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Chapter 16: Paint Analysis
16.1: Paint
Paint – a suspension of pigments and additives intended to color or protect a surface.
Pigment – a fine powder that is insoluble in the medium in which it is dispersed; are e intended to color and/or cover a surface; they may be organic, inorganic, or a mixture.
Binder – that portion of the coating, other than the pigment, that allows the pigment to be distributed across the surface.
Vehicle – refers to the solvents, resins, and other additives that form a continuous film, binding the pigment to the surface.
Solvents – dissolve the binder and give the paint a suitable consistency for application.
Four Categories of Paints:
Architectural Paints – household paints, and are those coatings most often found in residences and businesses.
Product Coatings – applied in the process of manufacturing products including automobiles.
Special-Purpose Coatings – fulfill some specific need beyond protection or aesthetic improvements, such as skid resistance, waterproofing, or luminescence.
Art Paints – are occasionally encountered in forgery cases. It is somehow the same as Architectural Paints, but many artists formulate their paints, leading to potentially unique sources.
Other Coating Definitions
Enamel – A pigmented coating that has a high gloss when it dries.
Lacquer – Clear or pigmented coatings that dry quickly through evaporation of the solvent.
Latex – A suspension of a pigment in a water-based emulsion of several resins.
Shellac – A solution of melted lac, a resinous excretion of the Lac insect dissolved in alcohol used as a sealant, adhesive, or insulating varnish.
Stain – A solution of dye or a suspension of a pigment designed to color, but not protect, a wood surface.
Varnish – A clear solution of oils and organic or synthetic resins in an organic solvent.
16.2: Paint Manufacturing
The automotive finishing process for vehicles consists of at least four separate coatings.
Pretreatment – applied to the steel body of the vehicle to inhibit rust.
Primer – usually an epoxy resin with corrosion-resistant pigments; the color of the primer is coordinated with the final vehicle color to minimize contrast and “bleed-through.”
Topcoat – the form of a single-color-layer coat, a multilayer coat, or a metallic color coat; this is the layer that most people think of when they think of a vehicle’s color.
Clear coats – unpigmented coatings applied to improve the gloss and durability of a vehicle’s coating.
16.3: Paint Collection
The collection of paint samples should proceed with caution.
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.
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
Lifting or prying out loose flakes is one of the ways to remove flakes of paint from a surface.
When a painted object strikes a glancing blow to another object, it can transfer paint in the form of a smear.
It is important when collecting known paint samples, therefore, that they are collected from areas as close as possible to, but not within, the point(s) of damage or transfer.
All paint samples should be clearly labeled as per origin, with drawings or photographs as documentation.
16.4: Analysis of Paint Samples
A combination of microscopes (stereo, transmitted light, and polarized light) at magnifications of 2× to 100× is used to examine the layers in paint.
Microtome – a minivice that holds a sample in place while a heavy and very sharp glass- or diamond-edged knife slices off sections of a few tens of microns thick.
Polarized light microscopy (PLM) is appropriate for the examination of a layer structure as well as the comparison and/or identification of particles present in a paint film including, but not limited to, pigments, extenders, additives, and contaminants.
Solvent and microchemical tests have been used to discriminate between paint layers of different pigment and binder compositions that are otherwise visually similar.
Infrared (IR) spectroscopy can identify binders, pigments, and additives used in paints and coatings.
FT-IR – measures the absorption of IR energy, over a range of wavelengths, as different bonds in the molecule vibrate and move in characteristic fashions.
The analysis of paints by FT-IR can be done in transmittance and reflectance.
Raman Spectroscopy – it is based on light scattering rather than absorption. It provides complementary information to that obtained from IR spectroscopy.
Absorption spectroscopy, using a microspectrophotometer (or MSP for short), has been used to categorize and discriminate between otherwise visually similar paints.
Metamerism – the condition in which two colors appear similar under one set of conditions but different under others.
Scanning electron microscope – can be used to characterize the structure and elemental composition of paint layers.
A consensus of forensic paint examiners is that the following factors strengthen an association between two analytically indistinguishable paint samples:
The number of layers;
The sequence of layers;
The color of each layer;
Cross-Transfer of paint between items.
Paint Data Query (PDQ) – run by the Royal Canadian Mounted Police (RCMP) and is the largest international automotive paint database.
It contains nearly 20,000 samples of paint systems, which represent over 74,000 individual paint layers used on most domestic and foreign vehicles being sold in North America, Australia, and Asia.
