MODULE 7 GLASS AND GLASS FRAGMENTS AND FRACTURES

INTRODUCTION

Glass is important as physical evidence because it breaks and pieces are scattered at the crime
scene and on the suspect. It is a common type of thing carried away evidence in and burglary and
vehicle hit and run cases. The evidence maybe fragments of a headlight leads found at the scene of
a hit and run accident, window glass from the scene of robbery or glass through which a bullet was
fired.
At the end of this chapter, the students must be able to:
1. Define glass and identify its composition
2. Determine how glass fragments are analyze in the scene of crime
3. Identify cases related to glass fragment and fractures

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LEARNING RESOURCES
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EXPLORE
Glass has been shown to be very useful evidence because it is often encountered in criminal
investigations. For example, when a burglar breaks a windowpane, small fragments of glass are often
showered onto his or her chair, clothing, or shoes, and these fragments can be later found on the
suspect as transfer evidence.
Types of Glass
Glass is a solid that is not crystalline but rather has an amorphous structure. The atoms of an
amorphous structure. The atoms of an amorphous solid have a random, disordered arrangement,
unlike the regular, orderly arrangement that is characteristic of crystalline solids. Another
characteristics property of glass is that it softens over a wide temperature range rather than melting
sharply at a well-defined temperature.
Soda-lime glass is commonly used in windows and bottles. It consists of 70% silicon dioxide
(SiO2), 15% Sodium oxide (Na2O), 10% Calcium oxide (CaO), and 5% of other oxides. This type of
glass is made by heating together sodium carbonate (baking powder), calcium oxide (lime) or calcium
carbonate (limestone), and silicon dioxide (sand). Soda-lime glass has a green to yellow tint, which
is most easily seen by looking at the edge of the pane. This color is caused by an iron impurity that
is present in the sand. Soda-lime glass starts to soften when it is heated to a temperature of more
than 650° C, a fact that can prove useful when investigating fires. For example, if the windows of a
burned building are found to be deformed (melted), the temperature of the fire must have exceeded
650° C. Common window glass fractures when its surfaces or edges are placed under tension, and
an edge fissure may propagate into visible cracks.
A variety of metal oxides can be added to this basic recipe to give glass a special appearance.
For example, the addition of Lead oxide (PbO) to glass made the resulting product more shock and
heat resistant. This borosilicate glass was given the trade name Pyrex and was subsequently found
to resist attack from virtually all chemicals except hydroflouric acid (HF), which etches its surface.
TEMPERED GLASS
Tempered glass (also known as safety glass) is more than four times stronger than window
glass. During its manufacture, the san, lime, and sodium carbonate are heated together, and the hot
glass that is formed is rolled into sheets, its upper and lower surfaces are then cooled rapidly with
jets of air. This process leaves the center of the glass relatively hot compared with the surfaces and
forces the surfaces and edged to compress. Tempered glass is stronger because wind pressure or
impact must first overcome this compression before there is any possibility of fracture.
When tempered glass breaks, it does not shatter into pieces with sharp edges, but rather
breaks into “dices” (i.e., small pieces without sharp edges). Tempered glass is used in the side and
rear windows of automobiles, in large commercial windows, in doors and even in shower doors and
home windows is less than 1 ft from the floor.
WINDSHIELD GLASS
Automobile windshields are made from the laminated glass. Today, windshields are made with
two layers of glass, with a high strength vinyl plastic film, such as polyvinyl butyral (PVB), being
sandwiched in between the layers. The three pieces are laminated together by applying heat and
pressure in a special oven called an autoclave.
This type of glass is ideal for automobile windshields because of its strength and shatter
resistance.
The plastic fil holds the glass in place when the glass breaks, helping to reduce injuries from
flying glass. The film can also stretch, yet the glass still sticks to it. Laminated safety glass is very
difficult penetrate compared with normal window-pane glass. The glass sandwich construction allows
the windshield to expand in an accident without tearing, which helps hold the occupants inside the
vehicle. Banks use a similar bullet-proof glass that has multiple layers of laminated glass.

FIGURE 1 Automobile windshields are made from

laminated glass whereas the side and rear windows
consists of tempered glass.

