The speed of light in a vacuum is a
Y
universal constant at 300 million meters per second and is designated by the symbol c. Frequency
LIGHT AS A WAVE The wave concept depicts light as having an up-and-down motion of a
and wavelength are inversely proportional to one another, as shown by the relationship expressed
in Equation (9–3):
1
5
(9–3)
6
THE ELECTROMAGNETIC SPECTRUM Actually, visible light is
8 only a small part of a large
family of radiation waves known as the electromagnetic spectrum. All electromagnetic waves
T
travel at the speed of light (c) and are distinguishable from one another only by their different
wavelengths or frequencies. Figure 9–7 illustrates the various types
S of electromagnetic waves in
ISBN: 978-1-323-16745-8
F5
wavelength
The distance between crests of
adjacent waves.
frequency
The number of waves that pass a
given point per second.
c
λ
order of decreasing frequency.) Hence, the only property that distinguishes X-rays from radio
waves is the different frequencies the two types of waves possess. Similarly, the range of colors
that make up the visible spectrum can be correlated with frequency. For instance, the lowest
frequencies of visible light are red; waves with a lower frequency fall into the invisible infrared
(IR) region. The highest frequencies of visible light are violet; waves with a higher frequency
extend into the invisible ultraviolet (UV) region. No definite boundaries exist between any colors
or regions of the electromagnetic spectrum; instead, each region is composed of a continuous
range of frequencies, each blending into the other.
Ordinarily, light in any region of the electromagnetic spectrum is a collection of waves possessing a range of wavelengths. Under normal circumstances, this light comprises waves that are
electromagnetic spectrum
The entire range of radiation
energy from the most energetic
cosmic rays to the least energetic
radio waves.
X-ray
A high-energy, short-wavelength
form of electromagnetic radiation.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
216
CHAPTER 9
FIGURE 9–7
The electromagnetic
spectrum.
Energy increases
Short wavelength
Gamma rays
Long wavelength
X rays
Ultraviolet
Infrared
Microwaves
High frequency
Radio waves
Low frequency
Visible light
FIGURE 9–8
Coherent and incoherent
radiation.
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Coherent radiation
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Incoherent radiation
1
laser
An acronym for light amplification
by stimulated emission of radiation;
light that has all its waves pulsating
in unison.
photon
T as electromagnetic radiation is moving through space, its
As long
behavior can be described as that
S of a continuous wave; however, once radiation is absorbed
by a substance, the model of light as a stream of discrete particles must be invoked to best
describe its behavior. Here, light is depicted as consisting of energy particles that are known
as photons. Each photon has a definite amount of energy associated with its behavior. This
energy is related to the frequency of light, as shown by Equation (9–4):
LIGHT AS A PARTICLE
E 5 hf
(9–4)
where E specifies the energy of the photon, f is the frequency of radiation, and h is a universal
constant called Planck’s constant. As shown by Equation (9–4), the energy of a photon is directly
proportional to its frequency. Therefore, the photons of ultraviolet light will be more energetic
than the photons of visible or infrared light, and exposure to the more energetic photons of X-rays
presents more danger to human health than exposure to the photons of radio waves.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
A small packet of electromagnetic
radiation energy; each photon contains a unit of energy equal to the
product of Planck’s constant and
the frequency of radiation: E 5 hf.
all out of step with each other (incoherent light). However, scientists can now produce a beam of
5
light that has all of its waves pulsating
in unison (see Figure 9–8). This is called coherent light
or a laser (light amplification by
the
stimulated
emission of radiation) beam. Light in this form
6
is very intense and can be focused on a very small area. Laser beams can be focused to pinpoints
that are so intense that they can8
zap microscopic holes in a diamond.
MATTER, LIGHT, AND GLASS EXAMINATION
217
Now that we have investigated various physical properties of objects, we are ready to apply
such properties to the forensic characterization of glass.
Forensic Analysis of Glass
Glass that is broken and shattered into fragments and minute particles during the commission of a
crime can be used to place a suspect at the crime scene. For example, chips of broken glass from
a window may lodge in a suspect’s shoes or garments during a burglary, or particles of headlight
glass found at the scene of a hit-and-run accident may offer clues that can confirm the identity of
a suspect vehicle. All of these possibilities require the comparison of glass fragments found on
the suspect, whether a person or vehicle, with the shattered glass remaining at the crime scene.
Composition of Glass
L
Glass is a hard, brittle, amorphous substance composed of sand (silicon oxides) mixed with variI at high temperatures, and
ous metal oxides. When sand is mixed with other metal oxides, melted
then cooled to a rigid condition without crystallization, the product
D is glass. Soda (sodium carbonate) is normally added to the sand to lower its melting point and make it easier to work with.
Another necessary ingredient is lime (calcium oxide), needed toDprevent the “soda-lime” glass
from dissolving in water. The forensic scientist is often asked to E
analyze soda-lime glass, which
is used for manufacturing most window and bottle glass. Often the molten glass is cooled on a
L typically used for windows.
bed of molten tin. This manufacturing process produces flat glass
This type of glass is called float glass.
L
The common metal oxides found in soda-lime glass are sodium, calcium, magnesium, and
, by substituting in whole or
aluminum. In addition, a wide variety of special glasses can be made
in part other metal oxides for the silica, sodium, and calcium oxides. For example, automobile
headlights and heat-resistant glass, such as Pyrex, are manufactured by adding boron oxide to the
oxide mix. These glasses are therefore known as borosilicates. T
Another type of glass that the reader may be familiar with isI tempered glass. This glass is
made stronger than ordinary window glass by introducing stress through rapid heating and cooling of the glass surfaces. When tempered glass breaks, it does notFshatter but rather fragments or
“dices” into small squares with little splintering (see Figure 9–9).FBecause of this safety feature,
tempered glass is used in the side and rear windows of automobiles made in the United States, as
A of all cars manufactured
well as in the windshields of some foreign-made cars. The windshields
in the United States are constructed from laminated glass. This
N glass derives its strength by
sandwiching one layer of plastic between two pieces of ordinary window glass.
