Paint as evidence

DEFINITION: Substance that is spread over surfaces and dries to leave a thin decorative or protective coating.

SIGNIFICANCE: Paint is present on a multitude of surfaces in many environments. If a painted surface is damaged during the commission of a crime, paint may be transferred from that surface onto the victim, the offender, or other persons or objects present. Forensic scientists most commonly encounter paint evidence in motor vehicle hit-and-run cases and in burglary cases.

The quantity of paint that is transferred in any particular incident in which a painted surface is damaged depends on the force of contact, the area of contact, and the paint’s characteristics, such as its softness and state of deterioration. In hit-and-run cases, where a vehicle has struck a person or another vehicle, a large amount of paint may be transferred, especially if a number of the striking vehicle’s panels have been damaged. In contrast, only a small amount of paint is likely to be transferred to a tool when it is used to pry open a window during a burglary.

Composition and Types

Different kinds of paint consist of mixtures of a number of different chemical compounds. Paint can broadly be described as being composed of pigment, binder (also called vehicle or resin), solvent, and other additives. The pigment imparts color and opacity (hiding power) to the paint. The binder, usually a polymeric compound, provides the medium to spread the pigment across surfaces. The solvent, which may be either organic or water-based, is designed to evaporate after the paint has been applied to the surface, leaving the dried paint film. A range of additives can be included in the formulation of paints, depending on the paints’ intended uses; these can include plasticizers, driers, and mildew-resistant agents.

Most of the work of forensic scientists in relation to paint evidence involves the comparison of either automotive paint or architectural paint, although other types of specialized paint, such as marine paint or art paint, are sometimes encountered. When an automobile is manufactured, its finishing system usually consists of four layers of paint, called the original manufacturer’s paint. First, the electrocoat primer layer is applied directly to the metal of the car body to provide corrosion resistance. This layer is usually black or gray. The primer layer is then applied. The purpose of this layer is to provide a smooth surface for the base-coat application. The base-coat layer imparts color and appearance to the final product and provides resistance to environmental conditions such as weather and ultraviolet radiation. The base coat may contain additives to affect the final look of the paint; for example, aluminum flakes give a metallic finish, and mica pigments coated with metal oxide produce interference colors. Finally, a clear-coat layer is applied to protect the base-coat layer and add a glossy appearance to the paint.

In contrast to automotive paint, architectural paint is not applied in any set sequence of paint layers. The choices of paints used on building interiors and exteriors are usually made by individuals and may include varying numbers of base- and top-coat layers. The walls of older buildings may be coated with large numbers of paint layers, reflecting the many times they have been repainted.

Depending on the cost and availability of individual components, paint manufacturers may alter the ingredients they add to their products at different times while still ensuring that the paint meets strict quality-control requirements. Such differences in batch formulation can provide forensic scientists with additional points of discrimination as they make paint comparisons.

Comparison of Paint Samples

The first step in the forensic examination of any paint evidence is a visual comparison between the paint sample recovered from the crime scene and a control sample. This comparison, often aided by a microscope, involves an analysis of the color and texture of each layer of paint. Inspection of the sequence of paint layers present is extremely important for evaluating the evidential significance of any match.

For automotive paints, the examiner should be able to establish whether the paint is the original manufacturer’s paint or whether additional layers of paint have subsequently been added. The presence of additional paint layers means that the sample of paint, and therefore the vehicle from which it came, is likely to be distinguishable from other vehicles of the same model made by the same manufacturer.

For architectural paints, an assessment of the commonness of the color and type of paint present is required. Thousands of different paint colors are available, and new colors are continually being marketed as fashions change. No set number of paint layers must be matched between samples for the examiner to determine a conclusive match, but matches that include the presence of uncommon paint colors or large numbers of corresponding layers are considered to be of higher evidential value.

The comparison of the visual appearance of paint samples can be complicated if the recovered sample is present only as a paint smear. Smears are frequently encountered on the clothing of hit-and-run victims and on tools used to pry open painted windows or doors during the commission of burglaries. The color of a paint smear may be altered slightly because of the influence of the substrate color or because a number of different paint layers have been crushed together.

If two samples of paint cannot be differentiated after a visual examination, the examiner’s next step is to compare the chemical compositions of the paint layers. As most paints contain mixtures of organic and inorganic compounds, the instrumental techniques used must be able to detect the full range of compounds present. In addition, each layer should be analyzed separately, if possible. For some techniques, this means that the examiner must section the paint, either by hand or using a microtome.

Pyrolysis gas (PyGC) is frequently used to compare the organic compounds present in the paint layers. This technique is particularly suited to the analysis of polymers, as the heat present in the pyrolyzer decomposes the polymer into smaller oligomers that are able to be chromatographed. Fourier transform infrared (FTIR) spectroscopy can also be used to compare the major organic compounds present and some of the inorganic compounds present (such as titanium dioxide).

The examiner may use a variety of techniques to compare the inorganic compounds present in each paint layer, such as X-ray fluorescence (XRF) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). SEM-EDS has the added advantage of allowing a visual examination of each layer at very high magnification. This can reveal the presence of additives such as aluminum flakes.

Evaluation of Paint Evidence

If the paint layers present in the recovered and control samples cannot be distinguished by their physical appearance and chemical composition, the next step is the assessment of the evidential value of these findings. This is usually a subjective exercise based on the experience of the analyst, who takes into consideration the number of corresponding paint layers and the commonness of the types of paint present. For automotive paint, it is important to establish whether the paint layers are the original manufacturer’s paint or whether additional layers of paint have been added.

The finding of a number of corresponding paint layers usually provides strong evidence to link the two samples of paint. For cases involving the collision of two vehicles, the finding of two-way paint transfer can provide very strong evidence, and in some cases a conclusive finding may be made. A conclusive match can also be reported when the analyst considers that the combination of corresponding paint layers is unique and therefore no alternative sources are possible.

Forensic scientists are also sometimes asked to identify the makes and models of automobiles from paint layers present in samples recovered from crime scenes. Such requests are commonly made to provide investigative evidence on the specific kinds of vehicles that were present at the scenes, whether hit-and-run or other kinds of crime scenes. To make such identifications, the scientists consult databases containing extensive information on the paints used by automobile manufacturers, such as the International Forensic Automotive Paint Data Query (PDQ) database established by the Forensic Laboratory Services of the Royal Canadian Mounted Police .

Bibliography

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Challinor, John. “Paint Examination.” In Expert Evidence, edited by Ian Freckelton and Hugh Selby. North Ryde, N.S.W.: Lawbook, 1993.

Duarte, Juliana Melo, et al. "Automotive Paint Analysis: How Far Has Science Advanced in the Last Ten Years?" TrAC Trends in Analytical Chemistry, vol. 132, Nov. 2020, doi.org/10.1016/j.trac.2020.116061. Accessed 16 Aug. 2024.

Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.

Inman, Keith, and Norah Rudin. Principles and Practice of Criminalistics: The Profession of Forensic Science. Boca Raton, Fla.: CRC Press, 2001.

Ryland, Scott. "Paint Analysis." American Society of Trace Evidence Examiners, 2024, www.asteetrace.org/paint. Accessed 16 Aug. 2024.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.