FORENSIC EXAMINATION OF GLASS EVIDENCE:AN OVERVIEW

For the Forensic Scientist, the goals in examining glass evidences are twofold:
1. To determine the broader class to which the glass belongs, thereby linking one piece of glass
to another
2. To individualize the glass to one source – a particularly difficult challenge given that glass is
so ubiquitous in modern society.
To pinpoint the source of the glass evidence, the forensic examiner needs the two usual samples:
glass fragments collected from the crime scene and glass fragments taken from some item belonging
to the suspect. The examiner must then compare these samples (often side-by-side via a
stereomicroscope) by identifying their characteristics -for example, their color, fracture pattern,
scratches and striations (irregularities) from manufacturing, unevenness of thickness, surface wear
(outside versus inside surfaces), surface film or dirt, and weathering patterns. In particular, the
examiner tries to fit the “pieces of the puzzle” together by matching the irregular edges of the broken
glass samples and finding any corresponding irregularities between the two fragments. Finding a
perfect match is tantamount to individualizing the glass to a single source with complete certainty.

FIGURE 2 Matching broken pieces of

glass. Finding a perfect match is
tantamount to individualizing the
glass to a single source with
complete certainty

NON - OPTICAL PHYSICAL PROPERTIES OF GLASS

Many non-optical physical properties can be used to compare a questioned specimen of glass
to a known sample. These non – optical physical properties include surface curvature, texture, and
special treatments. Clearly, frosted glass cannot be a match to a clear window glass. Similarly, a
curved piece(such as a fragment from a bottle) cannot come from the same source as a flat piece
(such as from a window). And finally, laminated glass would not compare to wire reinforced glass.
Thus, these sorts of comparisons are most useful in proving that the two pieces cannot be associated.
SURFACE STRIATIONS AND MARKINGS
When sheet glass is rolled, the rollers leave parallel striation marks, called ream marks, on the
surface. Even polishing does not completely remove these marks, and their presence can be enhanced
by low – angle illumination and photography. These ream marks may hint at how various pieces
should be oriented in the case of an indirect physical match where an intervening piece may be
missing. The relative spacing might also be useful as a means of individualization. Surface scratches,
etchings, and other markings might be employed in a similar way as the forensic examiner tries to
piece together the puzzle.
SURFACE CONTAMINANTS
The presence of such impurities as paint and putty is useful in two ways. First, the patterns
of the adhering materials might suggest how the pieces fit together. Second, chemical analysis of the
adhering materials might further individualize the pieces and prove their association.
THICKNESS
Thickness can be measured to a high degree of accuracy with a micrometer. One must be
careful, however, in assuming that the thickness is constant- it is not, particularly in curved pieces of
glass. For this reason, the forensic examiner must take several representative measurements of both
the known and the questioned samples. Determination of curvature can distinguish flat glass from
container, decorative, or ophthalmic glass. Thickness is a very useful way of proving that two pieces
of glass, which are otherwise extremely similar, are not actually from the same source.
HARDNESS
A number of sclaes are used to describe the hardness of substances. Geologists and
mineralogists often employ the Mohs scale, which indicates a substance’s hardness relative to other
substances. On the Mohs scale, the softest common mineral -talc – is assigned a relative value of 1,
and the hardest common mineral -diamond – is assigned a relative value of 10. Each of the remaining
values is assigned to another appropriate common mineral. For example, quarts is assigned the Mohs
value 7 and topaz is assigned the Mohs value 8.
The relative positions of the minerals on the Mohs scale reflect their scratching power. A
harder substance will scratch a softer one. Thus, diamond will scratch everything else on the list;
topaz will scratch quartz and everything lower on the Mohs scale, down to the talc. Talc, by contrast,
will not scratch anything else on the list. For an unknown mineral, or substance, its relative hardness
is determined by using it to try to scratch the benchmark minerals. Its position on the scale is between
the benchmark minerals. Its position on the scale is between the benchmark mineral, which it
scratches, and the next mineral on the list, which scratches it. For instance, an unknown mineral that
scratched by topaz, would be assigned a relative position between 7 and 8. In this same fashion, all
other materials can be ordered appropriately.
The Mohs scale is not very useful for glass samples, however, because all glasses tend to fall
in the same range, between 5 and 6. Thus, the Mohs scale is too insensitive for Forensic work, as are
all the of the other standard hardness scales. Generally, the forensic lab establishes relative hardness
by referring to glass samples in its collection. The relative scratching power of the known and
questioned samples is established by the trying to scratch these samples with glass in the lab’s
collection. Either the scratching powers of the known or unknown samples are similar or they are not.
GLASS FRACTURES
Elasticity is the ability of a material to return to its previous shape after a force is exerted on
it. For example, when a force is exerted on a pane of glass, it stretches (this bending may not be
visible to the naked eye). If the force is not too high, the glass will then return to its original state
and no damage occurs. However, if the force exceeds the glass’ elasticity, the glass fractures.
The forensic examiner may be able to analyze fractured windowpanes and determine the
direction of an impact and the amount of force applied to them, suggesting what happened at the
scene. For example, it is often important to establish whether a window was broken from the inside
or the outside. At the scene of a homicide, a broken window near the door latch may be an attempt
to disguise the crime as a burglary. In the case of burglary, the window would have been broken
from the outside. However, if the homicide was deliberate, the perpetrator may have broken the
window from the inside in an attempt to mislead investigators into thinking burglary was the intruder’s
primary goal.
CHARACTERISTICS OF GLASS FRACTURES
Glass may be subjected to three types of forces (strains):
a. Compressive force squeezes the material
b. Tensile force expands the material
c. Shear force slides one part of the material in one direction and another part in a different
direction.
Each of these forces causes a deformation, which is resisted by the internal cohesion (stress)
of the material. Glass breaks when a tensile strain that is sufficient to overcome the natural tensile
stress limit of the material is applied.
 Each of these forces causes a deformation, which is resisted by the internal cohesion (stress)
of the material. Glass breaks when a tensile strain that is sufficient to overcome the natural tensile
stress limit of the material is applied.

