Trace Evidence Paint

Introduction Manufactured products and even most natural materials contain small quantities of elements, or trace elements, in total concentrations of less than 1 percent. The presence of these trace elements is useful, because they: provide “invisible” markers that may establish the source of a material or additional points for comparison

Paint Paint spread on most surfaces will dry into a hard film. Paint contains pigments and additives suspended in a binding agent. Paint examined in the crime laboratory usually involves automobiles. Automobile manufacturers normally apply a minimum of 4 coatings to the steel parts of an automobile. These coatings include: electrocoat primer primer surfacer basecoat clearcoat.

Methods for Paint Comparison The huge supply of automotive paint helps to make-up the PDQ (automobile paint comparison database). Questioned and known paint specimens are best compared side by side under a stereoscopic microscope. Look for color, surface texture, and color layer sequence. Pyrolysis gas chromatography and infrared spectrophotometry can identify formula of paint binders. – Solid materials, such as paint, may be heated until they are pryolyzed (decompose) at very high temperatures – The gaseous products are measured as they flow through the GC column.

Stereoscopic microscope Used to compare questioned samples to known samples Analyzes color, surface texture, and color layer sequence

Methods for Paint Comparison The elements that are contained within paint pigments can also be identified by emission spectroscopy or X-ray spectroscopy.

PAINT IDENTIFICATION Crime laboratories are often asked to identify the make and model of a car from a small amount of paint and will make use of color charts for automobile finishes or the PDQ database.

Collection and Preservation Paint chips are most likely found on or near persons or objects involved in hit-and-run incidents. Paper druggist folds and glass or plastic vials make excellent containers for paint. Paint smeared or embedded in garments or objects require the whole item to be packaged and sent to the laboratory. The paint chips collected must be close to the area of the car that is being suspected of hitting the victim – Paint may be faded in other parts of the vehicle Uncontaminated standard/reference paint must always be collected. Smears on cars and other large objects should be scraped down to the metal to preserve all the layers

Evidence Collection (con’t) Tools used to gain entry into buildings or safes often contain traces of paint All evidence should be recorded in place before collecting (notes/pix) All containers should be labeled and sealed Paint chips must be kept intact

Paint is a mixture Pigment (color) – Blues and greens are organic – Reds, yellows, whites: inorganic Modifiers (change pigment) – Control the property of the paint Gloss, flexibility, durability, toughness Extenders (keep pigment) – Add bulk and covering capacity – Inorganic Binders (help paint stick to the surface) – Natural or synthetic – Stabilizes the mixture – Forms a film when spread

Automobile Coatings or Layers of paint Electrocoat Primer – the first layer applied to the steel for corrosion resistance. Primer surfacer – smooths out and hides imperfections Basecoat or Undercoat – provides color and represents the “eye appeal” of the finish Clearcoat or topcoat – resists UV radiation and acid rain

Which layer of paint is most informative? Undercoat more than any other property, gives paint its most distinctive forensic characteristics. contains most pigment

Each layer is unique Top coat, primer and undercoat all have different chemical compositions Exposure to chemicals, dirt and rain can complicate analysis