Marley, Ronald K. The Value of Glass in the Fire Investigation Process. Fire Engineering. Vol. 150. No. 1 (January 1997). p 79-84.

Abstract: Glass is valuable evidence in a fire investigation. Glass can be used in the analysis of cause and origin, and can link suspects to the scene.

The basic components of glass are quartz sand mixed with calcium oxide and sodium carbonate. This mixture, melted at 1,400°F, produces molten glass known as soda-lime glass, one of most common forms of glass used today. Other types of glass can be created by adding other compounds to the basic mixture or by altering the heating/cooling pattern. Fire investigators should understand that different kinds of glass have unique sets of physical and chemical properties.

Visually, glass can be identified as clear, frosted, and textured. Large samples can be evaluated based on thickness and shape. Other characteristics require a more in-depth laboratory analysis.

Density is a characteristic that when combined with other information, can be used to link fragments recovered from a suspect to evidence gathered at the scene. Density is defined as an objectís mass per unit of volume, and remains constant, no matter what the size of the sample. Density can be loosely determined by dividing an itemís weight by volume. Or, more accurate methods can be utilized in the lab.

Hardness can be measured by a sample's ability to scratch other materials. While there is no current system for accurately ranking samples with respect to hardness, the process can be useful in eliminating a sample from further investigation.

Refractive index, like density, is a constant physical property. It is defined as the bending of light caused by a change in velocity. To obtain valid comparisons, samples must be measured at the same temperature and with a constant wavelength light source.

When pressure is applied to glass, it will bend. When it reaches its elastic limit it will shatter or break in a predictable manner. The direction in which glass fragments fall depends on many factors including type of frame and window size. Therefore, one cannot judge the direction of the force based on the fragments' position on either side of the frame. Forensic scientists studying the pieces can normally determine the direction from which the pressure was applied. If such information is required in an investigation, the investigator should collect the fallen glass and the glass remaining in the window, and possibly the frame as well. The pieces in the frame should be marked, indicating which way they were facing. The examiner will use these pieces as controls as the window is rebuilt in the lab.

Glass fragments scattered more than 10-15 feet probably indicate and explosion. Clean fragments indicate a prefire explosion such as natural gas or propane. Heavily stained fragments indicate a post-fire development. All of the glass surrounding a structure must be examined, as it might vary.

Also, the original height of fallen glass must be considered because the higher it was, the farther it can spread when it hits the ground. Glass can also be projected during suppression, ventilation and overhaul. Interviewing the suppression crew is recommended. Keep in mind that if glass is going to be an issue, it may be necessary to chart, photograph and collect every piece.

Melted glass and its patterns are indicators of temperature. Crazing, or small, tightly spaced cracks, is often reported as the result of very rapid heat buildup such as in an accelerated fire. However, significant evidence disproving this indicator has been gathered.

Soot deposits on glass surfaces were once thought to result from hydrocarbon-accelerated fires. But the extensive use of petroleum-based home products can yield the same result. It is possible, yet costly and questionable to link soot deposits on glass to a flammable accelerant.

Light bulbs which are still intact within their fixtures, can indicate direction of a fire, as they present with a distinct "pushed out" shape on the side which faced the fire.

Finally, research shows that when a person breaks a window, glass will be projected onto that person's clothing, where, depending on the weave of the material, it will most likely remain. Clothing and shoes should be collected from a person suspected of breaking a window to start a fire.

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