Figure 3 Radial cracks grow from

the loaded point outward and
from the unloaded side to the
loaded side. Tangential (also
known as concentric) cracks grow
from one radial crack to another
and from the loaded side to the
unloaded side

If a person places a weight on a horizontal sheet of glass, the pane will experience
compressive strain where the load meets the pane. The side holding the weight is called the
loaded side, designated as side L, and the unloaded side is designated as side U. The deformation
induced by the load will cause side U to expand, so side U will experience a tensile strain. If the
tensile strain is sufficient to overcome the tensile strength of the pane, the pane will develop
cracks on the unloaded side. Several of these cracks may appear, and they will grow or travel in
two directions simultaneously. First, they will grow from the unloaded to the loaded side. Second,
they will radiate outward, away from the load point; they are, therefore, called radial cracks. The
radial cracks form several pie-shaped (or triangular) sectors radiating from the point of loading.
If the load is suddenly removed, these sectors will stay in place because the third side of each of
the triangular sections is still solid glass.
If the load persists, however, each sector will continue to be forced outward. This movement
causes compressive strains on side U and concurrent tensile strains on side L. These strains will
cause new cracks to develop on the loaded side. As before, these cracks grow in two ways: first
from the loaded to the unloaded side, and second, until they connect two radial cracks. These
new cracks are called tangential cracks or concentric cracks, and the resulting pattern has a spider
web appearance.
Note that radial cracks grow from the load point outward and from the unloaded side to the
loaded side. In contrast, tangential cracks grow from one radial crack to another and from the
loaded side to the unloaded side. This is the case if the weight was placed statically on a pane of
glass.
By contrast, when a bullet is shot at the pane of glass, the load is a projectile. The load side
is known as the entrance side, and the unloaded side is known as the exit side. The same cracking
occurs, and the same hole formation happens, when a static load is applied. However, as the
initial velocity of the projectile increases, the central hole becomes smaller, the cracking patterns
become simpler, and the central hole develops a pattern wherein the exit hole is invariably wider
than the entrance hole.
 Figure 4 The bullet entered from
the backside (entrance side),
making a smaller hole, and
passed through the glass pane,
leaving a wider hole at the front
surface (exit side).

