Forensic Class in the Philipines

4. What is a Glass?
A super cooled liquid which possesses high
viscosity and rigidity. It is a non-crystalline inorganic
substance.
Importance in the study of glass:
 as physical evidence, breakage of pieces are
scattered at the crime scene and on the suspect
 a common type of carried away evidence in
burglary (window glass), hit and run (fragments of
headlights), and a case which a bullet was fired

6. It is composed of oxides like SiO (silica), BO
(boric oxide), PO (phosphorous pentoxide).
1. Silica – most important oxide for commercial use
- the base of commercial glasses
2. Oxide – for fluxing, durability and reduction of
viscosity
Glass, like window and plate which are made in
mass production is fairly uniform in composition.
This may contain impurities and the presence of
these substances is invaluable for the identification
and comparison of glass by spectrographic analysis.

7. Gas have presence of trace elements which
maybe sufficient to establish or negate the fact of a
common source for two samples of glass.
OXIDES FUNCTION
1.) Silica (SiO2) -------------------- base of commercial glass
2.) Soda Na2O) --------------------- acts as flux for silica
3.) Lime (CaO) --------------------- gives the glass chemical
durability which it otherwise lack
because of the water-soluble Na2O

8. 4.) Magnesia (MgO) ----------------- present as impurity or
substitute for CaO
5.) Alumina (Al2O3) ----------------- gives the glass greater
chemical durability lower coefficient
expansion, and greater freedom from
devitrification.
The most important problem commonly
referred to a forensic chemist is the comparison of
two or more samples of glass.

9. 1.) Spectrographic Test – an instrumental method
of analysis which determines the presence of
trace elements.
2.) X-ray Diffraction Test – determines the type of
pattern of the glass but not as effective as
spectrographic test. Depends upon the
composition of glass.
3.) Physical Properties Examination – the most
sensitive method of determining differences of
composition in glass samples and depends upon
the physical properties of glass.

10. 4.) Ultraviolet Properties Examination –
determines the differences in the
appearance of the fluorescent thus
indication of physical and chemical
differences.
5.) Polish Marks Test – optical glass and other
fine glass wares are usually polished. Fine
marks are left for the basis of comparison.

11. Floatation Method – a rapid and convenient method of
determining the density of small fragments. Procedure and
principle is the same as in soil.
Immersion method – use to measure the refractive index of
a glass
- difficult to distinguish between two
samples of glass by density and
refractive index

12. The surface is cleaned with alcohol and then
etched by spraying with 20 to 25% hydrochloric
acid. The acid is permitted to remain on the
surface for several minutes. The glass is again
washed with alcohol and dried. If the surface is
illuminated by oblique light, a photograph can be
made to show the polish marks.

13. In the field of forensic chemistry the emphasis is placed on:
1.) Automobile glass in case of hit and run.
2.) Broken windows cause by pressure, blow or bullet in
case of robbery
3.) Broken bottles, drinking glasses, spectacles found at the
scene of an assault or other crimes of violence, which would
suggest examination of the soles and heels of a suspect for
embedded fragments.

14. (HOW GLASS FORMS CRACKS WHEN A BLOW OR
PRESSURE IS APPLIED ON ONE OF ITS SURFACE)
When the blow strikes the glass on one of its surface,
the front for example, the glass first bends a little owing to
its elasticity. When the limit of elasticity is reached, the glass
breaks along radial lines starting form the point where the
portion or surface which is more subjected to stretching by
bending. The front surface is only pushed. While the radial
fractures are taking place, the newly created glass triangle
between the radial rays also bend away from the direction of
the destroying force. By this bending the bend is stretched
along the front surface and when the limit of elasticity is
reached, the glass breaks in concentric cracks. These
originate on the front glass because of stretching.

15. Hit and run accidents represent a good percentage of
crimes. If an automobile or ay vehicle from that vehicle for
that matter discovered in which fragments of the lens can
be found, a comparison maybe with the fragments found at
the scene of accident employing the methods of analysis of
glass.
Examination of window fragments in robbery
cases is important when there is a question of “as to
whether the glass was broken from the outside or the
inside.” Since our penal law specifically provides the
mode of entrance before a crime maybe classified as
robbery, this particular kind of examination becomes

16. very important. The general procedure to determine whether
the glass was broken from the outside or inside or to
determine the side from which a pane of glass was broken is
to collect and piece together as much of the glass as possible
in order to study the patterns of the cracks and to be able to
orient the pieces in their original position.
1.) On one side of the hole, numerous small flakes of glass will
be found to have been blown away giving the hole the
appearance of a volcano crater. Such appearance indicates
that the bullet was fired from the opposite direction of the
hole from which flakes are missing.

17. 2.) If the shot was fired perpendicular to the window
pane the flake marks a re evenly distributed around
the hole.
3.) If the shot was fired at an angle from the right,
the left side will suffer more flaking from the right.
4.) Excessive flaking on the right side of a
windowpane would indicate a shot fired at angle
from the left.
(The direction is taken from the person shooting)

18. low-speed impact high-speed projectile

19. - states “stress lines on a radial crack will be at
right angle to the rear side of the glass”.
- states “stress lines on a concentric crack will be at a right
angle to the front side”, that is the side from which the blow
came rather than the rear side.
* the rule for concentric crack is the reverse of the 3R’s
rule provided the concentric cracks can be examined is
near preferably adjacent to the point of impact.

20. Where there are two bullet holes in a
window, one from each side, the problem of
which one was first becomes important to
determine who is the aggressor. It will be
found that the fractures caused by the first
will complete especially the radial cracks,
whereas the fractures from the second will be
interrupted and end-stopped at point where
they intersect those from the first.

21. Laminated glass that is now being used
in automobiles does not shatter when
struck sharply. Frequently the cracking of
safety glass is not complete. The radial
cracks do not extend to the side of impact
and the spiral cracks do not extend to the
other side.

22. The cracking into radial lines divides the pane into a
number of triangles. These triangles are pushed out from a
point of impact by initial impulse. The main body of a glass that
is fairly rigid resists the pushing out. The effect of a torqued is
produced, and if the force is sufficient, the glass is now pushed
in the opposite direction until again the limit of elasticity is
exceeded and the glass begin to break on the side where the
blow was struck. The cracking now takes place along the quasi-
circle concentric with the point of impact. It was demonstrated
that the number of spiral cracks present in a fracture depends
upon the nature of impringing force. A rapid dynamic force
produces more spiral pattern than a slow, relative static force
even though the total energy involved is the same in both cases.
If radial or concentric cracks cannot be definitely
established the side from which the blow came cannot be
determined.