Glass analysisDistinguishing 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.