COLLECTION
Because it is possible a physical match exists between known and questioned paint samples, as demonstrated in Figure 1, the collection of paint samples should proceed with caution. This type of evidence carries great significance and care should be taken to preserve any potential physical matches.
FIGURE 1 The strongest evidence of an association between a paint sample and a source is a physical match, considered unique and individualizing. These are somewhat uncommon and therefore carry strong probative value. Here, two paint chips are aligned to show the common border demonstrating that they were at one time one continuous coating.
Samples from the crime scene (questioned) should include all loose or transferred paint materials. Nearly any object or surface may retain a paint transfer and may include things as varied as tools, architectural structures and elements (floors, wainscoting), glass fragments, fabrics, hairs, fingernails, roadways and signs, and, of course, vehicles. Evidentiary items with paint transfers should be packaged and submitted to the laboratory in their entirety, if possible. Depending on the size of the object, packaging and submitting could prove problematic, so often sampling of paint transfers must take place in the field. It is also important to remember that cross-transfer could have occurred. Known and questioned samples should be collected from both surfaces. Paint evidence should be first photographed and then removed manually with nonmetallic tools, such as small wooden sticks, toothpicks, or plastic forceps. If tape lifts are to be used, the paint evidence should be collected first. Because of their structure, fragility, and size, if paint samples are collected with tape lifts, it could be very difficult to impossible to easily manipulate paint samples that are sticky from the tape’s adhesive. Additionally, the adhesive’s components, or material stuck to the adhesive, could contaminate the paint sample and change its apparent chemistry (see Figure 2). Flakes of paint can be removed from a surface in a variety of ways. Lifting or prying out loose flakes, cutting samples of the paint with a clean knife or blade, and sampling that took place at the crime scene and, because of sample source variation, may find that the paint samples are analytically distinguishable when the proper samples would have been the same in all tested respects. All paint samples should be clearly labeled as per origin, with drawings or photographs as documentation. Because paint is a multilayer composite material, the known samples should contain all layers of the undamaged paint. Differences in the thickness and sequence of layers can be significant over even short distances on a painted surface. This concept is very important with architectural paints, where substantial reworking of the surface (sanding, damage, overpainting) may have occurred. It could be important to collect known paint samples from several areas of an object if variation is noticeable or suspected; these samples from different areas should be packaged separately and labeled appropriately. Depending on what it is made of, the subcoating surface under the suspected transfer area should be included for analysis when possible. This may extend to portions or objects near the evidentiary location, such as portions of walls, doors, window frames, handles, fenders, and decorations. These additional samples may be useful to assess any difference that may exist between the known and questioned samples.
sampling that took place at the crime scene and, because of sample source variation, may find that the paint samples are analytically distinguishable when the proper samples would have been the same in all tested respects. All paint samples should be clearly labeled as per origin, with drawings or photographs as documentation. Because paint is a multilayer composite material, the known samples should contain all layers of the undamaged paint. Differences in the thickness and sequence of layers can be significant over even short distances on a painted surface. This concept is very important with architectural paints, where substantial reworking of the surface (sanding, damage, overpainting) may have occurred. It could be important to collect known paint samples from several areas of an object if variation is noticeable or suspected; these samples from different areas should be packaged separately and labeled appropriately. Depending on what it is made of, the subcoating surface under the suspected transfer area should be included for analysis when possible. This may extend to portions or objects near the evidentiary location, such as portions of walls, doors, window frames, handles, fenders, and decorations. These additional samples may be useful to assess any difference that may exist between the known and questioned samples.
FIGURE 2 Paint fragments must be handled with care; being too aggressive in their collection can damage or contaminate them.
