Forensic Examination of Paint

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Presentation transcript:

Forensic Examination of Paint Pg. 213-223

Paint is… Pigments (organic and inorganic) and additives suspended in a polymeric binder solution One of the most prevalent types of physical evidence Most often recovered in burglary and hit-and-run cases

Paint comparisons Need 2 paint samples for comparison OR Can examine uniqueness of one paint sample Comparison analysis can Link a person or vehicle to a crime scene OR Identify the color, make, and model of the vehicle that left the paint sample at the scene

Automobile paint Each manufacturer applies a specific variety of coats For steel auto finishes: 4 coats Electrocoat primer Primer surfacer Basecoat Clearcoat

Electrocoat primer Black to gray in color Electroplated to the steel surface Uniform in appearance and thickness Provides corrosion (rust) resistance Made of epoxy-based resins

Primer surfacer Originally used for corrosion control Now used for smooth out and hide any seams or imperfections Highly pigmented – different colors (gray to red) used to minimize the contrast between the primer and the basecoat Made of epoxy-modified polyesters

Basecoat (or colorcoat) Provides the color for aesthetic appeal Integrity based on resistance to the elements (UV radiation, acid rain, weather) Contains acrylic-based polymer binder Pigments dictated by toxicity – no lead, chrome, or other heavy metals Organic-based pigments used now – mica, pearl luster, aluminum flakes

Clearcoat Unpigmented Provides glossy appearance, improves durability Mostly acrylic-based but some polyurethane

Comparing paints Stereoscopic microscope (stereoscope) Color is most distinctive characteristic Also compare surface texture, shape, and color layer sequence

Color layer sequence Compare Number of layers Sequence of colors Matching samples is dependent on the uniqueness of each layer (color, thickness, etc.) No definite number of layers to guarantee a match

Chemical composition Of pigments or binders Pyrolysis gas chromotography Paint samples are decomposed by heat into gases and then identified through gas chromatography The “pyrogram” or chromatogram gives the unique chemical composition of the pigment or binder Can also use infrared spectrophotometry (unique spectrum), emission spectroscopy, X-ray diffraction or spectroscopy

If samples are compared by layer structure, solvent tests, pyrolysis gas chromatography, and x-ray spectroscopy… The chance of an incorrect match is 16,000 to 1

Paint databases Law Enforcement Standards Laboratory collection distributed through Collaborative Testing Services in McLean, VA Royal Canadian Mounted Police Forensic Laboratory’s PDQ (Paint Data Query) http://www.rcmp.ca/factsheets/pdfs/pdq_e.pdf