This document describes the determination of color, fluorescence,
surface features, curvature, and thickness as they relate to forensic
2. Reference Documents
|Scientific Working Group for Materials
||Trace evidence recovery guidelines
Quality assurance guideline
|American Society for Testing and Materials
||C1036 Standard Specification for Flat Glass
Colorimetry is an analytical method for measuring the color
intensity of a substance.
A conchoidal type of fracture is observed when glass breaks
to give irregularly curved and usually striated surfaces.
Etching is a surface design or pattern produced by the application
of a corrosive acid.
Frosting is a finely grained, slightly roughened surface
Interferometry is the use of light interference patterns
for the evaluation of very small linear displacements (e.g., the
assessment of a glass fragment surface for flatness or curvature).
Isotropy is the property of having the same refractive index
regardless of the direction of vibration of light passing through
the material. An isotropic particle will exhibit extinction at all
orientations between crossed polars.
Mirror region is the smooth portion of the crack surface
that exhibits a mirror-like reflection when viewed using low power
Mold marks are the transfer impressions present on the surface
of a molded glass object resulting from direct contact between the
mold and the molten glass.
Ream is an imperfection, nonhomogeneous layers in flat glass.
Rouge pits are defects on the glass surface containing
residual rouge from polishing.
Polarized light microsope (PLM) is a microscope equipped
with two polarizing elements, one (polarizer) located between the
light source and the sample and the other (analyzer) between the
sample and the observer.
Polish lines are striation marks produced on the glass
surface by polishing.
4. Summary of Guideline
Color, fluorescence, surface features, curvature, and thickness
can be measured by microscopic and macroscopic methods. The measurements
can be used to discriminate among different glass sources.
5. Significance and Use
At a minimum, a forensic glass examiner must identify an unknown material
as glass because many materials can be mistaken for glass. Following
identification, as many features as possible should be determined
to characterize the glass and discriminate between sources. For example,
two fragments differing in color can be excluded as originating from
the same source, and no further examinations are necessary.
6. Sample Handling
Proper sample preparation and technique are prerequisites for obtaining
reliable results. See the Scientific
Working Group for Materials Analysis Collection, Handling, and Identification
7.1.1. Condition of the glass
The general condition of the glass should be visually inspected,
using a stereomicroscope prior to cleaning. For many of the
comparative examinations that follow, it is desirable to select
glass fragments with edges that appear freshly broken. The relative
sharpness of the edge, the appearance of conchoidal fracture,
and transparency serve to recognize a freshly broken surface.
The presence of freshly broken glass fragments on an item can
be important when interpreting the significance of the evidence.
Comparing color can distinguish between two or more sources
of glass. Sample size may affect apparent color. Therefore,
side-by-side comparisons should be made with fragments of approximately
equal and sufficient size. The fragments should be visually
compared on edge, over a white surface, using natural light.
Viewing the glass in this manner allows for optimal color observation
through the fragment. This will also allow the examiner to distinguish
between the true color of the glass and the color of any coatings
or thin films that may be on the surface of the glass. Observing
the glass using both fluorescent and incandescent lighting may
be useful in distinguishing different colors or tints. The use
of a stereomicroscope may aid in observing the color of small
fragments of glass. Significant color differences between glass
fragments can be used as the basis for exclusion. It is usually
not possible to compare the color of microscopic glass fragments
due to insufficient color density. Colorimetric analysis is
typically not performed due to this limitation.
Fluorescence is determined by observing a glass fragment over
a nonfluorescent background while illuminating it consecutively
with short-wave (254nm) and long-wave (~350nm) ultraviolet light.
The surface of float glass that was in contact with molten tin
during manufacturing should fluoresce under short-wave ultraviolet
light. Different colors of fluorescence have been observed from
glass fragments under short-wave and long-wave ultraviolet light,
consecutively. These should be noted for comparison purposes (Lloyd
7.3. Surface Features
Surface features of glass can be formed either intentionally or
accidentally as a result of manufacturing and fabrication processes
or during use. Surface features form another basis of comparison
that can distinguish one source of glass from another. They may
also serve as identifying features when examining the glass fragments
for a fracture match when the feature is present on both fragments.
Surface features imparted by the manufacturing process include
coatings, thin films, and mirrored backings. A forensic glass
examiner typically encounters thin films or coatings on architectural
and automotive glass. These films are placed there to increase
durability, to provide solar shading, for aesthetic reasons,
or other purposes (Greenberg 1997). Thin films are composed
of a wide variety of materials in single or multiple layers.
They can be amorphous or polycrystalline. The thickness of coatings
can range from nanometers, as in the case of thin films, up
to one millimeter, as in the case of mirrored backings. Coatings
are generally applied to the nonfloat side of flat glass but
can be applied to either or both surfaces. It may not be possible
to distinguish or identify films and coatings visually. Additional
instrumental methods of analysis, such as X-ray fluorescence
methods, X-ray diffraction, scanning electron microscopy, transmission
electron microscopy, or differential interference contrast microscopy
may be necessary to detect invisible coatings or distinguish
between coatings that appear visually similar. Conductivity
testing may be used to detect the presence of coatings on low
emissivity glass. For a more complete treatment on coatings,
see Bach and Krause (1997).
7.3.2. Manufacturing features
Examples of intentional surface features include etching, frosting,
and texturing. Accidental surface features imparted to glass
as a result of the manufacturing process include mold marks,
ream, rouge pits, and polish lines. Container glass can have
a distinct "orange peel" texture or other surface features that
are imparted to the glass from the manufacturing process. All
of the above can be compared visually, usually with the aid
of a stereomicroscope.
7.3.3. Postmanufacturing surface features
Additional surface features that can serve to distinguish between
two fragments of glass include surface scratches, abrasions,
pitting, and extraneous materials adhering to the glass that
may have their own evidentiary value. Any of these features
present on glass fragments should be examined and characterized
prior to cleaning.
Determination of a curved surface can help distinguish flat glass
from other types of glass including container, decorative, and
ophthalmic. If the fragments are large enough, this can be done
with the aid of a stereomicroscope. Smaller fragments may require
the use of interferometry to determine if a surface is not flat
(Fox 1981; Locke 1984). Small glass fragments from the mirror
region of the fracture may mimic surface fragments.
Thickness can be a useful property in distinguishing between flat
glass products because many glass products have thickness as specified
by American Society for Testing and Materials C1036. Thickness
measurements should be made only on fragments possessing both
original parallel flat surfaces. To measure thickness, a micrometer
or caliper with a precision of +/- 0.02mm or better should be
used. Numerous fragments should be measured, if possible, to determine
the variation in measured thickness. When two glasses show significant
differences in thickness, as determined by the variation in the
known-source glass, they can be eliminated as originating from
the same source. When using thickness to classify glass into product
categories, see American Society for Testing and Materials specifications.
|These properties can be used to exclude fragments from originating
from a given source. When the initial examinations have not
excluded fragments, then further examination is required.
|Documentation should also include condition of recovered fragments,
approximate size range, presence of original surfaces, indication
of the quantity of debris collected from different locations,
and the presence of nonglass material of potential evidential
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