Contributed by Scott Ryland
Modern paint is a manufactured product typically consisting of a mixture of numerous materials (components). Its most apparent feature is the variety of colors available. Different paint manufacturers will usually use different components in their products. Any given manufacturer also offers a variety of grades or types of paint depending on its projected end use or its cost. These different grades or types also differ in the components or the relative amounts of components put into them. As such, even for a single layer of paint, there is a tremendous amount of variation from product to product and there are literally thousands of different kinds of paint in our environment.
Paint is usually encountered as evidence of association in a cured form often consisting of multiple intact layers, called a paint chip. It can be found in homicide, assault, vehicular manslaughter, hit-and-run, and burglary investigations, most often involving vehicle paints, architectural paints, or maintenance paints. It is commonly the most chemically complex type of trace evidence encountered. As noted above, each layer of paint in these chips carries the features distinct to that paint; morphological characteristics, organic chemicals and inorganic chemicals. Obviously, the more layers of paint present in a chip, the less likely it is for one to randomly encounter another source of paint with the same characteristics (layer sequence and individual layer components).The basic thrust of a forensic paint examination is to try to differentiate between paint samples and eliminate the possibility that they have the same source. The approach uses the scientific method and hypothesis testing. Paint is usually mass-produced using a recipe and sometimes in rather large batches. Accordingly, one has to consider the possibility that a given paint could be applied to a number of different sources. It is therefore often impossible to definitively associate a given paint sample's origin with one source to the exclusion of all others. There are exceptions to this, as in the case of paint chips with fractured edges or surface configurations that physically correspond to the paint at the source.
Using established forensic techniques, however, can lead to scientifically based conclusions as to the possibility that given paint samples originated from the same source. The techniques used in forensic paint comparisons are classical microscopical and analytical instrumental chemistry techniques taught in universities worldwide. They include such methods as stereomicroscopy, polarized light microscopy (PLM), Fourier Transform infrared microspectroscopy (FTIR), pyrolysis gas chromatography in conjunction with mass spectrometry (PyGC-MS), scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (SEM-EDS), and UV-Vis microspectrophotometry (MSP). ASTM International has published several consensus guidelines on these topics, to include a Standard Guide for Forensic Paint Analysis and Comparison (ASTM E1610-2014), a Standard Guide for Using Infrared Spectroscopy in Forensic Paint Examinations (ASTM E2937-13), a Standard Guide for Using Scanning Electron Microscopy/X-ray Spectrometry in Forensic Paint Examinations (ASTM E2809-13), and a Standard Guide for Microspectrophotometry and Color Measurement in Forensic Paint Analysis (ASTM E2808-11). In addition, the Scientific Working Group for Materials Analysis has posted a Standard Guide for Using Pyrolysis Gas Chromatography and Pyrolysis Gas Chromatography-Mass Spectrometry in Forensic Paint Examinations on their website (www.swgmat.org). If one applies to two samples in question a thorough analytical scheme which differentiates between the various physical and chemical features in most paints, one can deduce whether or not they are like one another. If significant differences are found, the results lead the examiner to the conclusion that the paints are dissimilar and did not originate from the same source. If no significant differences are found, the results lead the examiner to the conclusion that the paints are alike in all their measured significant characteristics and that it is possible that they originated from the same source. The evidentiary significance of the correspondence is reflected by the ability of the analytical scheme to differentiate between most paints. This ability can be demonstrated by published discrimination studies.
1. Forensic Examination of Glass and Paint: Analysis and Interpretation, B. Caddy, ed., Taylor and Francis, NY, NY, 2001.
2.Thornton, J., "Forensic Paint Examinations," Chapter 8, in Forensic Science Handbook, Vol. I, 2nd ed., Saferstein, R., ed., 2002, pp. 430-478.
3. Ryland, S., Jergovich, T. and Kirkbride, P., “Current trends in forensic paint examination,” Forensic Science Review, Vol. 18, No. 2, July 2006, pp. 97-117.
4. Ryland, S.G. and Suzuki, E.M., “Analysis of Paint Evidence,” Chapter 5, in Forensic Chemistry Handbook, L. Kobilinsky, ed., John Wiley and Sons, Inc., 2012, pp. 131-224.
Due to the nature of the cases and the amount of evidence involved, many of the cases involving paint evidence are adjudicated prior to trial.
1.United States vs. Benjamin Williams and James Williams, CR. No. S-00-139 GEB, United States District Court for the Eastern District of California, domestic terrorism, 2002..
2.State of Florida vs. Daniel Conahan, Jr., CR No. 97-0166CF, 20th Judicial Circuit, serial homicides, 1996.
3. State of Alabama vs. Williamson Samual Cecil III, CC 2007 002721 00 W012, in the Circuit Court of Tuscaloosa County, first degree assault and first degree theft, 2010.
4. State of Florida vs. Jerrold Baron, CR No. 06-418CF, 15th Judicial Circuit, vehicular homicide, 2007.
5. United States vs. Frankie Maybee, case number 3:11-CR30006-001, United States District Court for the Western District of Arkansas, attempted murder/hate crime, 2011.
6.State of Florida vs. Ryan Welch, 2012CF2181, Division F, 1st Judicial Circuit, vehicular manslaughter and leaving scene with death, 2013.