Y
ISBN: 978-1-323-16745-8
Comparing Glass Fragments
tempered glass
Glass that is strengthened by
introducing stress through rapid
heating and cooling of the glass
surfaces.
laminated glass
Two sheets of ordinary glass
bonded together with a layer of
plastic.
1 measuring the properties that
For the forensic scientist, comparing glass consists of finding and
will associate one glass fragment with another while minimizing or
5 eliminating the possible existence of other sources. Considering the prevalence of glass in our society, it is easy to appreciate
6 its greatest evidential value
the magnitude of this analytical problem. Obviously, glass possesses
when it can be individualized to one source. Such a determination,
8 however, can be made only
when the suspect and crime-scene fragments are assembled and physically fitted together. ComT glass as well as matching
parisons of this type require piecing together irregular edges of broken
all irregularities and striations on the broken surfaces (see FigureS
9–10). The possibility that two
pieces of glass originating from different sources will fit together exactly is so unlikely as to
exclude all other sources from practical consideration.
Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type. In such instances, the search for individual properties has proven fruitless.
For example, the general chemical composition of various window glasses within the capability
of current analytical methods has so far been found relatively uniform among various manufacturers and thus offers no basis for individualization. However, as discussed in Chapter 13,
trace elements present in glass have been shown to be useful for narrowing the origin of a glass
specimen. The physical properties of density and refractive index are most widely used for
characterizing glass particles. However, these properties are class characteristics, which cannot
provide the sole criteria for individualizing glass to a common source. They do, however, give
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
L
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FIGURE 9–9
When tempered glass breaks, it usually
holds together without splintering.
Courtesy Sirchie Fingerprint Laboratories, Youngsville, NC, www.sirchie.com
CHAPTER 9
Thinkstock
218
FIGURE 9–10
Match of broken glass. Note the physical
fit of the edges.
T
the analyst sufficient data to evaluate the significance of a glass comparison, and the absence of
I
comparable density and refractive index values will certainly exclude glass fragments that originate from different sources. F
F
Measuring and Comparing
Density
Recall that a solid particle will A
either float, sink, or remain suspended in a liquid, depending on
its density relative to the liquid.N
This knowledge gives the criminalist a rather precise and rapid
method for comparing densities of glass. In a method known as flotation, a standard/reference
Y a mixture of bromoform and bromobenzene may be used.
glass particle is immersed in a liquid;
The composition of the liquid is carefully adjusted by the addition of small amounts of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium. At this point,
the standard/reference glass and1liquid each have the same density. Glass chips of approximately
the same size and shape as the 5
standard/reference are now added to the liquid for comparison.
If both the unknown and the standard/reference particles remain suspended in the liquid, their
6and to that of the liquid.1 Particles of different densities either
densities are equal to each other
sink or float, depending on whether
8 they are more or less dense than the liquid.
The density of a single sheet of window glass is not completely homogeneous throughout. It
T by as much as 0.0003 g/mL. Therefore, in order to distinguish
has a range of values that can differ
between the normal internal density
S variations of a single sheet of glass and those of glasses of
different origins, it is advisable to let the comparative density approach but not exceed a sensitivity value of 0.0003 g/mL. The flotation method meets this requirement and can adequately
distinguish glass particles that differ in density by 0.001 g/mL.
Once glass has been distinguished by a density determination, different origins are immediately
concluded. Comparable density results, however, require the added comparison of refractive
1
As an added step, the analyst can determine the exact numerical density value of the particles of glass by transferring
the liquid to a density meter, which will electrically measure and calculate the liquid’s density. See A. P. Beveridge
and C. Semen, “Glass Density Measurement Using a Calculating Digital Density Meter,” Canadian Society of Forensic Science Journal 12 (1979): 113.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
Determining and Comparing Refractive Index
MATTER, LIGHT, AND GLASS EXAMINATION
219
Courtesy of Chris Palenik of Microtrace LLC, Elgin, IL
FIGURE 9–11
Hot-stage microscope.
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indices. This determination is best accomplished by the immersion
L method. For this, glass particles are immersed in a liquid medium whose refractive index is adjusted until it equals that of the
Lnotes the disappearance of the
glass particles. At this point, known as the match point, the observer
Becke line and minimum contrast between the glass and liquid medium.
The Becke line is a bright
,
halo that is observed near the border of a particle that is immersed in a liquid of a different refractive index. This halo disappears when the medium and fragment have similar refractive indices.
The refractive index of an immersion fluid is best adjusted by
T changing the temperature of
the liquid. Temperature control is, of course, critical to the success of the procedure. One apI known as a hot stage. The
proach to this procedure is to heat the liquid in a special apparatus
glass is immersed in a liquid, usually a silicone oil, and heated F
at the rate of 0.2°C per minute
until the match point is reached. Increasing the temperature of the liquid has a negligible effect
F at the rate of approximately
on the refractive index of glass, whereas the liquid’s index decreases
0.0004 per degree Celsius. The hot stage, as shown in FigureA
9–11, is designed to be used
in conjunction with a microscope, through which the examiner can observe the disappearance
of the Becke line on minute glass particles that are illuminatedNwith sodium D light or other
wavelengths of light. If all the glass fragments examined have Y
similar match points, it can be
concluded that they have comparable refractive indices (see Figure 9–12). Furthermore, the
examiner can determine the refractive index value of the immersion fluid as it changes with
temperature. With this information, the exact numerical value of1the glass refractive index can
be calculated at the match point temperature.2
5 of plate glass may not have
As with density, glass fragments removed from a single sheet
a uniform refractive index value; instead, their values may vary6by as much as 0.0002. Hence,
for comparison purposes, the difference in refractive index between a standard/reference and
8 to differentiate between the
questioned glass must exceed this value. This allows the examiner
normal internal variations present in a sheet of glass and those present
T in glasses that originated
from completely different sources.
