Trace Evidence: Paint
Criminalistics
Paint: binder, pigments and
additives suspended in a solvent
Major Components
 Binder
 Pigment
 Extenders
 Modifiers
Binder

Polymer (vehicle) that holds paint components
together, forming a hard surface film as
solvent evaporates
– Lacquer (evaporation of organic solvent)
– Enamel (chemical reaction of components)
– Latex (evaporation of water)
» Architectural
Pigment, Extenders, Modifiers

Pigment
– Color
» Organic: Blues & Greens
» Inorganic: Whites, Yellows, Reds

Extenders
– ↑ solid content, hiding ability/opacity
– Titanium dioxide acts as extender & pigment

Modifiers
– Gloss, hardness, flexibility, UV resistance,
application, adhesion, drying
Automotive Paint Layers

Electrocoat Primer on bare steel body


Primer Surfacer smoothes imperfections



Corrosion resistance, black, gray
Increases adhesion of basecoat to primer
Pigment added to minimize color contrast between
primer and basecoat
Basecoat /Color Coat
– Pigments, aluminum flakes

Clear Coat
– Modifiers for durability, UV resistance
Paint as Physical Evidence
Hit and Run/MVAs- Paint fragments or
smears transferred to clothing (pedestrian)
or another vehicle (vehicle to vehicle)
Burglaries- Paint smears transferred onto a
tool or point of entry (tool mark)
Scrape down
clothing for
Trace Evidence
Paint Chips from Scene
Paint Analysis
 Jigsaw
Match
 Microscopic
 Chemical
Examination
Analysis
Jigsaw Match
Known
Questioned
Suspect vehicle
Paint chips fm scene
Microscopic Examination of Paint
 Stereomicroscope
or Comparison
Microscope
 Physical Characteristics
– Color
– Surface Texture
– Number of layers
– Color sequence of layers
– Thickness of layers
Clearcoat
Basecoat/ Colorcoat
Primer Surfacer
Electrocoat Primer
Chemical/Instrumental Analysis of Paint
 Chemical
Analysis
– Solubility tests
– Pyrolysis GC- decomposes paint
– IR Microspectrophotometry
» IR absorption spectrum for each layer
– SEM-EDX or X-Ray Fluorescence (elemental
composition)
Solubility Testing
Pyrolysis Gas Chromatography
Microspectrophotometer
IR Absorption Spectra of Paint
Investigative Information
Paint Data Query (PDQ)
Database of Chemical & Color Information of
Original Automotive Paints (>13,000 vehicles)
Upload IR spectra from Q sample
 Receive:

– Make (Toyota, Ford, Chrysler, Chevy)
» GM- Chevy, Buick, Pontiac, Cadillac
– Model (Corolla, Mustang, Sebring, Malibu)
» Corolla, Camry, Solara
– Years of production
Paint Evidence Collection

Collect entire item, if possible


Vehicle to vehicle transfer


All layers of contact
Containers




Tools, clothing, bicycle, vehicle
Paper fold into envelope
Vial (glass, plastic)
Small cardboard box (larger paint chips)
Keep paint chips intact

Jigsaw match sometimes possible
Paint Evidence Collection
 Known/Reference





Sample
Clean scalpel or knife blade
Remember possibility of Jigsaw Match
Near area of damage down to bare metal
Near area of toolmark impression
Package separately, note location
Collecting Known Sample