(FPGS) Properties of Glass Evidence Lesson

Properties of Glass Evidence

Glass evidence is commonly encountered at a crime scene; especially those involving a burglary or a hit and run car accident. For example, broken glass at a crime scene can be used to place a suspect at the crime scene or may become lodged in the shoes or garments of a victim of a hit and run and matched to a certain vehicle. When glass evidence is available, it is typically examined for its physical properties. While glass itself can't be linked to an individual source in most cases, it can contain other types of evidence which are individual evidence, such as fingerprints, blood or hair! Glass evidence can also often be linked back to a common source by a combination of density, refractive index and any production or other irregularities on the surface of the glass.

The Structure of Glass

Glass appears crystalline to the naked eye, which means that it resembles crystal in transparency and appears to have a regular structure. However, glass actually has an amorphous structure. Amorphous solids have a disorderly, random distribution of the atoms or molecules that make them up rather than a standard repeating pattern of organization as found in true crystalline solids such as Sodium Chloride, or salt.

Solid glass is hard, brittle, and composed of silicon (sand) and metal oxides. The ingredients used to make glass varies by the type of glass being made and the application for which it is intended. Regardless, the manufacturing process for making glass is essentially the same in the beginning. The raw materials used to make the glass are put into a large furnace and heated until they melt into a liquid glass mixture. Some glass will be poured out flat to make sheets of glass for applications, such as windows, whereas other types of glass, such as measuring cups, are poured into molds so that when the glass cools and hardens, it will be in the desired shape. Glass is cooled very gradually using ovens that slowly decrease the temperature; this is to prevent the glass from cracking as it cools. During production, many glass makers add various materials to the molten glass for coloring, as well as for unique physical properties (such as the super durability of Pyrex glass). These additives can be traced and measured to differentiate between different types of glass in a crime scene.

Interactivity: Types of Glass

Forensic Comparison

The use of glass evidence relies on the investigator's ability to match glass from an unknown source to a crime scene. Chemical analysis is usually not unique enough to accomplish this task, so the focus is to associate one kind of glass with another while minimizing or eliminating other possible sources. The easiest way to accomplish this is when glass fragments can be pieced together with matching irregularities; similar to putting pieces of a puzzle together. If the pieces can be put back together; the original shape and dimensions of the piece of glass may be determined.

Glass is also compared on the basis of certain physical characteristics; namely those are glass color, thickness, fluorescence, curvature, surface characteristics, density and index of refraction.

Color

Color is typically observed visually with the glass fragments against a white background in natural light. Glass fragments of the same size are typically placed side by side for color comparison.

Thickness

Thickness is measured when glass fragments have both sides of their original surfaces. A small tool, such as a caliper or micrometer, is used to measure the thickness of the glass. Glass thicknesses are kept in a database for Forensic comparisons and are generally the same in various types of glass within one thousandth of an inch! The thickness of the glass is due to the materials used to make the glass and the method in which it is made. If it is not kept at a uniform thickness, the glass will be irregular with ripples which lessens the value of the glass when sold.

Fluorescence

Fluorescence of glass is caused by either materials added to the glass, such as uranium in a certain type of green glass, or the process in which it is produced such as the method of producing float glass in which liquid glass is poured into molten tin. The tin will cause the glass to fluoresce in UV light on the surfaces in which the tin touched the liquid glass. Fluorescence of glass is evaluated by shining UV light on the glass.

Curvature

Curvature of glass refers to whether or not the glass is flat or curved into shapes such as containers or glass lenses. The curvature of a lens can be evaluated using low-power magnification.

The surface characteristics of glass can vary greatly and be quite useful in an examination of glass evidence. Some examples of surface characteristics include scratches, decorative etching, roller marks from production, polish marks, frosting of the glass and coatings on the glass. Most of these surface characteristics can be observed and evaluated with the naked eye or stereoscopic Microscope. Some evaluations, such as those involving coatings on glass, require more intensive methods such as Transmission Electron Microscopy.