Becke line
A bright halo that is observed near
the border of a particle immersed
in a liquid of a different refractive
index.
S
ISBN: 978-1-323-16745-8
Classification of Glass Samples
A significant difference in either density or refractive index proves that the glasses examined
do not have a common origin. But what if two pieces of glass exhibit comparable densities and
comparable refractive indices? How certain can one be that they did, indeed, come from the same
source? After all, there are untold millions of windows and other glass objects in this world. To
provide a reasonable answer to this question, the FBI Laboratory has collected density and refractive index values from glass submitted to it for examination. What has emerged is a data bank
2
A. R. Cassista and P. M. L. Sandercock, “Precision of Glass Refractive Index Measurements: Temperature Variation and Double Variation Methods, and the Value of Dispersion,” Canadian Society of Forensic Science Journal 27
(1994): 203.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
220
CHAPTER 9
FIGURE 9–12
Determination of the
refractive index of glass.
(a) Glass particles are
immersed in a liquid of
a much higher refractive
index at a temperature of
20°C. (b) At 68°C the liquid
still has a higher refractive
index than the glass. (c) The
refractive index of the liquid
is closest to that of the glass
at 100°C, as shown by the
disappearance of the glass
and the Becke lines. (d) At
the higher temperature
of 160°C, the liquid has a
much lower index than the
glass, resulting in significant
edge contrast. The reference glass fragments shown
here has a refractive index
of 1.529.
(a)
(b)
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correlating these values to their frequency of occurrence in the glass population of the United
F to all forensic laboratories in the United States.
States. This collection is available
Once a criminalist has completed
a comparison of glass fragments, he or she can correlate
F
their density and refractive index values to their frequency of occurrence and assess probability
A same source. Figure 9–13 shows the distribution of refractive
that the fragments came from the
index values (measured with sodium
N D light) for approximately two thousand glasses analyzed
by the FBI. The wide distribution of values clearly demonstrates that the refractive index is a
Y and is thus useful for defining its frequency of occurrence and
highly distinctive property of glass
3
G. Edmondstone, “The Identification of Heat Strengthened Glass in Windshields,” Canadian Society of Forensic
Science Journal 30 (1997): 181.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
hence its evidential value. For example, a glass fragment with a refractive index value of 1.5290
is found in approximately only 1 out of 2,000 specimens, whereas glass with a value of 1.5180
1 out of 2,000.
occurs approximately in 22 glasses
Although refractive index and
5 density have been routinely used for the comparison of glass for
some time, forensic scientists have long desired to extract additional information from glass frag6
ments that would make their comparison
more meaningful. The trace elemental composition of glass
held a longtime attraction to forensic
scientists
for this purpose. However, until recently, the analyti8
cal instrumentation sensitive enough to develop a trace elemental profile from a glass fragment was
T This handicap has been overcome with the introduction of a
too costly for most crime laboratories.
technique that aims a high-energy
Slaser pulse to vaporize a microscopic amount of glass, raising its
temperature by thousands of degrees. As a result, the elements present in the glass are induced to
emit light whose wavelengths correspond to the identity of the elements present (see Figure 9–14).
The distinction between tempered and nontempered glass particles can be made by slowly
heating and then cooling the glass (a process known as annealing). The change in the refractive
index value for tempered glass upon annealing is significantly greater when compared to nontempered glass and thus serves as a point of distinction.3
MATTER, LIGHT, AND GLASS EXAMINATION
221
Inside the Science
GRIM 3
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Photo courtesy of Foster & Freeman
An automated approach for measuring the refractive index of glass fragments by temperature control using
the immersion method with a hot
stage is with the instrument known as
GRIM 3 (glass refractive index measurement) (see the figure). The GRIM 3
is a personal computer/video system designed to automate the measurements
of the match temperature and refractive
index for glass fragments. This instrument
uses a video camera to view the glass
fragments as they are being heated. As
the immersion oil is heated or cooled, the
contrast of the video image is measured
continually until a minimum, the match
point, is detected (see figure). The match
point temperature is then converted to a
refractive index using stored calibration
data.
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Courtesy Foster & Freeman Limited, Worcestershire, U.K.,
www.fosterfreeman.co.uk ASKED JANE FOR UPDATED CREDIT LINE
An automated system for glass fragment identification.
GRIM 3 identifies the refraction match point by monitoring a video image of four different areas of
the glass fragment immersed in an oil. As the immersion oil is heated or cooled, the contrast of the image
is measured continuously until a minimum, the match point, is detected.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
222
CHAPTER 9
120
100
60
40
20
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, 1.5200
0
1.5100
1.5120
1.5140
1.5160
1.5180
1.5220
1.5240
1.5260
1.5280
1.5300
1.5110
1.5130
1.5150
1.5170
1.5190
1.5210
1.5230
1.5250
1.5270
1.5290
Refractive index
Federal Bureau of Investigation
Number of specimens
80
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FIGURE 9–13
Frequency of occurrence of refractive index values (measured
with sodium D light) for approximately two
I
thousand flat glass specimens received by the FBI Laboratory.
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Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
Photo courtesy of Foster & Freeman
FIGURE 9–14
The elemental profile of a
glass fragment is obtained
by aiming a high-energy
laser beam at a glass particle, inducing the emission of light wavelengths
corresponding to the identity of the elements present
in the glass.
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MATTER, LIGHT, AND GLASS EXAMINATION
223
Glass Fractures
Courtesy Sirchie Fingerprint Laboratories, Youngsville, NC,
www.sirchie.com
ISBN: 978-1-323-16745-8
Glass bends in response to any force exerted on any one of its surfaces; when the limit of its
elasticity is reached, the glass fractures. Frequently, fractured window glass reveals information
that can be related to the force and direction of an impact; such knowledge may be useful for
reconstructing events at a crime-scene investigation.
