Glass Analysis Lab
Forensics 352 – O’Dette
Introduction
Crime scenes often contain broken glass from windows, cars, glass vases, and
numerous other glass items. Matching glass pieces with other glass pieces can often
reveal patterns that will help explain a crime sequence. Numerous glass analysis
techniques can be used. One specific technique, comparison of density, will be examined
in this laboratory.
Concepts
 Density
 Density gradient
 Flotation
Background
Forensic scientists compare glass pieces in order to associate one glass fragment
with another and at the same time eliminate the possibility that other sources may be
responsible for the glass fragments. Glass analysis has its greatest impact as evidence
when the glass can be identified with only one source. Such a determination can only be
made if fragments can be collected and physically pieced together in a perfect fit like a
jigsaw puzzle. Since the probability of two pieces of glass from two different sources
fitting together perfectly is low, it becomes compelling evidence when a perfect match
is found.
In many forensic settings, however, the physical pieces often do not fit together
easily. The forensic scientist is then left with other techniques that may not be as
conclusive or compelling. One such technique involves determining the density of glass
pieces and then comparing the density to those of known glass sources.
The simplest comparative density technique is known as flotation (not floatation).
It is based on the principle that a solid particle will float in a liquid medium of greater
density, sink in a liquid of lower density, and remain suspended in a liquid of equal
density. In the crime lab, this is usually done in a density gradient. Solutions of
decreasing densities are placed in a long column and then the object is placed into the
column. By comparing the point of suspension, the density can be identified and
different glasses can be compared.
Density is a characteristic property of materials and pure elements or compounds.
Materials can often be identified by their density. Density is defined as the mass of a
substance per unit of volume.
Density = Mass/Volume
Density is commonly expressed as g/cm3 or g/mL. The density of pure water is 1.00
g/cm3 at 20°C. Objects with a density greater than 1.00 g/cm3 will sink in pure water.
Objects with a density less than 1.00 g/cm3 will float in pure water. Alcohols have a
density less than 1.00 g/cm3 and some objects that float in water will sink in alcohol.
Solutions, such as sugar water, have a density greater than 1.00 g/cm3. The density of a
sugar solution will increase as the concentration of sugar in the solution increases.
In this experiment, the principle of density will be used to distinguish simulated glass
pieces (plastic beads). Once the properties of various beads are established, a bead
found at the crime scene will be analyzed.
Materials
Simulated glass A, 5 pieces
Isopropyl alcohol, 40 mL
Simulated glass B, 5 pieces
50% sucrose solution, 20-40 mL
Simulated glass D, 5 pieces
Crime scene beads, 5
Simulated glass F, 5 pieces
Paper Towel
Graduated cylinder, 100-mL
Distilled water, 80-100 mL
Beral-type pipets, 2
Small beakers, 2
Procedure
1. Fill a clean 100-mL graduated cylinder with 20 mL of distilled water.
2. Add five simulated glass beads A to the water in the graduated cylinder.
3. Slowly swirl the water in the cylinder to make sure there are no air bubbles
attached to the beads that would cause them to float instead of sink. Establish
whether the A beads sink or float in water. Why are five beads used instead of
one?
4. Record the floatability of the Ad beads on the Density Principles Worksheet.
5. If the beads float, go on to step 6. If they sink, go on to step 11.
6. Remove the beads from the water and blot them on a dry paper towel.
7. Pour the water out of the graduated cylinder and dry the inside of the cylinder
with a paper towel.
8. Carefully pour exactly 20 mL of isopropyl alcohol into the graduated cylinder.
9. Place the beads in the alcohol. Do they float or sink? Record the result on the
Density Principles Worksheet.
10. If the beads sink, use a Beral-type pipet to add distilled water to the graduated
cylinder about 1 mL at a time. Swirl the cylinder to mix the water and the alcohol.
Keep adding water until all of the sample beads float. When all the beads are
floating, stop adding water and note the level of the liquid in the graduated
cylinder. How much water did it take to get the beads to float? What
concentration (mixture ratio) of alcohol and water caused the beads to float?
Record the results on the worksheet.
11. If the beads sank, obtain a Beral-type pipet and some 50% sugar solution.
12. Start adding sugar solution 1 mL at a time, swirling between additions, until the
beads are all floating on the top of the solution in the graduated cylinder.
13. Note the level of the liquid in the graduated cylinder when all the beads are
floating. How much sugar solution did it take to get the beads to float? What
concentration (volume ratio) of sugar water and water caused the beads to float?
Record the result on the worksheet.
14. Clean and dry the graduated cylinder between each key step and repeat steps 113 for simulated glass samples B, D, and F.
15. When all the tests are completed on the four types of bead, complete the flow
chart on the Density Principles Worksheet. The Flow chart should depict a
complete strategy for identifying bead types A, B, D, and F.
16. Clean the equipment thoroughly and discuss your flow chart with your instructor
or as a class.
17. Test the Unknown Beads from the crime scene. Determine which bead type was
found at the crime scene.