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Forensic Paint Analysis and Comparison Guidelines by SWGMAT Paint Subgroup, Part 1 (FSC, July 1999)

Forensic Paint Analysis and Comparison Guidelines by SWGMAT Paint Subgroup, Part 1 (FSC, July 1999)

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July 1999 - Volume 1 - Number 2

Forensic Paint Analysis and Comparison Guidelines

Scientific Working Group on Materials Analysis (SWGMAT)
(formerly the Technical Working Group on Materials Analysis [TWGMAT])
Paint Subgroup


May 2000 Revision
(Originally Published as the January 1999 Revision)

Read about …
1.0. Scope
2.0. Reference Documents
3.0. Terminology
4.0. Quality Assurance Considerations
5.0. Summary of Practice
6.0. Significance and Use
7.0. Collection of Suitable Samples
8.0. Procedure
Figure 1: Paint Comparison Guide
Figure 2: Guide to Motor Vehicle Identification
9.0. Other Techniques
10.0. References
11.0. Endnotes
Document Comments Form

This document is intended to be a suggested revision to the original ASTM E1610-94 Guide. This revision is the product of the Paint Subgroup of the Scientific Working Group for Materials Analysis (SWGMAT). Its ultimate acceptance as an ASTM document will be the responsibility of ASTM Committee E-30 on Forensic Sciences. You are invited to submit constructive feedback on the Document Comments Form.

1.0. Scope

1.1. Forensic paint analyses and comparisons are typically distinguished by sample size that precludes the application of many standard industrial paint analysis procedures or protocols. The forensic paint examiner must address concerns such as the issues of a case or investigation, sample size, complexity and condition, environmental effects, and collection methods. These factors require that the forensic paint examiner must choose test methods, sample preparation schemes, test sequence, and degree of sample alteration and consumption suitable to each specific case.

1.2. This document is an introduction for the forensic examination of paints and coatings. It is intended to assist personnel who conduct forensic paint analyses in the evaluation, selection, and application of tests that may be of value to the investigation. The guidelines that follow describe methods to develop discriminatory information using an efficient and reasonable order of testing. The need for validated methods and quality assurance guidelines is also addressed. This document is not intended to be a detailed methods description or rigid scheme for the analysis and comparison of paints but a guide to the strengths and limitations of each analytical method. The goal is to provide a consistent approach to forensic paint analysis.

1.3. Some of the methods discussed in these guidelines involve the use of dangerous chemicals, temperatures, and radiation sources. This document does not address the possible safety hazards or precautions associated with its application. It is the responsibility of the user of these guidelines to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.

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2.0. Reference Documents

2.1. ASTM D16 Terminology Relating to Paint, Varnish, Lacquer, and Related Products1

2.2. ASTM D1535 Method for Specifying Color by Munsell System

2.3. ASTM E308 Test Method for Computing the Colors of Objects by Using the CIE System

2.4. ASTM E1492 Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory2

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3.0. Terminology

3.1. For definitions of terms used in these guidelines other than those listed, see ASTM D16 Terminology Relating to Paint, Varnish, Lacquer, and Related Products.1

3.2. Descriptions of Terms Specific to this Guide:

3.2.1. Binder: A nonvolatile portion of the liquid vehicle of a coating, which serves to bind or cement the pigment particles together.

3.2.2. Coating: A generic term for paint, lacquer, enamel, or other liquid or liquefiable material that is converted to a solid, protective, or decorative film or a combination of these types of films after application.

3.2.3. Discriminate: To distinguish between two samples on the basis of significant differences; to differentiate.

3.2.4. Discriminating Power: The ability of an analytical procedure to distinguish between two items of different origin.

3.2.5. Known Sample: A coating sample of established origin.

3.2.6. Paint: Commonly known as a pigmented coating.

3.2.7. Pigment: A finely ground, inorganic or organic, insoluble, and dispersed particle. Besides color, a pigment may provide many of the essential properties of paint such as opacity, hardness, durability, and corrosion resistance. The term pigment includes extenders.

3.2.8. Questioned Sample: A coating sample whose original source is unknown.

3.2.9. Significant Difference: A difference between two samples that indicates that the two samples do not have a common origin.

3.2.10. Additive (modifier): Any substance added in a small quantity to improve properties. Additives may include substances such as driers, corrosion inhibitors, catalysts, ultraviolet absorbers, and plasticizers.

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4.0. Quality Assurance Considerations

4.1. A quality assurance program must ensure that analytical testing procedures and reporting of results are monitored by proficiency tests and technical audits. General quality assurance guidelines may be found in Trace Evidence Quality Assurance Guidelines (1).

