Objectives
· 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
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)
Pipette
Teaspoon
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
Teaspoon
Funnel
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 and 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
Science
Children carry out the Grinding Chalk activity in which they investigate a range of grinding methods.
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Grinding pigments
You need:
· 3 small jars with lids
· white paint
· water
· 2 funnels
· glass beads
· weighing scales
· teaspoon
· dowelling rod
· timer
· 4 paintbrushes.
What to do:
1. Weigh 70g of glass beads into each jar.
2. Pour the beads through a funnel into each jar.
3. Pour just enough white paint into the bottles to cover the beads.
4. Add a level teaspoon of the pigment through a clean funnel in each jar.
5. Using a dowelling rod, mix the pigment into the white paint for about 30 seconds.
6. Dip a dry paintbrush into one of the jars and smoothly and evenly paint a strip on your paper.
7. Put the lid on the bottle and shake it as hard as you can (like a cocktail shaker!) for 10 seconds. Let everyone in your group do this.
8. Dip a clean dry brush again and smoothly and evenly paint a strip next to the first one on your paper, so the strips are just touching (without them mixing together).
9. Repeat the shaking and painting for each jar, but everyone in the group counts to 15 seconds when shaking the second jar, and to 20 seconds when shaking the third jar.
You should now have four strips of paint on your paper.