GLASS AND GLASS

FRAGMENTS

CHAPTER 7

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

COMPOSITION OF GLASS

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 glasses2. 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.

Gas have presence of trace elements which maybe sufficient to establish or negate the fact of a common source for two samples of glass.

COMMON OXIDES USED IN GLASS MANUFACTURE

OXIDES FUNCTION

1.) Silica (SiO2) -------------------- base of commercial glass2.) Soda Na2O) --------------------- acts as flux for silica3.) Lime (CaO) --------------------- gives the glass chemical durability which it otherwise lack because of the water-soluble Na2O

4.) Magnesia (MgO) ----------------- present as impurity or substitute for CaO5.) Alumina (Al2O3) ----------------- gives the glass greater chemical durability lower

coefficient expansion, and greater freedom from devitrification.

ANALYSIS OF GLASS

The most important problem commonly referred to a forensic chemist is the comparison of two or more samples of glass.

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.

Test/Analysis for glass

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.

Floatation Method – a rapid and convenient method of determining the density of small fragments. Procedure and principle is the same as in soil.

METHOD OF MEASURING THE REFRACTIVE INDEX OF GLASS

Immersion method – use to measure the refractive index of a glass

- difficult to distinguish between two samples of glass by density and

refractive index

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.

PROCEDURE FOR THE DETERMINATION OF FINE MARKS

GLASS AS EVIDENCE OF CRIMEIn 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 robbery3.) 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.

HOW GLASS BREAKS(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.

ANALYSIS OF GLASS FROM VEHICLESHit 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.

ANALYSIS OF BROKEN WINDOWS

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

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.

BROKEN WINDOW CAUSE BY BULLET HOLES

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.

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)

low-speed impact high-speed projectile

THE PRINCIPLE OF 3R’s RULE FOR RADIAL CRACK

- states “stress lines on a radial crack will be at right angle to the rear side of the glass”.

THE PRINCIPLE OF RFC RULE FOR CONCENTRIC CRACK

- 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.

THE BULLET HOLES IN A WINDOW

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.

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.

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.

PREPARED BY:JUDY ANN ALIGUIN

&AZRIEL PAGAYUNAN