Paints and Pigments

Question 1

What is the difference between paints and pigments?

Answer 1

Dyes are soluble (they dissolve) and pigments are insoluble

Dyes are primarily organic based while pigments are equally represented by organic and inorganic pigments

Question 2

What are their purposes?

Answer 2

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.

Question 3

What is the colour index?

Answer 3

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

Question 4

What is Hue?

Answer 4

Colour, dependent upon wavelength

Question 5

What is Chroma?

Answer 5

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

Question 6

What is Value?

Answer 6

Refers to the lightness or darkness of that colour.

Also known as Brightness or luminance

Question 7

What can be affected by value?

Answer 7

This is tint, the addition of black or the addition of black.

Question 8

What is colour determined by?

Answer 8

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

Question 9

What do pigments provide?

Answer 9

They provide both colour and opacity

Question 10

What are the advantages of inorganic pigments?

Answer 10

they are resistant to heat, light, weathering, solvents are are cheaper

Question 11

What are the advantages of organic pigments?

Answer 11

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

Question 12

What is a paint?

Answer 12

Most common surface coating

like fibres, there is a huge amount of complexity and variability in paint

Question 13

Paint is a complex mixture. What is it made of?

Answer 13

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

Question 14

Where are paints found?

Answer 14

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

Question 15

What are the recovery considerations of paint?

Answer 15

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.

Question 16

What is the analytical workflow for paint?

Answer 16

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)

Question 17

What is physical fit?

Answer 17

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

Question 18

What are you interpreting with the paint evidence?

Answer 18

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

Question 19

What is simultaneous contrast?

Answer 19

The perception of colour based on context
colour perception begins with retinal cells known as cone cells

Question 20

What is Microspectrophotometry?

Answer 20

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.

Question 21

How is the microscope useful in trace analysis?

Answer 21

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.