The penetration of ordinary window glass by a projectile, whether a bullet or a stone, produces a familiar fracture pattern in which cracks both radiate outward and encircle the hole, as
shown in Figure 9–15. The radiating lines are appropriately known as radial fractures, and the
circular lines are termed concentric fractures.
Often it is difficult to determine just from the size and shape of a hole in glass whether it
was made by a bullet or by some other projectile. For instance, a small stone thrown at a comparatively high speed against a pane of glass often produces a hole similar to that produced by a
bullet. On the other hand, a large stone can completely shatter a pane of glass in a manner closely
L
resembling the result of a close-range shot. However, in the latter instance, the presence of gunI caused by a firearm.
powder deposits on the shattered glass fragments points to damage
When it penetrates glass, a high-velocity projectile such asDa bullet often leaves a round,
crater-shaped hole surrounded by a nearly symmetrical pattern of radial and concentric cracks. The
D examining it is an important
hole is inevitably wider on the exit side (see Figure 9–16), and hence
step in determining the direction of impact. However, as the velocity
E of the penetrating projectile
decreases, the irregularity of the shape of the hole and of its surrounding cracks increases, so that at
some point the hole shape will not help determine the direction of L
impact. At this time, examining
the radial and concentric fracture lines may help determine the direction
L of impact.
When a force pushes on one side of a pane of glass, the elasticity of the glass permits it to
, exceeded, the glass begins to
bend in the direction of the force applied. Once the elastic limit is
crack. As shown in Figure 9–17, the first fractures form on the surface opposite that of the penetrating force and develop into radial lines. The continued motion of the force places tension on
T cracks. An examination of
the front surface of the glass, resulting in the formation of concentric
the edges of the radial and concentric cracks frequently reveals Istress markings (Wallner lines)
whose shape can be related to the side on which the window first cracked.
F are perpendicular to one glass
Stress marks, shown in Figure 9–18, are shaped like arches that
surface and curved nearly parallel to the opposite surface. The importance
of stress marks stems
F
from the observation that the perpendicular edge always faces the surface on which the crack
A crack near the point of imoriginated. Thus, in examining the stress marks on the edge of a radial
pact, the perpendicular end is always found opposite the side fromNwhich the force of impact was
applied. For a concentric fracture, the perpendicular end always faces the surface on which the
Y
force originated. A convenient way for remembering these observations
is the 3R rule—Radial
cracks form a Right angle on the Reverse side of the force. These facts enable the examiner
to determine the side on which a window was broken. Unfortunately, the absence of radial or
1 to broken tempered glass.
concentric fracture lines prevents these observations from being applied
radial fracture
A crack in a glass that extends
outward like the spoke of a wheel
from the point at which the glass
was struck.
concentric fracture
A crack in a glass that forms a
rough circle around the point of
impact.
5
6FIGURE 9–15
8Radial and concentric
fracture lines in a sheet of
Tglass.
S
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
224
CHAPTER 9
FIGURE 9–16
Crater-shaped hole made
by a bullet passing through
glass. The upper surface
is the exit side of the
projectile.
Don Farrall/ Getty RF Images Inc.
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Production of radial and concentric
T are
fractures in glass. (a) Radial cracks
formed first, commencing onSthe side of
Richard Saferstein, Ph.D.
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FIGURE 9–18
Stress marks on the edge of a radial
glass fracture. Arrow indicates direction
of force.
the glass opposite to the destructive force.
(b) Concentric cracks occur afterward,
starting on the same side as the force.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
When there have been successive penetrations of glass, it is frequently possible to determine
the sequence of impact by observing the existing fracture lines and their points of termination.
A fracture always terminates at an existing line of fracture. In Figure 9–19, the fracture on
the left preceded that on the right; we know this because the latter’s radial fracture lines terminate
at the cracks of the former.
MATTER, LIGHT, AND GLASS EXAMINATION
225
FIGURE 9–19
Two bullet holes in a piece
of glass. The left hole preceded the right hole.
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Collection and Preservation
of Glass Evidence
FIGURE 9–20
Presence of black tungsten
oxide on the upper filament
indicates that the filament
was on when it was exposed
to air. The lower filament
was off, but its surface was
coated with a yellow/white
tungsten oxide, which was
vaporized from the upper (“on”) filament and
condensed onto the lower
filament.
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The gathering of glass evidence at the crime scene and from the suspect must be thorough if the
examiner is to have any chance of individualizing the fragments to a common source. If even the
1 effort must be made to colremotest possibility exists that fragments may be pieced together, every
lect all the glass found. For example, evidence collection at hit-and-run
5 scenes must include all the
broken parts of the headlight and reflector lenses. This evidence may ultimately prove invaluable in
6 with glass remaining in the
placing a suspect vehicle at the accident scene by matching the fragments
headlight or reflector shell of the suspect vehicle. In addition, examining
the headlight’s filaments
8
may reveal whether an automobile’s headlights were on or off before the impact (see Figure 9–20).
When an individual fit is improbable, the evidence collectorT
must submit all glass evidence
found in the possession of the suspect along with a sample of S
broken glass remaining at the
crime scene. This standard/reference glass should always be taken from any remaining glass in
the window or door frames, as close as possible to the point of breakage. About one square inch
of sample is usually adequate for this purpose. The glass fragments should be packaged in solid
containers to avoid further breakage. If the suspect’s shoes and/or clothing are to be examined for
the presence of glass fragments, they should be individually wrapped in paper and transmitted to
the laboratory. The field investigator should avoid removing such evidence from garments unless
absolutely necessary for its preservation.
When a determination of the direction of impact is desired, all broken glass must be recovered and submitted for analysis. Wherever possible, the exterior and interior surfaces of the glass
must be indicated. When this is not immediately apparent, the presence of dirt, paint, grease, or
putty may indicate the exterior surface of the glass.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
chapter summary
The forensic scientist must constantly determine the properties
that impart distinguishing characteristics to matter, giving it a
unique identity. Physical properties such as weight, volume,
color, boiling point, and melting point describe a substance
without reference to any other substance. A chemical property
describes the behavior of a substance when it reacts or combines with another substance. Scientists throughout the world
use the metric system of measurement. The metric system has
basic units of measurement for length, mass, and volume: the
meter, gram, and liter, respectively. Temperature is a measure
of heat intensity, or the amount of heat in a substance. In science, the most commonly used temperature scale is the Celsius scale. This scale is derived by assigning the freezing point
of water a value of 0°C and its boiling point a value of 100°C.
