Paints and Pigments Flashcards
What is the difference between paints and pigments?
Dyes are soluble (they dissolve) and pigments are insoluble
Dyes are primarily organic based while pigments are equally represented by organic and inorganic pigments
What are their purposes?
Dyes are of greater use in textiles; while pigments are used in paints, inks, plastics, cement, ceramics, glass
Pigments have a much broader application making them more applicable trace evidence analysis.
What is the colour index?
Comprehensive list of all commercially available dyes and pigments globally (regularly updated)
Each colourant is assigned a generic name, which incorporates it’s application, its hue and its CI number
What is Hue?
Colour, dependent upon wavelength
What is Chroma?
This refers to the purity of the colour
how vibrant it is
how close to the true colour it is
Also known as saturation, strength, intensity
What is Value?
Refers to the lightness or darkness of that colour.
Also known as Brightness or luminance
What can be affected by value?
This is tint, the addition of black or the addition of black.
What is colour determined by?
light absorption or reflection - the colour that we see is reflected while the colour we cannot see is absorbed. (peak of the wavelength)
crystal lattice - how atoms are packed within the crystal structure
particle size and shape - can be tweaked/changed
opacity - determining the degree of light scattering
the higher the RI, the higher the opacity
What do pigments provide?
They provide both colour and opacity
What are the advantages of inorganic pigments?
they are resistant to heat, light, weathering, solvents are are cheaper
What are the advantages of organic pigments?
Also known as “lakes”
they offer better colour intensity and brightness, and have a lower refractive index (high translucency and lower opacity)
hence they are used in printing inks
What is a paint?
Most common surface coating
like fibres, there is a huge amount of complexity and variability in paint
Paint is a complex mixture. What is it made of?
Binders - polymers - film former such as acrylic
pigments - TiO2
fillers - increases the thickness of the coating e.g talc
drying agents - speed up polymerisation
Texturisers - smooth surface e.s solvents
emulsifiers - prevent separation, increases shelf life
fungicides, biocides, insecticides
UV stabilisers and corrosion inhibitors
Plasticisers - increases flexibility
Where are paints found?
Automotive and clear coats (hit and run)
Architectural - burglaries, forced entries
Vehicular coatings e.g bike, marine, aircraft
Tool coatings - burglaries
Spray paints
Road paints - RTC, distinctive glass beads
Works of art - forgeries
Nail varnish - associate trace evidence
What are the recovery considerations of paint?
Do not lift with J-Lar or acetate. This is because it is difficult to remove the delicate fragments from the tapes
Also, adhesive tape is made of polymer and this will interfere with the subsequent analysis.
the best option is to pick it up with a pair of forceps and place it into a plastic vial and then place it in an evidence bag to maintain the integrity of the sample.
take control samples close to the area of impact to facilitate side by side comparison.
Do not refrigerate or freeze, this introduces moisture, which may chemically alter the paint.
What is the analytical workflow for paint?
Gross examination, recovery and collection
Preliminary examination
Physical fit examination
All microscopic techniques
Microspectrophotometry - colour determination
Infrared spectroscopy - organic binders
Raman spectroscopy - inroganic pigments
SEM-EDX, XRF - elemental composition of layers
XRD - crystal structure, polymorphs e.gWh titanium dioxide
(Pyrolysis - GC/MS and microchemical tests)
What is physical fit?
This is the most important step. If you can show that two paint samples are complementary like pieces of a jigsaw, then you don’t need to proceed with the analytical workflow
What are you interpreting with the paint evidence?
These characteristics add to the discriminating factor of the paint
The number of fragments and their physical dimensions
The number, sequence, thickness and colour of layers
Application method - brush strokes?
Surface topography - hammered finish, wrinkle finish
Size and distribution of pigments
Defects, weathering, delamination, corrosion
What is simultaneous contrast?
The perception of colour based on context
colour perception begins with retinal cells known as cone cells
What is Microspectrophotometry?
this is the technique of combining a microscope with a spectrophotometer
It gives the precise characteristic colour described as a distribution of wavelengths, therefore is independent of colour perception.
How is the microscope useful in trace analysis?
It locates minute traces or shows how light interacts with the material under analysis.
Allows a capture of both a magnified visual and spectroscopic pattern at the same time, increasing the number of identifying characteristics of any given material.
the instrument combines UV-VIS NIR digital imaging with colorimetry.
