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  • To appreciate that particle size can, alone, affect the tinting strength (depth) of a colour.
  • To break down (by grinding) particle size to observe the changing tint.
  • To know that this test is used in industry for quality control of the product.

Curriculum links
English National Curriculum: Science 3: 3a

Gateway story

When a pigment is mixed with a white liquid, such as emulsion paint, the 'tinting strength' can be seen. (Children may use the word 'shade'.) A pigment manufacturer produces 4 or 5 grades of particles from coarse to fine particles. These are sorted and separated by very fine mesh filters.
Gateway elements

This gateway consists of:

  • Examples of coloured pigments
  • Two pots of blue pigments
  • Microscopic images of the two blue pigments
  • Chameleon mixing blue and white paint.

Two pots of blue pigments and the microscopic images: These pigments are of different grades, produced by the same company. The three differences between the pigments are:

- pigment grade on the pot (5 and 10)
- colour of blue in the pot
- size of the pigment particles (as seen in the microscopic images).

Approximate time required: 30 - 40 minutes

Resources needed

Per group or for 1 demonstration:

Part 1
2 blue pigment samples of different particle size, in transparent containers (see List of Suppliers)
100ml (approx.) White emulsion paint
Samples of three different particle size of pigments (blue) in transparent containers (from Supplier)
Mixing palette

Part 2
Grinding pigments sheet
Red pigment (see List of Suppliers)
100 ml (approx.) white emulsion paint
3 small jars &lids (approx. 120 ml, wide necked and squat)
210 g glass beads (see List of Suppliers)
4 paint brushes
Stop clock or stop watch
Weighing scales (sensitive enough to weigh 50 g)
Brush cleaner, to remove emulsion paint from brush.

Suggested organisation

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.

Background information

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 problem.
The manufacturer produces 4 or 5 grades of particles from coarse particles, which are 5 or 6 microns in size, to fine particles of only 1 micron.

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.

Companies use sieves with very fine mesh sizes, maybe 45 microns (0.0045 cm), to sieve their pigments during quality control as a means of removing impurities and gritty bits (which would show up as specks in the finished coloured product). The majority of a high quality pigment will pass through a sieve with this size of holes. Ideally, all of the pigment would pass through the sieve.

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.

Extensions / links

Children carry out the Grinding Chalk activity in which they investigate a range of grinding methods.