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5.0. Summary of Practice

5.1. Physical and Chemical Features
Paint films are characterized by a number of physical and chemical features. The physical characteristics may include color, layer sequence and thickness, surface and layer features, contaminants, and weathering. Chemical components may include pigments, polymers, and additives. These features can be determined and evaluated by a variety of macroscopical, microscopical, chemical, and instrumental methods. Limited sample size and sample preservation requirements mandate that these methods be selected and applied in a reasonable sequence to maximize the discriminating power of the analytical scheme.

5.2. Questioned and Known Samples
Searching for differences between questioned and known samples is the basic thrust of forensic paint analysis and comparison. However, differences in appearance, layer sequence, size, shape, thickness, or some other physical or chemical feature can exist even in samples known to be from the same source. A forensic paint examiner's goal is to assess the significance of any observed differences. The absence of significant differences at the conclusion of an analysis suggests that the paint samples could have a common origin. The strength of such an interpretation is a function of either or both the type or number of corresponding features.

5.3. Motor Vehicle Identification
An important aspect of forensic paint analysis is the identification of the possible makes, models, and years of manufacture of motor vehicles from paint collected at the scene of a crime or an accident. The color comparison and chemical analysis of the undercoat and topcoat systems requires a knowledge of paint formulations and processes, collections of paint standards, and databases of color and composition information.

5.4. Sample Documentation
The test procedure selected in a paint analysis and comparison begins with thorough sample documentation. Some features of that documentation are described in ASTM E1492 Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory.2 Analysis generally begins with appropriate nondestructive tests. If the initial tests are inconclusive or not exclusionary, the examination may proceed with additional tests that are selected on the basis of their potential for use in evaluating or discriminating the samples of interest or both.

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6.0. Significance and Use

6.1. These guidelines are designed to assist the forensic paint examiner in selecting and organizing an analytical scheme for identifying and comparing paints and coatings. The size and condition of the sample or samples will influence the selected analytical scheme.

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7.0. Collection of Suitable Samples

7.1. The potential for physical matches between known and questioned samples must be considered before selecting the method of paint sample collection. Care should be taken to preserve the potential for a physical match.

7.2. Questioned Samples
7.2.1. Questioned samples should include all loose or transferred paint materials. Sources of questioned samples can include tools, floors, walls, glass fragments, hair, fingernails, roadways, adjacent structures, transfers or smears on vehicles, or transfers to or from individuals such as damaged fabric with paint inclusions. Items with paint transfers should be appropriately packaged and submitted in their entirety for examination whenever possible. If sampling is necessary, the procedures listed in Trace Evidence Recovery Guidelines (2) may be used. When paint evidence is recognized, every effort should be made to manually remove it before using tape lifts to collect other types of evidence. If paint is collected with tape lifts, be aware of the possible difficulty encountered when attempting to manipulate paint samples bearing adhesive residues. In addition, components of the adhesive could contaminate the paint sample and change its apparent chemistry.

7.2.2. Smeared transfers can exhibit mingling of components from several layers or films that could preclude application of some of the analytical methods discussed in these guidelines. Because of the difficulties associated with collecting smeared or abraded samples, the entire object bearing the questioned paint should be submitted to the laboratory whenever possible.

7.2.3. When contact between two coated surfaces is indicated, the possibility of cross transfers must be considered. Therefore, if available, samples from both surfaces should be collected.

7.3. Known Samples
7.3.1. When feasible, known paint samples must be collected from areas as close as possible to, but not within, the point or points of damage or transfer. These damaged areas are usually not suitable sources of known samples. The collected known samples should contain all layers of the undamaged paint film. Substantial variations in thickness and layer sequences over short distances can exist across a painted surface. This is particularly true in architectural paint and automotive films where the curves, corners, and edges are often impact points and may have been subjected to previous damage, sanding, or overpainting. If necessary, several known paint samples should be taken to represent all damaged areas. Known paint samples collected from different areas should be packaged separately and labeled appropriately.

7.3.2. The surface underlying the suspected transfer area should be included for analysis when possible. Adjacent sections removed from a wall, ceiling, door, window, implement handle, and automobile door, fender, and hood are examples of items that can be valuable for assessing questioned and known sample differences and for evaluating the possible cross transfer of trace materials.

7.3.3. Paint flakes can be removed from the parent surface by a number of methods. These include but are not limited to the following: lifting or prying loosely attached flakes, cutting samples of the entire paint layer structure using a clean knife or blade, or dislodging by gently impacting the opposite side of the painted surface. When cutting, it is important that the blade be inserted down to the parent surface. It should be noted that no one method of sampling should be relied upon exclusively.

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To Part 2 (Section 8.0)

To Part 3 (Sections 9.0-11.0)

You are invited to submit constructive feedback on the Document Comments Form.