Density

Density of glass, as discussed earlier, can be determined in several ways including calculation or a special method known as the flotation method. The flotation method is a quick and easy method for comparing glass densities. Glass particles are immersed in a liquid; the density of the liquid is adjusted until one or more glass particles remain suspended in the liquid. When the glass is suspended, it has the same density as the liquid and the density can then be inferred from the liquid. The comparison pieces of glass will either suspend, float or sink depending on their density relative to the liquid. One form of analysis that uses the principles of the floatation method is the Density Gradient Column that was mentioned earlier. Remember that density is an intensive property, so the density of a glass fragment will remain the same no matter how many times it is broken into pieces. This is why glass fragments can be reliably matched using their densities.

Refractive Index

Refractive Index is another physical characteristic that can be used to compare fragments of glass evidence in a method known as Immersion. This method involves immersing a glass particle in a liquid medium whose refractive index is varied until it is equal to that of the glass particle. When the refractive index is matched, a line around the interior or exterior perimeter known as the Becke Line, will disappear. The Becke Line is defined as a bright halo near the border of a glass particle that is immersed in a liquid of a different refractive index. Becke Lines on the inside of a glass particle indicate that the refractive index of the glass is higher than the liquid it is immersed in. Becke Lines on the outside of a glass particle indicate that the refractive index of the liquid is higher than that of the glass particle. Forensic technicians will continue to vary the refractive index of the liquid until Becke Line around the glass particle disappears. At that point, the technician will note the exact refractive index of the liquid as it will be the same as the refractive index of the glass fragment. While this is a quick and easy evaluation of glass particles, it is not typically used to determine exact measurements of the index of refraction. It is used only as a screening test to discern between glass particles that have very different refraction indices. If the refraction indices are very close on two samples of glass, this test will not be able to definitively tell if they are from the same glass object or not. The most reliable, but less often used method of precise glass analysis, is that of Elemental Analysis. In this method, glass composition is analyzed for the specific elemental compounds contained within it so that glass samples can be differentiated based on their chemical composition. Due to the expensive, complicated machinery needed to conduct the analyses, this method is rarely used in Forensic Labs. This begs the question; how do Forensic Scientists usually analyze glass particles? The answer is a combination of the above techniques combined with specially trained glass evidence examiners. Generally the physical characteristic tests offer information that can be combined to generate a professional opinion as to whether the glass particles came from the same particular source. The next step is to try to fit pieces of the glass samples together if enough are present. If the glass particles fit together like a puzzle, it is considered a match because glass never shatters in the same pattern. The reason that glass never shatters in the same pattern is due to the amorphous structure of glass; the particles are randomly distributed and thus will cause the glass unit to break in the weaker areas between those random distributions.

Glass Fractures

When a force is applied to one side of glass, the elastic property of the glass will allow it to bend to a certain degree. When the force surpasses the limits the glass can withstand, cracks will form. These cracks are characterized as radial fractures or concentric fractures.

Radial, or primary fractures, are cracks in the glass which radiate outward from the point of impact. These cracks form on the opposite side of the impact and radiate outward like spokes on a wheel.

Concentric, or secondary fractures, are fractures that form an approximately circular pattern around the point of impact. These fractures always form on the same side as the point of impact.

Examining the edges of radial and concentric fractures can help investigators determine the rate of impact and the direction that a projectile was traveling as it passed through the glass.

Radial fractures follow the 3R Rule, which means that Radial Cracks form a Right Angle on the Reverse side of the force.

When more than one projectile or object has impacted the glass, it is often possible to determine which impact occurred first, as well as the order of the subsequent impacts. New fractures will always terminate at an existing line of fracture because the stress placed on the glass (causing it to crack) will be transferred along the existing crack rather than across it. The crack line of the first impact will, in effect, stop the crack line of the second impact from crossing. Finding where this occurs will give you information as to which impact occurred first. In the image you can see in the red circle that impact A occurred first because the radial fracture from impact B was stopped by the radial fracture in impact A.

It is hard to determine if glass was broken by a bullet or some other projectile. One thing that can be determined is the direction the projectile traveled. This is because the hole that forms in the glass will be cone shaped with the exit side always appearing larger than the entry side.

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