INVESTIGATOR EXAMINATION

You should evaluate glass fragments and fractures as you do other items of evidence, considering their importance and their relationship to all other evidence. This evaluation begins, of course, in the initial stages and continues until the investigation is completed or discontinued.

In the evaluation process, the meaning and value of certain materials found on the scene are not always easy to determine in the initial stages. This raises the question as to whether you should collect and preserve such materials. The safest decision is to do so. If you do not, it is likely that the broken glass will be quickly discarded as trash, and neither you nor a laboratory will have an opportunity to evaluate or examine it. Material that proves valueless can be disposed of or discarded at a later time.

In your evaluation, you should also consider the need or desirability y of scientific laboratory analysis. Should you decide upon such action, it is usually best to request it as early as possible. In some instances, speed may be essential, as in cases involving fragments of glass containers which are suspected of having contained inflammables or explosives so odors or residue may be retained, or to provide further investigative leads.

General Examination

Examination of fractured glass found at the scene of a crime may reveal the direction from which the blow was struck.

When broken glass is reconstructed, some of the fractures (the primaries) in the glass will resemble the spokes of a wheel in that the fractures will radiate outward from the point of impact. These spokelike fractures, called radial fractures, originate on the surface opposite to that on which the fracturing blow or pressure was applied. They tend to lengthen after a period of time because of internal stresses setup by the original shock. The original radial fracture will appear as a wavy line. Extensions to the original fracture will run in a straight line. Temperature changes cause extensions to develop more rapidly.

Another force working in the opposite direction causes the glass to break in secondary fractures, called concentric fractures, which form a series of broken circles, or arcs, around the point of impact. These fractures are made in the opposite manner from radial fractures, the glass bending on the opposite side and then stretching and breaking on the side from which the original blow was applied. They extend from one radial fracture to another.

Edges of broken pieces of glass bear a number of curved lines, called stress lines. (See fig. 16-9.) These stress lines are almost parallel to one side of the glass and perpendicular to the other. Stress lines are usually visible to the naked eye. If the lines are difficult to detect, turn the glass at various angles to the light so that the reflection will reveal them. Stress lines indicate the increase in stress setup in the glass until it breaks, and are always perpendicular to the side that broke first.

In radial fractures, the stress lines are perpendicular (at right angles) to the side opposite from which the blow

Figure 16-9.-Stress line.

was struck. In concentric fractures, the stress lines are perpendicular to the side on which the blow was struck.

From these facts the 3-R rule was developed-in RADIAL fractures, the stress lines are at RIGHT angles to the REAR surface.

Accordingly, if the examiner is sure of the type of fracture being examined it can be determined from which direction the blow was struck. The direction of the blow cannot usually be determined by examining a single piece of broken glass. When it is necessary to prove the manner in which a pane of glass broke, sufficient glass fractures must be assembled to determine which are the radial edges.

Determination of the outside and inside of the glass may be aided by examining the surface of the glass, noting the amount of dirt on the surface, putty marks, and other indications that may help to replace the pane in its original form. After sufficient pieces are put back in place to enable an identification of radial and concentric fractures, the stress lines on the edges can be examined to determine the direction of the blow.

Glass and glass fragments found at the scene of a crime involving explosives or inflammables may provide you with valuable information.

Door or windowpanes broken inwardly may suggest a means of entry, or they may have burst from _{exposure to heat.} Such panes broken outwardly may indicate the direction, force, and limits of an explosion or blast, and may indicate the point of origin.

Glass fragments, especially of bottles or jars, may bear odors or chemical traces of explosive or inflammable agents that you may readily identify or may be subjected to laboratory analysis for identification.

Safety Glass

Safety glass, commonly used in automobiles, consists of a transparent binding agent, such as a sheet of vinyl plastic sandwiched between two sheets or ordinary glass. The binding agent prevents shattering of the ordinary glass when it is struck.

Due to the structure of safety glass, cracking is frequently incomplete, in that neither the radial nor the concentric cracks penetrate completely from one side to the other.

In such cases, the side of impact maybe determined. If the concentric cracks appear only on one side, and no radial cracks are found on that side, that is obviously the side of impact. If only radial, and no concentric cracks are found on one side, this is the side away from the impact.

Determination can usually be made by sliding the fingernail, or a sharppointed instrument, along the glass surface across the apparent cracks.

Another test for side of impact is based on the property of safety glass that causes it to bend and remain somewhat bent, instead of shattering when struck. Since the bending will result in a concave surface on the impact side and a convex surface on the opposite side, determination may be made by placing a ruler, level, or other straightedge on the plane surface and observing the result.

Paint Spots on Glass

Glass on automobiles frequently bears traces of the paint used on the automobile body. Such traces can be of value particularly in cases of fleeing-the-scene accidents, since they may indicate the color of the vehicle. While these traces may be plainly visible to you, and identifiable at least as to color, better results can be obtained in a laboratory.

It is of primary importance that you examine glass in such cases very carefully, as not to disturb any specks, flakes, chips of paint, or other foreign matter on the glass, and that you make specific reference to them in your request for laboratory examination.