Examination of the edges of broken pieces of glass will reveal a set of curved lines known as
rib marks (or “stress” marks). These arcs are always nearly perpendicular to the surface at the
side on which the break started, and they curve until they are nearly parallel to the surface on
the opposite side (e.g., the side to which the break grew). In a radial crack, the rib marks will be
nearly perpendicular to the unloaded (or exit) side and nearly parallel to the loaded (or entrance)
side. Things will be exactly reversed for a tangential crack, which grows in the opposite way. The
3R rule helps in remembering this pattern:
■Radial cracks give rib marks, which make
■Right angles on the
■Reverse side from where the force was applied.
The direction of lateral propagation of the crack is always from the concave sides of the rib
marks toward their convex sides. Thus, in a radial fracture, the rib marks will be oriented with
their concave sides “cupped” toward the load (or entrance) point
Forensic Examination of Glass Fractures
If all of the glass pieces are present, the First thing to check for is the hole made by the load
or projectile (e.g., bullet, hammer), which will be wider on the exit side. As can be seen in FIGURE
5-5, the angle at which a bullet pierces a pane of glass can help identify the position of the shooter.
If the bullet came at an acute angle from the le, glass fragments will be sprayed to the right and the
exit hole will be an irregular oval. If the bullet came from an acute angle from the right, glass
fragments will be sprayed to the le and the exit hole will be an irregular oval. This test works best
when the hole is made by a high-speed projectile. In the event that the hole was made by a low-
speed projectile (such as a hammer), this test will not be very meaningful.
Therefore, for low-speed projectiles, it is usually best to examine the rib marks. Of course, to
make this examination meaning-full, each edge must be determined to be either a radial or a
tangential crack (which is why it is so important that all pieces be collected), and interior and exterior
sides of the pieces must be identified (which is why it is so important that the investigators mark the
proper orientation of each piece directly on the item, as well as documenting all orientations in their
notes and photos).Therefore, if a forensic scientist is examining the edge of a radial fracture,
whichever side shows nearly perpendicular rib marks will be the unloaded (or exit) side, that is, the
side away from the force that caused the break. Alternatively, if the forensic scientist is examining a
tangential fracture, the side showing the nearly perpendicular rib marks will be the loaded (or
entrance) side, that is, the side from which the original breaking force was applied.
 In the event that the investigator or evidence technician neglected to mark which side of the
glass was inside and which side was outside, it is sometimes possible to figure out this information in
the lab. Traces of window putty, for example, would indicate an exterior side, and paint traces of
different colors might also be used to distinguish between the two sides.

Figure 5 The angle at which a bullet pieces

a pane of glass can help identify the
position of the shooter

Of course, the preceding discussion assumes that the glass is not tempered. When tempered
glass breaks, it produces small pieces; the fractures cannot be categorized as radial or tangential, so
the kind of analysis mentioned previously is not applicable.
When there are several bullet holes, analysis can determine the sequence of the impacts. The
first shot will cause fractures that simply “run out” (terminate) wherever the original strains have
been sufficiently relieved in the material. The radial fractures associated with a second shot will run
out when they meet a fracture from the First shot, and so on for all subsequent shots (FIGURE
5-6).The majority of fragments recovered from a suspect’s clothing or hair will likely be very small
(0.25 to 1 mm). Most glass evidence adhering to a suspect is lost fairly rapidly, depending on the
suspect’s subsequent activities and the texture of his or her clothing. For example, wool sweaters will
retain glass fragments longer than a leather jacket. The size of a fragment may be so small that
individual characteristics cannot be found. In such cases, the forensic examiner turns to
measurements of density and refractive index to characterize glass evidence.
Glass Density Tests
Density tests are often performed on glass fragments. When a forensic scientist measures the
density of a glass fragment, he or she is measuring one of its physical properties. Density is a class
characteristic, so it cannot serve as the sole criterion used for individualizing the glass evidence to a
single source. Such measurements can, however, give the forensic scientist enough data to warrant
further testing of other evidence or to pro-vide enough evidence to exclude the glass fragments as
having originated somewhere other than the crime scene. In addition, if a sufficient amount of
separate class characteristic evidence can be gathered against a suspect, the evidence collectively
may make a strong circumstantial case, which may result in conviction.
To see how this works, consider decorative glass. This type of glass is made by adding
different minerals to the glass recipe as the basic ingredients—sand, lime, and sodium carbonate—
are being heated. The density of the resulting glass will vary with the type and amount of minerals
added. If a recovered glass fragment is placed in a liquid that has higher density than the glass, the
glass fragment will coat. If the liquid is less dense than the glass fragment, the glass will sink. When
the density gradient column method is used to determine the density of glass, the forensic scientist
uses a density gradient tub filled with a liquid that has been specially prepared to have a density
gradient.
The gradient is prepared such that the density at any level is less than that of any level lower
in the tube and greater than that of any level higher in the tube. The gradient is prepared by mixing
bromoform and bromobenzene, two dense organic liquids, in different proportions. When glass
fragments are poured in the top of the column, they fall through the liquid until they become sus-
pended in the liquid at the level that is the same density as the glass fragment. Fragments of different
densities will, therefore, settle at different levels in the column. The questioned glass fragment’s
density may then be compared with a glass sample from the crime scene to prove (or disprove) that
it is a match.
Density measurements should not be performed on fragments of glass that are cracked or
contain an inclusion, because these flaws will make the glass seem less dense than it really is. (An
inclusion is a defect that forms when a particle or bubble becomes embedded in the main body of
the glass.) Window glass does not have a uniform density. For this reason, the variation in density of
the known sample should be determined with samples taken from different locations in the window,
or door, whenever possible. Likewise, because the surface or edge of tempered glass is denser than
at its interior, care must be taken with tempered glass to measure several known samples. Density
comparisons between known and questioned specimens should be made using fragments of
approximately equal size.
The Federal Bureau of Investigation (FBI) has reported density results for 1,400 glass samples.
From this information, it is known that the range of densities for flat glass, container glass, and
tableware glass all overlap.
 Figure 6 In these two bullet holes in one piece of glass, the formation of (B) preceded the formation of (A).