To compare glass fragments, a forensic scientist evaluates two
important physical properties: density and refractive index. Density is defined as the mass per unit volume. Refractive index is the
ratio of the velocity of light in a vacuum to that in the medium under examination. Crystalline solids have definite geometric forms
because of the orderly arrangement of their atoms. These solids
refract a beam of light in two different light-ray components. This
results in double refraction. Birefringence is the numerical difference between these two refractive indices. Not all solids are crystalline in nature. For example, glass has a random arrangement of
atoms that forms an amorphous or noncrystalline solid.
review questions
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9. A gas (has, has no) definite shape or volume.
10. During the process of ___________, solids go directly
to the gaseous state, bypassing the liquid state.
11. The attraction forces between the molecules of a liquid
are (greater, less) than those in a solid.
12. Different ___________ are separated by definite visible
boundaries.
13. Mass per unit volume defines the property of ___________.
14. If an object is immersed in a liquid of greater density, it
will (sink, float).
15. The bending of a light wave because of a change in velocity is called ___________.
16. The physical property of ___________ is determined by
the ratio of the velocity of light in a vacuum to light’s
velocity in a substance.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
1. Anything that has mass and occupies space is defined as
___________.
2. The basic building blocks of all substances are the
___________.
3. The number of elements known today is ___________.
4. An arrangement of elements by similar chemical properties is accomplished in the ___________ table.
5. A(n) ___________ is the smallest particle of an element
that can exist.
6. Substances composed of two or more elements are called
___________.
7. A(n) ___________ is the smallest unit of a compound
formed by the union of two or more atoms.
8. The physical state that retains a definite shape and volume is a(n) ___________.
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Dispersion is the process of separating light into its component colors. Each component bends, or refracts, at a different angle as it emerges from a prism. The large family of
radiation waves is known as the electromagnetic spectrum.
Two simple models explain light’s behavior. The first model
describes light as a continuous wave; the second depicts light
as a stream of energy particles.
The flotation and immersion methods are best used to
determine a glass fragment’s density and refractive index, respectively. In the flotation method, a glass particle is immersed
in a liquid. The density of the liquid is carefully adjusted by
the addition of small amounts of an appropriate liquid until
the glass chip remains suspended in the liquid medium. At
this point, the glass will have the same density as the liquid
medium and can be compared to other relevant pieces of glass.
The immersion method involves immersing a glass particle
in a liquid medium whose refractive index is varied until it is
equal to that of the glass particle. At this point, known as the
match point, minimum contrast between liquid and particle is
observed.
By analyzing the radial and concentric fracture patterns
in glass, the forensic scientist can determine the direction of
impact. This can be accomplished by applying the 3R rule:
Radial cracks form a Right angle on the Reverse side of the
force.
MATTER, LIGHT, AND GLASS EXAMINATION
17. True or False: Solids having an orderly arrangement of
their constituent atoms are crystalline. ___________
18. Solids that have their atoms randomly arranged are said
to be ___________.
19. The crystal calcite has two indices of refraction.
The difference between these two values is known as
___________.
20. The process of separating light into its component colors or frequencies is known as ___________.
21. True or False: Color is a usual indication that substances
selectively absorb light. ___________
22. The distance between two successive identical points
on a wave is known as ___________.
23. True or False: Frequency and wavelength are directly
proportional to one another. ___________
24. Light, X-rays, and radio waves are all members of the
___________ spectrum.
25. Red light is (higher, lower) in frequency than violet
light.
26. A beam of light that has all of its waves pulsating in
unison is called a(n) ___________.
27. One model of light depicts it as consisting of energy
particles known as ___________.
28. True or False: The energy of a light particle (photon) is
directly proportional to its frequency. ___________
29. Red light is (more, less) energetic than violet light.
30. A hard, brittle, amorphous substance composed mainly
of silicon oxides is ___________.
31. Glass that can be physically pieced together has
___________ characteristics.
32. The two most useful physical properties of glass for forensic comparisons are ___________ and ___________.
33. True or False: Automobile headlights and heat-resistant
glass, such as Pyrex, are manufactured with lime oxide
added to the oxide mix. ___________
ISBN: 978-1-323-16745-8
review questions for inside
1. A(n) ___________ property describes the behavior of a substance without reference to any other
substance.
2. A(n) ___________ property describes the behavior of
a substance when it reacts or combines with another
substance.
3. The ___________ system of measurement was devised by the French Academy of Science in 1791.
227
34. ___________ glass fragments into small squares, or
“dices,” with little splintering when broken.
35. ___________ glass gains added strength from a layer of
plastic inserted between two pieces of ordinary window
glass; it is used in automobile windshields.
36. Comparing the relative densities of glass fragments is readily accomplished by a method known as
___________.
37. When glass is immersed in a liquid of similar refractive
index, its ___________ disappears and minimum contrast between the glass and liquid is observed.
38. The exact numerical density and refractive indices of
glass can be correlated to ___________ in order to asL
sess the evidential value of the comparison.
I 39.
D
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E 41.
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The fracture lines radiating outward from a crack in
glass are known as ___________ fractures.
A crater-shaped hole in glass is (narrower, wider) on the
side where the projectile entered the glass.
True or False: It is easy to determine from the size and
shape of a hole in glass whether it was made by a bullet
or some other projectile. ___________
True or False: Stress marks on the edge of a radial crack
are always perpendicular to the edge of the surface on
which the impact force originated. ___________
T 43.
I 44.
F
F 45.
A
N 46.
