LEARNING OBJECTIVES: Describe how glass or glass fragments may provide information in the investigation of offenses. Explain the nature and properties of glass and how glass should be collected and handled. Explain how the investigator should examine glass fragments and fractures. Describe the process of laboratory examination of glass, the techniques used, and the possible outcomes of laboratory examination of glass.
Glass, or glass fragments, will frequently be important factors in the investigation of offenses such as burglary, housebreaking, fleeing the scene of vehicle accidents, and others. The ultimate value of such material, either as evidence in itself or in the investigative leads which may be obtained from it, depends on your knowledge and training in the nature of glass, proper procedures of collection, preservation, and examination, and what you and the scientific laboratory technician can learn from it.
Glass fractures and glass fragments can provide information from which the following types of determinations may be possible: . That a fragment of glass did or did not originate from a particular glass object that has been broken (such as a fragment of headlight lens found at the scene of a vehicle accident did or did not originate from a broken headlight of a suspect vehicle).
. That a fragment of glass originated from a particular kind of glass object, such as a headlight lens, spectacle lens, or windowpane.
. The origin and direction of a fracture; that is, what caused it and the direction from which the causative force came.
. In the event of multiple fractures, including bullet holes, the order in which the fractures occurred.
. The angle from which a bullet struck a glass object.
. That a particular glass object, such as a bottle or jar, contained an inflammable or explosive substance.
NATURE OF GLASS
Before discussing specific matters related to glass as evidence, a general understanding of the nature and properties of glass is required.
Glass is normally a fused mixture of silica usually in the form of natural sand and two or more alkaline bases such as soda, lime, or potash. It also contains quantities of various other elements and metals, present either as incidental impurities in the basic constituents or added to them for color, degree of hardness, heat resistance, and other specific purposes.
These constituents are melted in a melting pot under very high temperature, and the molten mass is then either rolled, blown, or molded into desired sizes and shapes. It may later be polished, ground, or cut for useful or decorative purposes, or it may be combined with other materials. For example, sheet vinyl plastic is fused between sheets of plain glass to form safety glass.
PROPERTIES OF GLASS
The value of glass to you, either as evidence or in the development of investigative leads, lies mainly in its physical properties. These properties make it possible to determine that glass fragments did or did not originate from the same source, or to determine the manner in which a piece of glass was broken.
Differences in the amounts of mineral composition of ingredients used in one batch of molten glass from those used in another will produce variations that can be detected and proved under laboratory procedures if a continuous batch process was not used.
Other properties of glass provide for determinations that may be made by you or a laboratory examiner. These properties include the fact that glass seldom breaks squarely across but leaves convex/concave edges, or stress lines, on the fractured edges; that it bends and stretches before breaking; and that breaks produce both radial (primary) and concentric (secondary) fractures. (See fig. 16-8).
COLLECTING AND HANDLING
The procedures used in the search for and collection of glass and glass fragments are, in general, the same as those used for other types of evidence. Differences are due to the nature of glass itself and to the investigative and evidentiary findings to be sought from it.
Figure 16-8.-Radial and concentric fractures.
As with other types of evidence, glass and glass fragments must be photographed and their locations noted on the crime scene sketch before they are touched or moved. Pertinent data must be recorded in your notes concerning the glass and any obvious, suspected, or hypothetical relation it has to the incident under investigation.
In collecting glass or glass fragments, carefully avoid smudging any fingerprints or disturbing any other substance such as dust or dirt, bloodstains, or other foreign matter which may be on the glass, since any or all of these may provide investigative leads or be evidence in themselves.
Rubber or fabric gloves should be worn, and rubber-tipped tweezers or a similar device should be used for handling small fragments so as not to scratch the glass. Metal tweezers with adhesive tape placed over the inner surface of the points make a good field expedient.
Glass should be picked up by the edges, avoiding the plane surface as much as possible. All available fragments should be collected to provide as complete a reassembly as possible. Even particles too small to permit matching or reconstruction should be collected and preserved, since they can be analyzed in a laboratory for their physical properties.
In cases where the glass has been broken out of a window, door, or similar frame, and pieces remain therein, the frame should be removed and kept intact, if this is practicable. (It may be necessary to provide interim protection for the premises.) This procedure will also make reassembly of the broken pieces easier.
If removal of the article is not practicable, the pieces remaining in the frame should be carefully marked (inside and outside surfaces must be designated) and removed to avoid further damage to the glass or disturbance of any depositor substance thereon. If the frame is not removed, samples of the wood, paint, putty, and any other materials should be collected from it.
Broken glass may provide a valuable clue in the identification of a suspect.
Frequently, in the commission of an offense such as burglary, the perpetrator will break a window or other glass object. Particles or fragments of the glass may lodge in, or adhere to, his or her clothing or fall into a pocket or trouser cuff. Collection and laboratory analysis of such fragments for comparison with glass found on the scene is a worthwhile effort. The soles and the sewn edges of the shoes should not be overlooked in the search for this type of evidence.
This type of thorough search may also be productive in the investigation of a fleeing-the-scene vehicle accident where glass was broken. Fragments or particles of glass maybe found adhering to, or imbedded in, the tires of the suspect vehicle. These should also be submitted for laboratory analysis.
Glass fragments of sufficient size are marked with a diamond point or carborundum pencil, a piece of properly marked adhesive tape, or a grease pencil. Markings are placed in an area that is of value as evidence. Markings should include your initials, the date, and time.
To aid in reassembly of the fragments and in reconstruction of the incident, markings are placed on the side which was up (or inside the building or room if taken from a window frame or door) when found, and include a sequence number which, when keyed with your notes, photographs, and sketches, serves to identify the location where found. Fragments too small to permit such markings are placed in suitable containers, and both the container and lid marked.
Glass or glass fragments should be wrapped in soft paper, cotton, or similar material which will prevent breakage, in a manner which will avoid damage to fingerprints or other substance to be examined by the laboratory or preserved as evidence. The wrapped glass is placed in suitable containers, properly fastened so that it will not shift. Wrappings and containers should be marked "Fragile."
Property tags and property receipts should be prepared by you, and the property should be released to the designated custodian at the earliest possible time. The letter of transmittal and request for laboratory examination should also be prepared by you.