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
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
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
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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