Y
A fracture line (will, will not) terminate at an existing
line fracture.
Glass fracture lines that encircle the hole in the glass are
known as ___________ fractures.
When glass’s elastic limit is exceeded, the first fractures
develop into radial lines on the surface of the (same, opposite) side to that of the penetrating force.
Collected glass fragment evidence should be packaged
in ___________ containers to avoid further breakage.
47. Glass-containing shoes and/or clothing should be individually wrapped in ___________ and transmitted to
the laboratory.
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science
4. The basic units of measurement for length, mass, and
volume in the metric system are the ___________,
___________, and ___________, respectively.
5. A centigram is equivalent to ___________ gram(s).
6. A milliliter is equivalent to ___________ liter(s).
7. 0.2 gram is equivalent to ___________ milligram(s).
8. One cubic centimeter (cc) is equivalent to one
___________.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
228
CHAPTER 9
9. True or False: One meter is slightly longer than a yard.
___________
10. The equivalent of 1 pound in grams is ___________.
14. There are ___________ degrees Celsius between the
freezing and boiling points of water.
15. The amount of matter an object contains determines its
___________.
16. The simplest type of balance for weighing is the
___________.
11. True or False: A liter is slightly larger than a quart.
___________
12. ___________ is a measure of a substance’s heat
intensity.
13. There are ___________ degrees Fahrenheit between the
freezing and boiling points of water.
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application and critical thinking
1. An accident investigator arrives at the scene of a hitand-run collision. The driver who remained at the scene
reports that the windshield or a side window of the car
that struck him shattered on impact. The investigator
searches the accident site and collects a large number
of fragments of tempered glass. This is the only type
of glass recovered from the scene. How can the glass
evidence help the investigator locate the vehicle that fled
the scene?
2. Indicate the order in which the bullet holes were made in
the glass depicted in the accompanying figure. Explain
the reason for your answer.
(a)
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3. The accompanying figure depicts stress marks on the
edge of a glass fracture caused by the application of
force. If this is a radial fracture, from which side of the
glass (left or right) was the force applied? From which
side was force applied if it is a concentric fracture? Explain the reason for your answers.
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(c)
ISBN: 978-1-323-16745-8
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
MATTER, LIGHT, AND GLASS EXAMINATION
229
further references
Bottrell, M. C., “Forensic Glass Comparison: Background
Information Used in Data Interpretation,” Forensic
Science Communications 11, no. 2 (2009), http://www
.fbi.gov/about-us/lab/forensic-science-communications/
fsc/april2009.
Caddy, B., ed., Forensic Examination of Glass and Paint.
Boca Raton, Fla.: CRC Press, 2001.
Koons, R. D., J. Buscaglia, M. Bottrell, and E. T. Miller,
“Forensic Glass Comparisons,” in R. Saferstein, ed., Forensic Science Handbook, vol. 1, 2nd ed. Upper Saddle
River, N.J.: Prentice Hall, 2002.
Thornton, J. I., “Interpretation of Physical Aspects of Glass
Evidence,” in B. Caddy, ed., Forensic Examination of
Glass and Paint. Boca Raton, Fla.: CRC Press, 2001.
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Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
headline news
Jeffrey MacDonald: Fatal Vision
The grisly murder scene that confronted
police on February 17, 1970, is one that cannot
AP Im
ages
be wiped from memory. Summoned to the Fort Bragg
L of Captain Jeffrey MacDonald, a physician, police
residence
found the
I bludgeoned body of MacDonald’s wife. She
had been
D repeatedly knifed, and her face was smashed
to a pulp.
D MacDonald’s two children, ages 2 and 5, had
been brutally and repeatedly knifed and battered to
E
death. Suspicion quickly fell on MacDonald. To the
L
eyes of investigators, the murder scene had a staged
L
appearance.
MacDonald described a frantic effort
,to subdue four intruders who had slashed at him
with an ice pick. However, the confrontation left
TMacDonald with minor wounds and no apparent
I defense wounds on his arms. MacDonald then
F described how he had covered his slashed
F wife with his blue pajama top. Interestingly,
when the body was removed, blue threads
A
were observed under the body. In fact, blue
N
threads matching the pajama top turned
Y
up throughout the house—nineteen in
one child’s bedroom, including one beneath
1 and two in the other child’s bedroom.
her fingernail,
Eighty-one blue fibers5were recovered from the master bedroom, and
two were located on a bloodstained
6 piece of wood outside the house. Later forensic
examination showed that the 48 ice pick holes8in the pajama top were smooth and cylindrical,
a sign that the top was stationary when it was slashed. Also, folding the pajama top demonstrated that
T
the 48 holes actually could have been made by 21 thrusts of an ice pick. This coincided with the number
S
of wounds that MacDonald’s wife sustained. As described in the book Fatal Vision, which chronicled the
murder investigation, when MacDonald was confronted with adulterous conduct, he replied, “You guys are
more thorough than I thought.” MacDonald is currently serving three consecutive life sentences.
ISBN: 978-1-323-16745-8
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
chapter
10
hairs and fibers
Learning Objectives
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After studying this chapter you should be able to:
T and medulla
t Recognize and understand the cuticle, cortex,
I
areas of hair
F
t List the three phases of hair growth
t Appreciate the distinction between animalFand human hairs
A
t List hair features that are useful for the microscopic
N
comparison of human hairs
Y evidence
t Explain the proper collection of forensic hair
t Describe and understand the role of DNA typing in hair
1
comparisons
KEY TERMS
anagen phase
catagen phase
cortex
cuticle
follicular tag
macromolecule
manufactured fibers
medulla
mitochondrial DNA
molecule
monomer
natural fibers
nuclear DNA
polymer
telogen phase
5 and manufactured
t Understand the differences between natural
fibers
6
8
t List the properties of fibers that are most useful
for forensic
comparisons
T
ISBN: 978-1-323-16745-8
t Describe the proper collection of fiber evidence
S
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
232
CHAPTER 10
The trace evidence transferred between individuals and objects during the commission of a crime,
if recovered, often corroborates other evidence developed during the course of an investigation.
