TINTS OF COLOUR
Approximate time required: 30 - 40 minutes
Per group or for 1 demonstration:
This can be a teacher demonstration, or groups of children can try this activity as part of a circus of related activities, where the groups move on from one to the next, in rotation.
Carrying out the activity
Parts 1 or 2, or both, can be carried out to demonstrate the relationship between particle size and the tinting strength of a colour.
Part 1 - observing tinting strength of colour
The white paint is mixed with each blue pigment at a ratio of 1:4. Therefore, in a mixing palette, mix about 7 pipettefuls of white paint (about 20 ml) with a teaspoon (about 5 ml) of each pigment in turn. Two different tints of the blue colour should be clearly seen. The children can keep a permanent record of this by painting a stripe of each tint on paper and recording which pigment was used to produce each sample.
Note: Children should paint the strips of colour so that they touch. If a white space is left between the colours, it is harder to see the difference in tint.
Tell the children that pigment 1 has the largest particles and pigment 2 the smallest. Industry sells different particle sizes of pigments to different markets.
Part 2 - grinding the pigment
The children (or teacher, if using this as a demonstration) follow the instructions provided on the activity sheet. If the children are carrying out the activity, dilute the emulsion paint 50:50 with water before the lesson. Otherwise, this can be done during the teacher demonstration.
The glass beads act to grind the red pigment into smaller and smaller particles. Therefore, each strip of colour should have a slightly different tinting strength.
When a pigment is mixed with a white liquid, such as emulsion paint, the 'tinting strength' can be observed.
Industry tests the quality of a pigment in several
ways. One of these is to mix the pigment with a liquid (sometimes linseed
oil rather than white emulsion paint) and match it to a sample of the
required colour, to ensure the particle size is as required. If it does
not match, further processes are carried out on the pigment to rectify
The coarse particles are cheaper and so get used for less demanding uses, such as school paints; the finer particles are used for industrial purposes, like making blue plastics.
Although finer particles are more expensive, they produce a high tinting strength, so less weight is needed to achieve the required colour.
Another test for quality involves mixing the particles with white paint ( called testing it 'in reduction') to observe and measure the resulting tint. (When mixed with clear liquid this is known as full shade.)
The particle size affects the tint of the colour, because the wavelength of light reflected from particles that are this small varies. The wavelength matches the actual particle size.
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