Paint can reveal vital evidence for the forensic scientist as it is present on many surfaces. The skill and techniques used by the scientist to analyse paint samples recovered from crime scenes can make a significant contribution to an investigation.
The examination of paint usually, although not exclusively, involves the comparison of a recovered paint sample from a crime scene with a control sample of paint which has been taken from a known source.
Some examples of paint being transferred include:
- Paint recovered from the clothing of a road traffic victim with paint from a vehicle.
- Paint from two or more vehicles involved in a collision.
- Paint recovered from a tool that may have been used in a housebreaking with paint from the point of entry.
Paint is generally encountered in vehicle accidents. Newly manufactured vehicles have a specific layer sequence that helps the forensic scientist in their analysis and conclusions of the origin of the paint samples.
This layer sequence can incorporate several different layers including basecoat, primer and topcoat. Each batch for a particular make and model of vehicle typically has the same paint layer sequence. If vehicles are re-sprayed or have damaged areas repaired, the paint layer sequence is changed. As more layers are added, fewer vehicles are likely to show the same layer sequence. Household paints, or indeed any painted item, may also have a layer sequence.
Examination and Analysis
The forensic scientist initially conducts a visual exam of the paint sample. Using a low power microscope, the forensic scientist examines the sample on its side - looking at the different layers of paint and for similarities in the layer structure.
Depending on the sample the forensic scientist may use an instrument called a microtome which can slice a thin section of the paint to use for further analysis.
After producing thin sections of paint, the different layers can be analysed using instrumentation such as FTIR (Fourier Transform Infra Red) and S.E.M. (Scanning Electron Microscope). Each layer of the recovered sample can be compared with the corresponding layer in the control sample to determine a match. Chemical tests can also be applied to the paint samples.
At all steps of the examination there are opportunities to identify differences in the samples. Only when the forensic scientist is completely satisfied with the results of their analysis will they conclude that the paint samples could have originated from the same source.