Mrs. Ashley
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Glass is made up primarily of three
substances
◦ Sand
◦ Soda
◦ Lime
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Most glass is Bottle Glass or also called SodaLime glass (90% of all glass)
Windows are made from Float Glass which is
soda lime glass that is allowed to cool on top
of a liquid
Radial
Concentric
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A projectile hole is
inevitably wider at
the exit side
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Backscatter- when window breaks, most of
the fragments will be carried forward, but
some will be projected backwards, called
backscatter
Heat fracture-In a fire, glass may break due
to heat fracture that looks like wavy fracture
lines towards the heat source
A fracture always terminates at an
existing line of fracture
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Radial cracks are at Right angles
to the Rear (side opposite the
impact)
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Exceptions
–tempered glass
“dices” without forming ridges
–very small windows held tightly
in frame
can’t bend or bulge appreciably
– windows broken by heat or
explosion
no “point of impact”
Do the glass particles from suspect and crime
scene fit together?
◦ Most of the time the glass evidence is too
fragmentary or too minute to permit a comparison
of this type.
◦ Most often it is the composition of the glass that is
examined and the method of how glass was
broken.
The general composition of most glass is the
same.
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Density
Refractive Index
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Each type of glass has a density that is
specific to that glass. One method of
matching glass fragments is by density
comparison.
Density (D) is calculated by dividing the mass
(m) of a substance by its volume (V). The
formula for calculating density can be written
as D = m
V
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Measure the density using a scale (be sure to
zero scale before using and put to correct
units – grams)
Measure the volume by measuring the
amount of water displaced using a graduated
cylinder in milliliters.
Use your calculator on phone to find M/V to
find the density
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Refractive Index
◦ When light travels from
one medium to another its
speed changes relative to
the density of the medium.
This can be observed as
the light bends when
traveling from one medium
to another.
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The speed of light in a vacuum is always the same,
◦ but when light moves through any other medium it travels
more slowly since it is constantly being absorbed and
reemitted by the atoms in the material.
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The ratio of the speed of light in a vacuum to the
speed of light in another substance is defined as
the index of refraction (aka refractive index or n)
for the substance. This measurement does not
have units
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The FBI has a database
off over 2000 refractive
indexes of different
types of glass which
shows that glass is very
distinctive and helps
assign an appropriate
statistical probability
that the two pieces of
glass share a common
source.
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Submersion method: Place glass fragment
into different liquids of known refractive
indexes. If a piece of glass and a liquid have
the same refractive index, the glass fragment
will seem to disappear when placed in the
liquid.
Submersion and Low Power of Microscope.
Submerge fragment of glass in a liquid and
then view it under low power using a
compound microscope. If the refractive
index (n) of the liquid medium is different
from the refractive index of the piece of
glass, a halo-like ring appears around the
edge of the glass. This halo-like effect is
called a Becke line
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Notice the halo of light
on the inside perimeter
of the glass sample.
When the Becke line is
inside the perimeter of
the glass fragment, the
refractive index of the
glass is higher than the
refractive index of the
surrounding medium.
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Notice the halo of light
(Becke line) is outside the
perimeter of the glass
fragment.
When the Becke line is
outside the perimeter of
the glass sample, the
refractive index of the
medium is higher than
the refractive index of the
glass.
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You will use the submersion method by placing
the piece of glass in substances with a known
refractive indices.
When the glass and the substance have the same
refractive index, the glass will appear to
disappear.
In reality, if a piece of glass is immersed in a
liquid with the same refractive index, the glass
usually does not completely disappear. The
reason is that there are other effects caused by
light passing through glass. These are more
pronounced when the glass is thick. The result is
that we can still see faint borders of the glass
even if they have the same refractive index.
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Remember to wear goggles and gloves until
your lab is complete and area is clean
Use forceps to pick up glass particles
Once lab is complete, clean up your area and
return all materials to the proper location.
Complete the Study Guide on Glass Analysis,
due Monday, April 28.
Test on Glass Analysis Monday April 28, after
we go over the study guide at beginning of
class.