Although in most cases physical evidence cannot by itself positively identify a suspect, laboratory examination may narrow the origin of such evidence to a group that includes the suspect. Using many instruments and techniques, the crime laboratory has developed a variety of procedures
for comparing and tracing the origins of physical evidence. This chapter and those that follow
discuss how to apply these techniques to the analysis of the types of physical evidence most often
encountered at crime scenes. We begin with a discussion of hairs and fibers.
Forensic Examination of Hair
Hair is encountered as physical evidence in a wide variety of crimes. However, any review of the
forensic aspects of hair examination
L must start with the observation that it is not yet possible to
individualize a human hair to any single head or body through its morphology. Over the years,
criminalists have tried to isolateI the physical and chemical properties of hair that could serve as
individual characteristics of identity.
D Partial success has finally been achieved by isolating and
characterizing the DNA present in hair.
D
The importance of hair as physical
evidence cannot be underemphasized. Its removal from
the body often denotes physicalEcontact between a victim and perpetrator and hence a crime of
a serious or violent nature. When hair is properly collected at the crime scene and submitted to
Lstandard/reference samples, it can provide strong corroborative
the laboratory along with enough
evidence for placing an individual
L at a crime site.
The first step in the forensic examination of hair logically starts with its color and structure,
, progresses to the more detailed DNA extraction, isolation, and
or morphology, and, if warranted,
characterization.
cuticle
The scale structure covering the
exterior of the hair.
cortex
The main body of the hair shaft.
medulla
A cellular column running through
the center of the hair.
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Morphology of I Hair
F that grows out of an organ known as the hair follicle. The
Hair is an appendage of the skin
length of a hair extends fromFits root or bulb embedded in the follicle, continues into the
shaft, and terminates at the tip end. The shaft, which is composed of three layers—the cuticle,
A to the most intense examination by the forensic scientist
cortex, and medulla—is subjected
(see Figure 10–1).
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Cortex
Cuticle
Follicle
Root
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Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
FIGURE 10–1
Cross section of skin showing hair growing out of a tubelike structure called the follicle.
HAIRS AND FIBERS
233
Cuticle
Two features that make hair a good subject for establishing individual identity are its resistance
to chemical decomposition and its ability to retain structural features over a long period of time.
Much of this resistance and stability is attributed to the cuticle, the outside covering of the hair.
The cuticle is formed by overlapping scales that always point toward the tip end of each hair. The
scales form from specialized cells that have hardened (keratinized) and flattened in progressing
from the follicle. There are three basic patterns that describe the appearance of the cuticle: cornal,
spinous, and imbricate (see Figure 10–2).
The scales of most animal hair can best be described as looking like shingles on a roof.
Although the scale pattern is not a useful characteristic for individualizing human hair, the variety of patterns formed by animal hair makes it an important feature for species identification.
Figure 10–3 shows the scale patterns of some animal hairs and of a human hair as viewed by the
scanning electron microscope. Another method of studying the scale pattern of hair is to make
L medium, such as clear nail
a cast of its surface. This is done by embedding the hair in a soft
polish or softened vinyl. When the medium has hardened, the hair is removed, leaving a clear,
I
distinct impression of the hair’s cuticle, ideal for examination with a compound microscope.
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Contained within the protective layer of the cuticle is the cortex. The cortex is made up of
spindle-shaped cortical cells aligned in a regular array, parallelEto the length of the hair. The
cortex derives its major forensic importance from the fact that it is embedded with the pigment
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granules that give hair its color. The color, shape, and distribution of these granules provide imL
portant points of comparison among the hairs of different individuals.
The structural features of the cortex are examined microscopically after the hair has been
,
mounted in a liquid medium with a refractive index close to that of the hair. Under these condiCortex
tions, the amount of light reflected off the hair’s surface is minimized, and the amount of light
penetrating the hair is optimized.
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Medulla
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Richard Saferstein, Ph.D.
The medulla is a collection of cells that looks like a central canal running through a hair. In many
F
animals, this canal is a predominant feature, occupying more than half of the hair’s diameter. The
Fthe diameter of the hair shaft
medullary index measures the diameter of the medulla relative to
(a)
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FIGURE 10–2 N
(a) The coronal, or crownlike, scale pattern
Y
resembles a stack of paper cups. (b) Spinous
or petal-like scales are triangular in shape
and protrude from the hair shaft. (c) The
1
imbricate, or flattened-scale,
type consists of
overlapping scales
5 with narrow margins.
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ISBN: 978-1-323-16745-8
(b)
(c)
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
Richard Saferstein, Ph.D.
Richard Saferstein, Ph.D.
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(c)
Richard Saferstein, Ph.D.
(a)
(d)
Continuous
FIGURE 10–4
Medulla patterns.
The initial growth phase during
which the hair follicle actively produces hair.
(f)
Interrupted
Fragmented
and is normally expressed as a fraction. For humans, the index is generally less than one-third;
1 is one-half or greater.
for most other animals, the index
The presence and appearance of the medulla vary from individual to individual and even
5
among the hairs of a given individual. Not all hairs have medullae, and when they do exist, the
degree of medullation can vary.6In this respect, medullae may be classified as being continuous,
interrupted, fragmented, or absent (see Figure 10–4). Human head hairs generally exhibit no me8
dullae or have fragmented ones; they rarely show continuous medullation. One noted exception
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is the Mongoloid race, whose members
usually have head hairs with continuous medullae. Also,
most animals have medullae that are either continuous or interrupted.
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Another interesting feature of the medulla is its shape. Humans, as well as many animals,
have medullae that give a nearly cylindrical appearance. Other animals exhibit medullae that
have a patterned shape. For example, the medulla of a cat can best be described as resembling a
string of pearls, whereas members of the deer family show a medullary structure consisting of
spherical cells occupying the entire hair shaft. Figure 10–5 illustrates medullary sizes and forms
for a number of common animal hairs and a human head hair.