When the density tests are concluded, any evidence that does not match the known specimen
can be excluded. However, if questioned and known samples are found to have comparable densities,
further testing is required. A refractive index test is usually performed to support the comparison. If
the density measurement indicates that the specimen from the crime scene matches the reference
material, a refractive index test that also indicates a match will improve the discrimination capability
by approximately twofold.
Optical Physical Properties of Glass
1. Color
Comparing the color of a suspect piece of glass with the color of a reference sample can
distinguish whether the two samples share a common source. As a consequence, significant color
differences between glass fragments can be used as the basis for exclusion of a suspect.
Given that sample size may affect the apparent color, side-by-side comparisons should be
made with fragments of approximately the same size. These fragments should be visually compared
by placing them on edge over a white surface using natural light. Viewing the glass in this way allows
for the optimal observation of color. It also allows the examiner to distinguish between the true color
of the glass and any coatings or films that might be present on the glass’s surface. In addition,
observing the glass using both fluorescent and incandescent light is often helpful in distinguishing
colors.
2. Refractive Index
Light has wave properties. That is, a beam of light traveling from a gas (such as air) into a
solid (such as glass) undergoes a decrease in its velocity, such that the beam bends downward as it
passes from the air into the glass. The application of this phenomenon allows the determination of
the glass’s refractive index, a measure of how much the light is bent (refracted) as it enters the glass.
The bending of a light beam as it passes from one medium to another is known as refraction. The
refractive index, η, is the ratio of the velocity of light in the air to the velocity of light in the glass
being measured. The velocities of light in both media are related to the angles that the incident and
refracted beams make with a theoretical line drawn vertically to the glass surface (FIGURE 5-7)
 FIGURE Refraction of light through
glass. The refractive index – a
measure of how much the light is
bent (refracted) as it enters the glass
– can be used as a basis of
comparison

The temperature and wavelength of the light being refracted influence the refractive index for
any substance. The temperature of the sample affects its density, and the density change affects the
velocity of the light beam as it passes though the sample. Therefore, the temperature at which the
refractive index is determined is always specified by a superscript in the notation of η. Likewise, the
wavelength of the light used affects the refractive index because light of differing wavelengths bends
at different angles. e bright yellow light from a lamp containing sodium, which produces a beam with
a wavelength of 589 nm is commonly called the sodium D line. is lamp provides the standard
wavelength of light, denoted as η D. Thus, the refractive index of a liquid measured at 20°C using a
sodium lamp that gave a reading of 1.3850 would be reported as η 20D=1.3850.

Single sheets of plate glass, such as those commonly used for making windows, usually do
not have a uniform refractive index value across the entire pane. Because the index of refraction can
vary as much as 0.0002 from one side to another, the difference in the refractive indices of the
questioned plate glass fragment and the reference sample must be smaller than 0.0002 if the forensic
scientist is to be able to distinguish the normal variations in a pane of glass from variations that would
rule out a match altogether.
 The refractive index is one of the most measured physical properties in the forensic laboratory,
because it gives an indication of the composition and the thermal history of the glass. Two methods
are used to measure the refractive index of glass: the oil immersion method and the Emmons
procedure.

