Forensic Analysis of Paints

Everyone has seen or used some kind of paint during their lives.  Anyone who has used it will realize that the paint goes on in liquid form, then dries to form a solid coating.  There is much more to paints than meets the eye.  Below are some definitions which will help in deciphering exactly why forensic scientists sound so sure of themselves when they match a paint chip from a crime scene to the vehicle of a suspect.
Paint Components Blue Paint

Forensic paint matching has many steps.  The steps taken depend on the details of the situation.  What kind of paint is being matched?  Is the unknown sample from a smear or from a flake or was it transferred to another painted surface?  How much paint is available for matching?  What is the reference sample? Does the sample need to be preserved?  All of these are questions that must be asked by the forensic scientist.  The FBI uses the following flowchart as a guideline for testing the paints: (9)

FBI Paint Flowchart

Once it is decided that chemical analysis is necessary, methods must be chosen to analyze the paint for the components used in the binders, pigments, and additives.

Binder Flowchart Pigment Flowchart Additives Flowchart       
paintf1a.htm                                                                                          paintf1b.htm                                              paintf1c.htm

Notice that FTIR is one of the options for each of these analyses.  Raman is only listed with the pigments, but it can be used for some of the additives as well.

Though FTIR methods have been used in analyzing paints for some time, Raman is relatively new to this forensic application. (11,12)

Binders, pigments and additives contain specific types of molecules.  Though many of the molecules may be analyzed using either FTIR or Raman spectroscopy, one of the methods may be more accurate than the other in this application.  Below is a summary of the types of molecules found in each paint component. For detailed spectroscopic information, click the icon to the left of each heading.

Numbers         BINDERS

Typical binders consist of polyurethane, epoxy, acrylic or silicone resins.  Some inorganic silicates are also used.  (10)  Melamine or styrene are often used to cross-link the binders.  All of these substances have vibrational frequencies that can be identified spectroscopically.

Numbers         PIGMENTS

The most common pigment is rutile (TiO2). (10)  This is used in a majority of white paints and in many primers.  Other pigments are usually organic molecules, but several inorganic compounds are used as extender pigments.  Extender pigments are used in primers to enhance the properties of the coating. Barium sulfate and calcite (CaCO3) are typical extenders.

Numbers         ADDITIVES

Additives can be used to gain a variety of properties.  Silicone is used to make the paint surface more resistant to marking and scratching.  Stabilizers like phthalate are used to prevent the aggregation of pigment molecules while the paint is drying. (13)  Anti-molding agents, anti-corrosion agents, and drying agents are just a few more of the additives used in modern paints.

In summary, both FTIR and Raman are necessary in order to best identify an unknown paint sample. Much research is being conducted (5, 11, 12, 15, 16) which will hopefully enhance the scientists' ability to confidently say, "it's a match"!

Forensic Spectroscopy


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