What Forensic Science Can Tell Us From Paint Chips

Paint fragments can appear as trace evidence in household burglaries and automobile collisions.  Regarding the burglaries, paint from a colored “tool” such as a crowbar can be left on the point of entry and, conversely, paint from the window or door frame can be left onto the tool.  This “cross transfer” of paint between the burglary tool and the home properly classifies paint as a form of “transfer evidence.”  This essay will discuss the composition of household paint and some techniques used to identify a particular match so that the trace evidence recovered can be used to link a suspect to a crime.

Generally speaking, paint has 3 components: pigment, binder and solvent.  In some instances, additives are used to improve the paint’s specific properties depending upon its intended use.

Pigments are simply the color chemical (typically a metal compound) in paint and is typically a solid.  The color is derived from the chemical’s ability to absorb and reflect light waves. The binder is a chemical causing the paint particles to stick to each other and the surface upon which someone is painting. Traditionally, binders were made from natural oils but now it is common to see binders made from synthetic plastics.  The solvent component of paint is the substance that makes the pigment-binder a thinner and less viscous liquid.  Water is a common binder but so too are various petroleum based chemicals.  The solvent will evaporate after the paint dries.

In layman’s terms, you can classify household paints as either exterior paints or interior paints.  Exterior paints have pigments able to withstand extreme weather conditions. Furthermore, interior paints can be grouped into either water-based or oil-based paints. There are a variety of ways you can analyze paints to see if there is a match.

The first, most obvious and probably least precise, is the “naked eye.” A careful description of features and properties should be noted.  In some instances, a visual inspection can be used to determine that there is not a color match.  That’s fine but unreliable when colors are very similar as this test is dependent upon the color sensitivity of the observer.  In one study reviewed for this essay, it was suggested that a microspectophotometer could be used to discriminate household paints closely similar in appearance.  In this analysis, the microspectophotometer allows for the comparison of the visible reflectance spectra of the paint.  While this tool can help determine whether paints are the same color, other tests will help determine the chemical composition of the paints: micro-Raman Spectroscopy; Infrared Spectroscopy; Pyrolysis Gas Chromatography; Scanning Electron Microscopy; and Solubility Tests.

The micro-Raman spectroscopy is used to determine pigment/dye content in paint samples. This is a non-destructive test that uses lasers which will interact with the vibrational frequencies of the paint molecules.

Infrared Spectroscopy is an effective tool used to characterize paints in terms of their organic and inorganic component.  Here, we can measure how the paint molecules react with infrared light. This can help determine the binder used with a particular paint.  Similarly, with a solubility test, the chemicals used in a paint can be determined by what substances they will or will not completely dissolve into to.

Pyrolytic Gas Chromatography is a destructive technique that can be used for binder classification and comparison.  In this procedure, the sample is heated until it breaks down and smaller molecules are separated.  The data from this process can be used to identify the materials found in the paint sample.

Scanning Electron Microscopy allows for the characterization of the paint sample’s morphology and its elemental composition.  In this test, emitted x-rays provide information on compositional elements.  Also, the electrons emitted can provide additional info on the paints topography.