lecture 5 - chemistry of colour

Question 1

what colours are the three colour cones in the human eye?

Answer 1

blue, green and red

Question 2

what happens to the three cones in the human eye when seeing white?

Answer 2

all cones are stimulated equally

Question 3

what happens to the three cones in the human eye when observing orange?

Answer 3

stimulation of only two cones

Question 4

where can you find the 3 colour cones in the human eye

Answer 4

the retina at the back of an eye

Question 5

what does the CIE system base colour on

Answer 5

the coloured cones in the human eye

Question 6

what is the drawback of the CIE system?

Answer 6

colour can only be described with this system, not quantified - due to the fact not all colours are represented equally in the diagram

Question 7

what do we use the Munsell system for in forensics?

Answer 7

used for the quantification of the colour of soil - when its wet/dry

Question 8

what is hue?

Answer 8

the actual colour

Question 9

what is value (in the Munsell system)

Answer 9

how light or dark a colour is - vertical axis - ranges from black (0) through to white (10)

Question 10

what is chroma (in the Munsell system)

Answer 10

how strong or weak your colour is - horizontal axis - varies continuously and ranges from neutral at the centre to maximum saturation at the outer edge

Question 11

name the 5 primary colours used in the Munsell system

Answer 11

red, yellow, green, blue, and purple

Question 12

when labelling using the Munsell system what’s the order we use?

Answer 12

hue, value, chroma

Question 13

what are dyes soluble in

Answer 13

the solvent they are contained in - it has an affinity for the substrate it is deposited on

Question 14

what are pigments

Answer 14

particles suspended in a solvent - with no affinity for the substrate

Question 15

what is opacity

Answer 15

the ability of your paint to stop the transmission of light

Question 16

what do pigments produce that causes opacity

Answer 16

scattering of light - the greater the scattering of the light the greater the opacity of a paint

Question 17

what is optimal scattering

Answer 17

ensuring the diameter of the pigments particles are half that of the wavelength of incident light that is directed at the pigment particles e.g. white pigment particles need to be around 200-400nm as that’s half the wavelength of incident light for white

Question 18

what is the definition of paint

Answer 18

a liquid that oxidises in the air or a coloured powder in a glue-like medium

Question 19

what are the 5 things that make up paint and why do we use them?

Answer 19

1. water
2. pigment - for colour and opacity
3. binder - to help bind the pigment to the wall
4. surfactant & antifoam - to reduce bubble
5. thickener - thickens the paint once its on the wall

Question 20

to maximise scattering and reflection what 3 things do you need to ensure?

Answer 20

1. pigment strength is high
2. the pigment RI is higher than the binder RI
3. the size of the pigments particles are half the wavelength of incident light

Question 21

give an example of an inorganic pigment

Answer 21

Titanium dioxide (TiO2)

Question 22

name the 3 crystal structure Titanium dioxide exists in

Answer 22

Rutile
Anatase
Brooktite

Question 23

which of the three crystal structures of Titanium dioxide is not used in the production of white paint

Answer 23

Brooktite

Question 24

which of the three crystal structures of Titanium dioxide produces the best opacity in white paint

Answer 24

Rutile as its RI is the greatest

Question 25

why is paint that contains Titanium dioxide photocatalytic (self cleaning)

Answer 25

sunlight will react with the TiO2 in the white paint and oxidise any dirt found in the paint

Question 26

what are luminescent pigments used in and how do they work

Answer 26

fluorescent inks and paints e.g. found in emergency exit signs. They work by absorbing light of a specific wavelength and emitting light of a longer wavelength

Question 27

what are pearlescent pigments used in and how do they work?

Answer 27

pearlescent paint used on cars. They work due to the differences in the RI of the pigment (high RI) and the mica (low RI)

Question 28

what are thermochromic pigments and how do they work?

Answer 28

pigments sensitive to temperature - they change colour when heated up or cooled down

Question 29

name 4 different dyes used in forensics (only one acid)

Answer 29

1. solvent black 2. gentian violet 3. acid yellow/black/violet 4. indigo

Question 30

name an example of an azo dye

Answer 30

solvent black

Question 31

what is solvent black used for in forensics?

Answer 31

the detection of oil based fingerprints

Question 32

Name a basic dye used in forensics

Answer 32

gentian violet

Question 33

what is gentian violet dye used for in forensics

Answer 33

the detection of oil based fingerprints on the adhesive side of tape

Question 34

Name the three acidic dyes used in forensics

Answer 34

acid black/ yellow/ violet

Question 35

what is acid black (and yellow & violet) used for in forensics

Answer 35

the detection of fingerprints in blood as they bind to the haem group of blood

Question 36

Name a carbonyl dye

Answer 36

indigo dye

Question 37

give a use of indigo dye

Answer 37

blue fibres - mostly found in jeans

Question 38

name the three less common dyes and their uses

Answer 38

1. Methyl violet - the main dye found in ballpoint pen ink 2. Basic red 40 - used to further enhance white superglue fumed fingerprints 3. Copper phthalocyanines - used in pigment paint and the detection of fingerprints

Question 39

name the three types of pigments

Answer 39

inorganic (TiO2), organometallic (contain a transition metal complex), and natural (lapis lazuli)

Question 40

name the 3 general components found in ink

Answer 40

1. Colourants - such as dyes and pigments 2. Solvents - the vehicle for the colourant 3. Additives

Question 41

what does the type of solvent used in inks depend on

Answer 41

the age of the pen - older inks used oil based solvents and newer inks use glycol based solvents

Question 42

name 3 additives to inks and what they are used for

Answer 42

1. fatty acids - lubricate the ball in the pen 2. natural resins/ synthetic polymers - help to dissolve your inks within the solvent to make the ink go further 3. organic additives - prevent corroding 4. glycols and Formamide - ensure a pen will not dry out

Question 43

name the three ways we analyse dyes in inks and what they do

Answer 43

1. UV-Vis spectroscopy - helps us differentiate between inks 2. IR spectroscopy - determines the composition of inks 3. Raman spectroscopy - helps us differentiate between inks

Question 44

name the 4 ways we analyse dyes in fibre in forensics

Answer 44

1. ATR-FTIR - non-destructive 2. HPLC and TLC 3. UV-VIS attached to a microscope - non-destructive 4. UV-Visible spectroscopy and Raman spectroscopy - UV is only tentative whereas Raman is confirmatory

Question 45

name the three ways we analyse pigments in paint forensically

Answer 45

1. Fourier transform infrared (FTIR) 2. Raman and FTIR spectroscopy 3. X-Ray fluorescence