LECTURE 17: PAINT AND GLASS Flashcards
automotive paint
- Electrocoat primer on metal surface for anti-corrosion
- Primer surfacer to make surface smooth
- Basecoat where pigments added
- Clearcoat to protect everything below
Can be individual/group evidence - paint evidence
- specialised coatings will have specific make-up
- individual manufacturers may have different formulations even for the same colour
- need to be aware that car manufacturers may use number of companies for supplies/use same paint on number of models
Destructive testing - art fraud
- Spot tests – for particular pigments
- Scrape off sample and do a chemical reaction to test for
- particular elements
- e.g. Naples Yellow Pb(SbO4)2H2
- Massicot PbO
- Lead-tin Yellow 2PbO.SnO2 used in Europe < 1750
- Crystal size/ structure
Semi Destructive - art fraud
- Laser microanalysis AES
- Fine laser beam ~ 100 µm across focussed on specimen and material evaporated
- Get small indents in metallic objects but quite large in ceramics, glass, organic materials
Non-Destructive - art fraud
- radiocarbon dating
- thermoluminescence (dating of fired objects)
- neutron autoradiography
- SEM to look at pigment particle shape/size
- Spectroscopic techniques
- X-ray
- infra red (IR)
float glass
- flat surface on both sides, used for windows
glass evidence is usually what kind of evidence
class evidence
analysis of glass
refractive index and density
glass fractures are
concentric and radial fractures
paint
a suspension of pigments & additives intended to colour or protect a surface.
pigment
is a fine powder, insoluble in the medium in which it is dispersed – the granules don’t dissolve & remain intact & are dispersed evenly across the surface. They may be organic, inorganic, or a mixture.
solvents
dissolve the binder & give the paint a suitable consistency for application (brushing, spraying, etc).
binders
is the portion of the coating, other than the pigment, that allows the pigment to be distributed across the surface.
vehicle
refers to the solvents, resins, & other additives that form a continuous film, binding the pigment to the surface. Used interchangeably with “binder”
paints are divided into 4 categories
- Architectural paints (household paints) = coatings most often found in residences
& businesses - Product coatings = applied in the process of manufacturing products include
automobiles. 2nd major category - Special-purpose coatings = fulfil some specific need beyond protection or
aesthetic improvement, such as skid resistance, water proofing,
or luminescence. - Art paints = similar in many respects to architectural paints, but
many artists formulate their own paints, leading to potentially unique sources.
microscopes used for paint
polarised light microscopy for layer structures
infrared (IR) spectroscopy for binder, pigments and additives
Raman spectrocopy based on light scattering rather than absorption
pryrolisis-gas chromatography disassembles molecules through heat (pyrolysis), uses the breakdown products for comparisons of paints & identification of the binder type.
energy dispersive X-ray spectrometer (SEM/EDS) is used to characterise the structure & elemental composition of paint layers, uses an electron beam rather than a light beam.
soil and glass are usually
class evidence
soil =
earth material, either natural or manmade that is transferred from a crime scene to a person or object or vice versa
glass can be defined as an
amorphous solid
mechanical fit or fracture match
- There is one set of circumstances where glass can be individualized to a particular object.
occurs where a piece of glass breaks into relatively large pieces that have at least one, good, intact edge that can be fitted to the edge of another piece from the same source // large pieces can be put together like a jigsaw puzzle, so the evidence can be individualized to a particular source.
examination of small glass partciles
density and refractive index
density
sink/float method
refractive index is the
ratio of the velocity of light in a vacuum to the velocity as it passes through the medium
becke line immersion method
- When a piece of glass is immersed in a liquid of different refractive index & observed under a microscope with transmitted light, a bright halo in the shape of the glass, will appear to surround the glass. This halo is called the Becke line, caused by the difference in refraction.
- If glass & liquid have the same refractive index, the Becke line will disappear.
