QUALITY TESTING OF DYES
English National Curriculum
Approximate time required: 1 - 2 hours (plus 5 mins observation per day for 2 weeks)
Per group of 5 children:
Children work in mixed ability groups of 5 children
Carrying out the activity
Fading in sunlight
Instructions for the test are provided on the activity sheet Fading in sunlight. Each group of children cuts a strip from the synthetically and naturally dyed sample (using the template). The samples are pinned to a piece of paper and hung in a sunny place. A strip is cut from both samples every 2-3 days and stuck onto their record sheets, which should be kept in the dark.
Fabric dyed with a natural dye such as turmeric fades very quickly. The synthetically dyed piece of fabric will show little or no fading, illustrating one good reason for using synthetic dyes.Fading due to washing
The children are provided with limited instructions, as they are encouraged to consider fair conditions for the test. They are also asked to produce their own record sheet. They can use the record sheet from the previous test as a stimulus to help design their own.
The pieces of unwashed fabric they cut out and stick on their record sheet act as the control for the experiment, i.e. nothing is altered on these samples.
Conditions the children may consider keeping constant are:
water temperature; volume of water; quantity of washing-up liquid; number of stirs/rubs to the fabric whilst in the water.
Cutting a small triangle from one corner of the undyed fabric pieces prevents confusion with the dyed fabric if they become a similar colour. After each wash a small strip is cut from the samples to stick onto the activity sheet, Fading with washing. The children then compare the fastness of the natural and synthetic dyes on the different fabric types, and the discussion of the results is left open-ended.
The teacher can prompt discussion with some of the following questions:
Show children pictures of industrial testing equipment:
The children compare the fastness of the natural and synthetic dyes on the different fabric types, and the discussion of the results is left open-ended. The teacher can prompt discussion with some of the following questions:
Synthetic dyes have many advantages over natural dyes:
A simplified scientific explanation of the dyeing process involves the dye and fabric particles and the bonds that form between them. Fibre particles attract dye particles. The fibre and dye particles are attracted to each other by strong forces and the dye sticks to the surface of the fibres. The dye particles form a coloured layer all around the fibres. The fibre-natural dye bonds are often weaker than, and of a different type to the fibre-synthetic dye bonds. The dye-fibre bonds vary with the dyestuff. When using natural dyes, therefore, cotton fabric dyed with turmeric fades more quickly than cotton fabric dyed with red cabbage.
Fading that occurs in sunlight is due to a chemical reaction between ultra violet light in sunlight and the dye in the fabric. After fading, the dye is still present in the fabric in a colourless form.
The fading that occurs when the fabric (dyed in this activity) has been washed is due to the dye being water-soluble. When washed, if dye comes out of the fabric, it is because the force of attraction between the water and dye particles pulls the dye away from the fabric and into the water.
In synthetic dyes, the dye-fibre bonds are either stronger than the water-dye bonds or the dyes are insoluble in water, so the dye remains attached to the fabric. At high temperatures the dye-fibre bonds can be broken, even with synthetic dyes.
Industrial testing of dyes and pigments is crucial in checking the quality of the products. The washing test is very similar, but it is mechanised, so a person does not have to physically agitate the samples. The samples are all the same size and washed under the same conditions.
The sunlight test is done using a machine which provides a very intense artificial sunlight, so that the test can be performed much more quickly. The samples are stuck to card, and face the strong light for several hours before the test is complete. The samples are then compared to accepted standards of fading.
Industry carries out other tests, such as the dye's resistance to perspiration and to friction and abrasion.
The rub test
Children devise a test to find out the natural and synthetic dyes' resistance to rubbing or abrasion.
They can mimic the kind of test used in industry, if they do not have suitable suggestions of their own:
In industry, plain white pieces of cotton are attached to a cork with a rubber band and rubbed against the dyed fabric a specified number of times, with a constant pressure and a constant speed.
The resulting piece of white cotton should NOT have any dye visible on it, to pass industry's quality control test.
The children can experiment by deliberately allowing paper to fade in strong sunlight to produce art works. They could temporarily attach a cutout shape to a piece of backing paper they suspect will fade in strong sunlight. By leaving it exposed on a sunny wall or windowsill for as long as they want, they can then remove the shape to show the preserved stronger colour behind. This idea could then be developed using multiple images or other shapes overlaid later.