A searchable database on CD-ROM of the 35 most common animal hairs encountered in
forensic casework is commercially available.1 This database allows an examiner to rapidly search
1
J. D. Baker and D. L. Exline, Forensic Animal Hair Atlas: A Searchable Database on CD-ROM. RJ Lee Group, Inc.,
350 Hochberg Rd., Monroeville, PA, 15146.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
anagen phase
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Richard Saferstein, Ph.D.
FIGURE 10–3
Scale patterns of various
types of hair. (a) Human
head hair (600×), (b) dog
(1250×), (c) deer (120×),
(d) rabbit (300×), (e) cat
(2000×), and (f) horse
(450×).
Richard Saferstein, Ph.D.
CHAPTER 10
Richard Saferstein, Ph.D.
234
(a)
(b)
235
Richard Saferstein, Ph.D.
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Richard Saferstein, Ph.D.
Richard Saferstein, Ph.D.
HAIRS AND FIBERS
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ISBN: 978-1-323-16745-8
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(d)
(e)
(f)
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FIGURE 10–5
Medulla patterns for various types of hair. (a) Human head T
hair (400×), (b) dog (400×), (c) deer (500×), (d) rabbit
(450×), (e) cat (400×), and (f) mouse (500×).
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for animal hairs based on scale patterns and/or medulla type using a PC. A typical screen presentation arising from such a data search is shown in Figure 10–6.
Root
The root and other surrounding cells within the hair follicle provide the tools necessary to produce hair and continue its growth. Human head hair grows in three developmental stages, and the
shape and size of the hair root is determined by the growth phase in which the hair happens to be.
The three phases of hair growth are the anagen, catagen, and telogen phases.
catagen phase
A transition stage between the
anagen and telogen phases of hair
growth.
telogen phase
The final growth phase in which
hair naturally falls out of the skin.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
Richard Saferstein, Ph.D.
Richard Saferstein, Ph.D.
Richard Saferstein, Ph.D.
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CHAPTER 10
RJ Lee Group
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FIGURE 10–6
Information on rabbit hair contained within the Forensic Animal Hair Atlas.
(b)
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(c)
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FIGURE 10–7
Hair roots in the (a) anagen phase,
(b) catagen phase, and (c) telogen phase (100×).
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follicular tag
In the anagen phase, which may last up to six years, the root is attached to the follicle for
continued growth, giving the root bulb a flame-shaped appearance (Figure 10–7[a]). When pulled
from the root, some hairs in the anagen phase have a follicular tag. With the advent of DNA
analysis, this follicular tag is important for individualizing hair.
Hair continues to grow, but at a decreasing rate, during the catagen phase, which can last
anywhere from two to three weeks. In the catagen phase, roots typically take on an elongated appearance (Figure 10–7[b]) as the root bulb shrinks and is pushed out of the hair follicle.
Once hair growth ends, the telogen phase begins and the root takes on a club-shaped appearance (Figure 10–7[c]). Over two to six months, the hair is pushed out of the follicle, causing the
hair to be naturally shed.
Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.
ISBN: 978-1-323-16745-8
A translucent piece of tissue surrounding the hair’s shaft near the
root; it contains the richest source
of DNA associated with hair.
Courtesy of Charles A. Linch
(a)
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HAIRS AND FIBERS
237
Identification and Comparison of Hair
Most often the prime purpose for examining hair evidence in a crime laboratory is to establish
whether the hair is human or animal in origin or to determine whether human hair retrieved at a
crime scene compares with hair from a particular individual. Although animal hair can normally
be distinguished from human hair with little difficulty, human hair comparisons must be undertaken with extreme caution and with an awareness of hair’s tendency to exhibit variable morphological characteristics, not only from one person to another but also within a single individual.
Considerations in Hair Examination
A careful microscopic examination of hair reveals morphological features that can distinguish
human hair from animal hair. The hair of various animals also differs enough in structure that the
examiner can often identify the species. Before reaching such a conclusion, however, the examiner must have access to a comprehensive collection of referenceL
standards and the accumulated
experience of hundreds of prior hair examinations. Scale structure, medullary index, and medulI
lary shape are particularly important in hair identification.
The most common request when hair is used as forensic evidence
D is to determine whether
hair recovered at the crime scene compares to hair removed from a suspect. In most cases, such a
comparison relates to hair obtained from the scalp or pubic area. D
Ultimately, the evidential value
of the comparison depends on the degree of probability with which
E the examiner can associate
the hair in question with a particular individual.
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In making a hair comparison, a comparison microscope is an
L and known hair together,
invaluable tool that allows the examiner to view the questioned
side by side. Any variations in the microscopic characteristics ,will thus be readily observed.
Because hair from any part of the body exhibits a range of characteristics, it is necessary to have
an adequate number of known hairs that are representative of all of its features when making
a comparison.
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In comparing hair, the criminalist is particularly interested in matching the color, length, and
diameter. Other important features are the presence or absence ofI a medulla and the distribution,
shape, and color intensity of the pigment granules in the cortex. AFmicroscopic examination may
also distinguish dyed or bleached hair from natural hair. A dyed color is often present in the cuticle as well as throughout the cortex. Bleaching, on the other hand,Ftends to remove pigment from
the hair and to give it a yellowish tint. If hair has grown since itA
was last bleached or dyed, the
natural-end portion will be quite distinct in color. An estimate of the time since dyeing or bleachN
ing can be made because hair grows approximately one centimeter per month. Other significant
but less frequent features may be observed in hair. For example,
Ymorphological abnormalities
may be present because of certain diseases or deficiencies. Also, the presence of fungal and nit
infections can further link a hair specimen to a particular individual.
HAIR CHARACTERISTICS
1
5 against standard/reference
an appropriate approach for including and excluding questioned hairs
hairs, many forensic scientists have long recognized that this approach is subjective and is
6
highly dependent on the skills and in...
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