Glass analysis Distinguishing Glass Fragments

What is Glass? Glass is a is a hard, amorphous material made by melting sand, lime (also called calcium oxide), and sodium oxide at very high temperatures. Silicon dioxide, also called silica, is the primary ingredient in glass.

Types of Glass Altering the compounds used to make glass changes the composition and produces different types of glass. The most common type of glass, soda-lime glass, is inexpensive, easy to melt and shape, and reasonably strong. Fine glassware and decorative art glass, called crystal or leaded glass, substitutes calcium oxide with lead oxide. Ovenware and laboratory glassware contain compounds that improve the ability of the glass to withstand a wide range of temperatures needed for cooling or heating glassware in a kitchen or lab. Different colors of glass are produced by adding certain metal oxides to the glass mixture.

Properties of Glass A physical property is a characteristic of a substance that can be observed or measured without changing the composition of matter. Physical properties of glass include its weight, volume, color, density, refractive index, and fracture patterns.

Fabulous, but Forensics? Different types of glass have different densities, thicknesses, and refractive indices. As such, comparing the glass fragments found at a crime scene with those found on the suspects or their belongings can help point to the perpetrator. Glass fragment patterns can reveal details about how the crime occurred.

Determining Glass Density Measure the mass (in grams) of the glass fragment. Carefully add 25mL of water to a 50-mL graduated cylinder. Add the glass fragment and measure the volume of water that has been displaced. This is the volume of your glass fragment. Divide the mass (g) by the volume displaced (mL) to find density.

Refractive Index Refraction, or the bending of light, is the change in the direction of light as it speeds up or slows down when moving from one medium into another. The refractive index is a tool used to study how light bends as it passes through one substance and into another.

Comparing Refractive Indices If evidence glass obtained from the clothing or shoes of a suspect is too small to easily measure the refractive index the submersion method can be used. The submersion method involves placing the glass fragment into different liquids of known refractive indices. Another technique involves submerging a fragment of a glass in a liquid and viewing it under a microscope. If the refractive index (n) of the liquid medium is different than the refractive index of the piece of glass, a halo-like ring (Becke line) appears around the edge of the glass.

Glass Thickness Not all glass is the same thickness and this provides another clue for identifying glass. Therefore, by determining the thickness, refractive index, and density of the glass collected, glass fragments can be matched.

Glass Fracture Patterns Glass is an amorphous solid; therefore glass will break into fragments, not into regular pieces with straight lines at the edges. The fracture patterns formed on broken glass can provide clues about the direction and rate of impact. Primary radial fractures and concentric fractures can both occur when glass breaks.

What Can Fracture Patterns Reveal? Putting the pieces back together, like a puzzle, can reveal the type of object that hit the glass: bullet, head, rock, etc. For example, as a bullet passes through glass, it pushes some glass ahead of it, causing a cone-shaped piece of glass, which makes the exit hole larger than the entrance hole of the bullet. The angle at which a bullet enters a piece of window glass can help locate the position of the shooter.

Can You Fracture This? Bulletproof glass is a combination of two or more types of glass, one hard and one soft. Safety glass, also known as tempered glass, used in windshields is composed of two layers of glass bonded together with a layer of plastic in the middle.