Oil Immersion Method

When using the oil immersion method, a forensic examiner places the questioned glass
fragments in specialized silicone oils whose refractive indices have been well studied. e refractive
index of the oil is temperature dependent: As its temperature increases, its refractive index decreases.
Silicone oils are chosen for this task because they are resistant to decomposition at high temperatures.
e refractive index of virtually all window glass and most bottles can be compared by using silicone oil
as the comparison liquid and by varying its temperature between 35°C and 100°C.

An easy way to vary the refractive index of the immersion oil is to heat it. e suspected glass
fragments and immersion oil are placed on a microscope slide, which is then inserted into a hot-stage
microscope. The stage of such microscopes is fitted with a heater that can warm the sample slowly,
while accurately reporting the temperature to ±2°C. A filter inserted between the lamp and the
sample allows light with a constant 589-nm wavelength to reach the sample. Increasing the
temperature has little effect on the refractive index of the glass but decreases the refractive index of
the oil by about 0.004 per 1°C.

When the glass fragments are initially observed through the microscope, they will produce a
bright halo around each fragment, known as the Becke line (FIGURE 5-9). As the temperature
increases, the refractive index of the oil decreases until the Becke line and the glass fragments
disappear from view. At this point (called the match point), the refractive indices of the oil and the
glass fragment are the same, so the exam-iner is no longer able to see the glass fragments that are
immersed in the oil. e examiner can compare suspect and known samples in this way to determine
whether they have the same match point; alternatively, he or she can estimate the refractive index
of the glass from graphs that report the refractive index of the oil as a function of temperature.

Automated systems are also available for making refractive index measurements using the
immersion method. The Glass Refractive Index Measurement (GRIM) system, for example, combines
a hot-stage microscope with a video camera that records the behavior of the glass fragments as they
are being heated . That is, the camera shows the contrast between the edge of the glass fragment
and the immersion oil as the temperature increases, until it reaches the match point. e GRIM system’s
computer then converts this temperature to a refractive index using reference information stored in
a database.

Emmons Procedure

The Emmons procedure, which was developed by the Association of Official Analytical
Chemists, uses a hot-stage microscope in conjunction with different source lamps. It measures the
index of refraction at a variety of wavelengths. Most often, the refractive index measurements are
recorded by first taking a measurement with a sodium lamp (the sodium D line at 589 nm) and then
by using a hydrogen lamp (which produces two lines, the C line at 656 nm and the F line at 486 nm).
e microscope converts the difference in the refractive indices between the particle of glass and the
silicone oil to a difference in brightness contrast, and it enhances the Becke line. This procedure
increases the precision of the refractive index measurements taken on the glass particles.

The questioned glass is crushed and placed in the silicone oil on the hot stage. As the
temperature of the hot stage increases, measurements are taken at the three different wavelengths
(486, 589, and 656 nm). Lines representing the refractive index of the glass as a function of
wavelength are recorded for each temperature. ese data are then superimposed on a complex graph,
known as the Hartmann net. The Hartmann net contains the correlation between the refractive index
and the wavelength at fixed temperatures for the silicone oil. The point at which the dispersion lines
for the glass samples intersect the dispersion lines for the silicone oil is where the refractive index of
the glass sample is determined. Three separate indices of refraction are recorded: ηC, ηD, and ηF.
Because three separate measurements are taken on each sample, this method, although more dicult
to carry out, gives more precise refractive index measurements.

Refractive Index of Tempered versus Non tempered Glass

Often, a forensic examiner needs to determine whether the questioned glass sample is
tempered or non-tempered glass. Tempered glass can be distinguished from non-tempered glass by
heating the glass fragments in a furnace at a temperature higher than 600°C in a process known as
annealing. If the questioned glass sample is large enough, it can be broken in two. Each piece is
heated separately in the oven, is allowed to cool, and then has its refractive index measured. Because
annealing alters the optical properties of the glass, the change in refractive index between the two
annealed pieces can be used to determine if it is tempered or non-tempered glass. Aer annealing, the
change in refractive index for tempered glass is much greater than the change observed for non-
tempered glass.

