Case Number 169100  The Case of the Broken Glass

During a burglary, a glass object has fallen off a table onto the floor.  The floor has no carpet and the glass object shattered.  If the burglar was near the object when it shattered, perhaps small fragments of glass can be found lodged in his clothing, especially in the lower trouser leg areas or the soles of his shoes.  A suspect is apprehended and his living quarters searched.  A pair of trousers is found which indeed does have glass fragments in the fibers, and a few fragments of glass are removed from one pair of shoes.  Density determinations are to be done and an attempt made to link the pieces of glass in the trousers and shoes to the fragment collected at the scene.  Your task is to determine if the density of the glasses match well enough to place the suspect at the scene of the crime. 

Glass from various sources such as windowpanes, automobile headlights, bottles, and plate glass doors, all have slightly different densities.  This makes it possible, in some cases, to help place a suspect at the scene of the crime if they have broken a glass object and small fragments have become lodged in their clothing.  The density of the glass may by only slightly different but careful analysis will make these differences apparent.  Table 1 gives the density of several types of glass and similar materials.

SAFETY

This forensic investigation will require you to work with broken glass.  The glass should be handled with forceps only.  Bromoform and acetone will be used in this experiment as well.  Prior to lab look up the material safety data sheets for these two chemicals and report them in your pre-lab.

Pre-lab:  Include the experiment name, the purpose of the experiment, the major experimental steps used to achieve this purpose and a summary of the safety precautions for dealing with bromoform and acetone.

Background information:  For your lab report please report on forensic uses of density differences.  (Density has been used in glass, plastics, and bones in forensic applications among other materials.)

Table 1 Densities of Various Common Materials (g/cm3)

Exploration

Before you request the actual evidence, you need to become proficient at measuring the density of other regular and irregularly shaped objects. 

Part 1: DENSITIES OF REGULARLY SHAPED SOLIDS

In this part of the experiment, the volumes of regularly shaped objects are calculated from measurements of their dimensions. In each case the appropriate formula is applied:

rectangular solid: V = length x width x height

cube: V = (side)3

cylinder: V = π x (radius)3 x height

sphere: V = ⁴/₃ x π x (radius)3

triangular solid: V = ½ x base x height x width

The mass is found by direct measurement with a balance and the density calculated:

density = mass/volume

PROCEDURE

1. Start with a rectangular solid. Weigh it and record the mass on the data table.

2. Measure the length, width and height with a micrometer and record in a data table in your notebook.

3. Calculate the volume of the rectangular solid and record the value on the data table:

volume = length x height x width

4. Calculate the density of the rectangular solid and record the value on the data table:

density = mass/volume

5. Choose another regularly shaped solid and determine its density.

6. Repeat for a third object.

7.  Based upon the calculated densities and table 1, determine the likely material each object is.  What limitations do you have in assigning object material with certainty?

Part 2:  The Density Of Irregularly Shaped Solids

You are now ready to measure the density of glass, an irregularly shaped object here since it has been shattered. 

Water displacement is a simple way to measure the volume of a solid object. It is especially useful with irregularly shaped objects. Water is poured into a graduated cylinder or measuring cup and its volume read and recorded. The solid is completely submerged in the water and the volume of water plus object read and recorded. The volume of the object is found by subtracting the volume of the water from the volume of the water plus object. The mass is found by direct measurement with a balance and the density of the object is calculated using the formula:

density = mass/volume

PROCEDURE

1. Broken glass fragments of non-evidentiary value are available for you to practice with.  Choose a fragment and record its shape and approximate size in your notebook.

2. Weigh the solid object and record the mass on the data table.

3. Fill a graduated cylinder partially full of water and record the volume.

4. Add the solid object to the graduated cylinder. If it sinks but is not completely covered with water, start over at step 3 using less water. If it floats, use your pencil to push it underwater but don’t submerge any more of the pencil than absolutely necessary. Read the new volume and record it on the data table.

5. Calculate the volume of the object by subtracting the volume of the water (step 3) from the volume of the water plus object (step 4). Record the volume of the object on the data table.

            6. Calculate the density of the object from its mass (step 2) and volume (step 5). Record the density on the data table.

            7. Once you feel comfortable with this procedure, sign out the evidence and determine the density of the glass fragments found at the crime scene and the density of glass fragments found in the possession of the suspect.

            8.  As you are probably aware, airtight cases can easily be blown apart by small mistakes.  To minimize the importance of such mistakes, repeat the density determinations at least three times, perhaps on different pieces within each evidence bag to strengthen your case.

Part 3: Physical Measurements of Glass Fragments

Recognizing that the more pieces of evidence that point toward a particular conclusion, the easier that conclusion is to defend, more evidence will be collected on the evidentiary glass fragments.

1. Physical matching:  Try to obtain a physical match between fragments if possible.  That is, match letters (if present) or two fragments by fitting together any scratches on the fragments, or by some other piecing together of the fragments.  Such a fracture match – a physical match of two fractures – is conclusive proof that they were once two pieces of the same piece of glass.  Record any physical matches in your notebook.  As you do this, be sure to keep the evidence collected from the suspect separate from the evidence collected at the scene.  Your evidence bags will be weighed as you check them in and out to verify evidence integrity.

2. Edge thickness:  Compare the fragments by measurement of the thickness of the edges of the fragments.  Use a micrometer for these measurements.  Although the results may not be conclusive themselves, they may help the decision making process later.  Record in your notebook.

Part 4:  Density comparison by Flotation

In part two, the absolute density was determined.  This can be useful but sometimes there is not enough accuracy in the measuring devices to tell that two samples are different statistically.  In this part of the experiment you will directly compare the densities of the suspect and scene glasses by floating them in a high density liquid.

1. Before adding the glass fragments to the test tube, it is important that each piece of glass be carefully examined and briefly sketched.  This will allow the examiner to identify each fragment when placed in the mixture of bromoform and acetone.
2. A cleaned and dried sample of the unknown glass particle should be placed in a test tube containing the 1-2 mL of bromoform.  The glass will float on the liquid’s surface.  This indicates that the density of the liquid is greater than that of the glass.  Slowly add a less dense liquid (acetone) dropwise, with stirring, until the particle is exactly suspended.  If too much acetone is added, so that the glass particle begins to sink, add more of the bromoform (dropwise).
3. Add a similarly sized, cleaned, and dried sample of the other glass.  Carefully stir the contents of the test tube.  If the two glasses are similar in density, each will remain suspended in the liquid.  Otherwise, one particle will tend to rise or sink relative to the other.
4. Repeat with several other fragments.  Record your observations.