• Chemistry of paint very complex
• Forensic role is to compare paint evidence from scene to known source
• Individual evidence only in cases where large enough paint samples exist to physically match

• PAINT: A suspension of pigment in a film former (also called vehicle)
• Usually not a solution. Pigment is suspended in film former
• Pigment is usually mixture of inorganic metal salts. Purpose is to impart color
• Film former is usually organic polymer(s). Purposes are to protect surface and hold pigments

• Varnish: A film former, commonly polyurethane, dissolved in a solvent, which normally doesn’t contain a stain. Purpose is to protect wood
• Stain: A mixture of organic dyes dissolved in solvent. Made to penetrate wood and stain in. Does not protect
• Enamel: originally a glossy, thermosetting paint. Now any paint which dries glossy

• Solvent evaporation
• Rust proof paints
• Heating (thermosetting)
• Automotive
• Oxidation
• Drying oils, artistic paints
• Linseed oil

• Paint chips: Pieces of paint which have come off surface. They usually contain all of the layers of the paint
• Paint smears: The top layer of paint which has loosened owing to being wet or air oxidation. Smears onto another surface after brushing contact. Layer structure is not present

• Color layer analysis
• Same number and order of layers
• Relative thickness must be same

• Solubility
• Use solvents such as acetone, dichloromethane, pyridine. Acrylic lacquers are soluble in acetone
• Pyrolysis GC
• Analysis of film formers
• Bulk technique, all layers analysed together

Analysis of Paint

• Pyrolysis GC

• FTIR
• Can make KBr pellet
• Will determine type of film former
• Is a bulk method: if more than one layer is present, IR will be a composite

• Electron microscopy
• Inorganic pigments
• Excellent for single, top layer analysis and paint smears
• UV-visible spectrophotometry
• Must be able to separate pigment from film former and dissolve it

FTIR of Automobile Paint

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