Variations in Density and Refractive Index

As with other types of evidence, the properties of glass are more often used to exonerate
suspects than to individualize samples and definitively prove a connection between a suspect and a
crime scene. Indeed, if either the densities or the refractive indices of a questioned glass specimen
and a reference glass sample do not match, the forensic scientist can easily prove that they did not
share a common origin. However, glass is so ubiquitous, and so many manufacturers use the same
processes to produce each type (e.g., rolling molten glass into at sheets to make windows), that
sometimes, even fragments from different sources may have similar indices of refraction or similar
densities. us, individualizing glass samples accurately is particularly challenging.

To assist crime labs in making such distinctions, the FBI has compiled density and refractive
index data about glass from around the world. ese data indicate how widespread the use of a glass
with a specific refractive index is. For example, a glass fragment having a refractive index of 1.5278
was found in only 1 out of 2,337 specimens in the FBI database, while glass with a refractive index
of 1.5184 was found in more than 100 of the 2,337 specimens. e forensic scientist can access this
FBI database whenever he or she needs to compare the refractive index of a questioned glass
fragment to refractive index information and, thereby, calculate the probability that two such samples
might be matches as a result of sheer chance.

The FBI has also correlated the relationship between their refractive indices and densities for
1,400 glass specimens. The results show that once the refractive index of a glass specimen is known,
the subsequent measurement of its density will improve the discrimination capability of the
measurements by approximately twofold. Most forensic examiners prefer to measure refractive index
simply because refractive index measurements are faster and easier to make than density
measurements, and often, the glass fragment size is too small to get an accurate density
measurement. If the glass fragment is large enough, both the refractive index and the density should
be determined unless other discriminating measurements, such as elemental analysis, are performed.

Elemental Analysis of Glass

 The physical and optical methods for forensic comparison of glass fragments are well
established in crime labs and widely accepted in courts throughout the world. ese analytical
methodologies have two other advantages: (1) ese tests are nondestructive, so the evidence is
preserved for additional testing, and (2) the tests are performed using inexpensive instruments. ese
advantages ensure that these tests will remain the principal methods for the comparison of glass.
Methods of elemental analysis— particularly those in which the specimen is consumed during the
analysis—should be used only after all nondestructive methods of examination have been completed
and in cases where additional discrimination is necessary.

The elemental composition of glass can be measured by surface techniques such as the use
of a scanning electron microscope (SEM) or X-ray fluorescence (XRF). e SEM has several
disadvantages that limit its value in the analysis of glass fragments. Primary among these is that,
because of the irregular shape of the glass fragments, precise quantitative determination of element
concentration is not possible. The XRF, by contrast, is routinely used for elemental analysis of glass.
For instance, the glass industry uses XRF as an accurate, precise method of enforcing quality control
during glass manufacturing. e XRF instrument focuses a beam of X-rays on the surface of the glass
and then measures the energy of the X-rays that are emitted from the glass. e energy of the emit-
ted X-rays can be correlated to the presence of specific elements. In one study, XRF was used to
measure the ratios of 10 elements in window glass samples that had virtually identical indices of
refraction. When the elemental ratios determined by XRF were compared, the source of 49 of the 50
glass specimens could be correctly determined. Also, a major advantage of XRF is that it does not
destroy the sample.

The elemental composition of glass can also be measured by flameless atomic absorption
spectrophotometry (FAAS) or inductively coupled plasma (ICP) methods. There are two major
disadvantages of using these methods for the analysis of glass fragments. First, the glass fragment
must be dissolved in acid and small samples of the resulting solution then injected into the instrument,
which means that the original sample is destroyed. Second, these methods entail the use of hazardous
chemicals, such as hydro fluoric acid.

Despite these disadvantages, the ICP method, when coupled with an optical emission
spectrometer (ICP-OES), has been shown by the FBI to be a dependable method for the determination
of 10 elements in glass: aluminum, barium, calcium, iron, magnesium, manganese, sodium,
strontium, titanium, and zirconium. The FBI studies also demonstrated that the determination of the
concentrations of these 10 elements provides a great degree of discrimination capability. An ICP-OES
study of the elemental distribution of automobile side-window glass, for example, found that the
probability of two glass samples from different cars being indistinguishable was 1 in 1,080, compared
with 1 in 5 when just the refractive indices were used as the basis of comparison.