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SCIENCE

QUALITY TESTING OF DYES

Objectives
  • To test and compare the fading properties (washing and in sunlight) of natural and synthetic dyes.
  • To appreciate the quality testing of dyes (and pigments) carried out in industry.

Curriculum links

English National Curriculum
Science Sc1: 1b, 2c, 2d, 2f, 2g, 2h & Sc3: 1a

Gateway story

Testing the quality of dyes and pigments is a crucial element of their manufacture. Industrial sites have laboratories and scientists dedicated to ensuring that the quality and precision of a colour has been attained. For example, a dye intended for use on a fabric will be tested to ensure minimum colour loss due to sunlight, washing and perspiration.

All the images on this gateway show aspects of the quality testing performed by the colour industry.

Gateway elements

This gateway consists of:

  • Matching cabinet
  • Colour standard
  • Results of testing
  • Dyed multifibre
  • Animation
  • Washing line.

Matching cabinet: This grey cabinet has four different kinds of light bulb, each representing a different quality of light. These are D65 (artificial daylight), A (tungsten), TL84 (fluorescent light), and UV (ultra violet light).

If a scientist is asked to check the colour of a particular batch of dye that has been made, he/she will dye a small sample of fabric or yarn and place it beside a 'standard' in the matching cabinet. The scientist can then find out whether a good match is achieved under all four lighting conditions. If the colours do not match, the scientist will advise the makers on how to improve the batch before testing it again and selling it.

Colour standard: The colour industry produces precise colour swatches and yarns that represent a specific range of colours. These internationally accepted standards are then used in matching cabinets to ensure precise colour matching.

For any further information on either the matching cabinet or the standards, please contact the Colour Museum.

Results of testing: This page of results was kindly donated by scientists from a laboratory based on a dye-making site in Bradford, West Yorkshire. The method of recording is strikingly similar to that employed in the primary classroom! By scrolling down the sheet, the results of a wide range of tests can be seen. The dyed fabric is mounted next to 'multifibre' that has gone through the relevant tests simultaneously. Any changes to the fabrix is thus recorded.

Animation: Here, the chameleon demonstrates the colour change that can occur (albeit, rather exaggerated) when a dyed fabric is viewed in tungsten and then daylight, if a dye has not been matched in a matching cabinet. Note: The animation can be paused to emphasise discussion points.

Gateway discussion

Whilst looking at the gateway, ask the children some of the following questions:

  • What has happened to the chameleon's t-shirt?
  • Why is the chameleon unhappy with the sun?
  • What might case the t-shirt to change colour?
  • What is a matching cabinet used for?
  • What kind of quality tests are carried out on dyed fabric?
  • How are results recorded?

 

Approximate time required: 1 - 2 hours (plus 5 mins observation per day for 2 weeks)

Resources needed

Per group of 5 children:

  • Fading in sunlight activity sheet
  • Results of fading in sunlight Group Record Sheet
  • Fading with washing activity sheet
  • 4 squares of fabric, 2 naturally and 2 synthetically dyed, 10 cm x 10 cm
  • Undyed Multi-fibre strip - enough for 12 strips, each about 2 cm x 10 cm (from Supplier)
  • Washing-up liquid
  • Source of warm water
  • Bowl / sink
  • Rectangular template (2 cm x 10 cm)
  • Photographs showing washing and sunlight tests in industry
Organisation

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:

  • Have both the fabric samples faded?
  • Have all the fabric types faded in the same way? What are the differences?
  • Is one dye better than the other? What differences have you observed?
  • Has one colour faded more than another?
  • Which dye would you choose to dye a white cotton T-shirt (or acrylic jumper, etc.)?
  • Before synthetic dyes were invented, what colours do you think clothes, blankets and curtains were? How can you find out whether you are right?
  • Do you think dye-making factories carry out tests like this? Why/why not?

Show children pictures of industrial testing equipment:

  • What do you think these pictures show?
  • How are the tests the same as/different from the tests we have done in school?
Plenary

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:

  • Have both the fabric samples faded?
  • Have all the fabric types faded in the same way? What are the differences?
  • Is one dye better than the other? What differences have you observed?
  • Has one colour faded more than another?
  • Which dye would you choose to dye a white cotton T-shirt (or acrylic jumper, etc.)?
  • Before synthetic dyes were invented, what colours do you think clothes, blankets and curtains were? How can you find out whether you are right?
  • Do you think dye-making factories carry out tests like this? Why/why not?

    Show children pictures of the industrial testing equipment

  • What do you think these pictures show?
  • How are the tests the same as/different from the tests we have done in school?
Background information for adults

Synthetic dyes have many advantages over natural dyes:

  • a wider range of synthetic dye shades is possible.
  • the colours tend to be brighter.
  • they display better colour fastness.
  • synthetic dyes are easier to apply.
  • it is cheaper to produce batches of uniform quality synthetic 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.

Extensions / links

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.

Art

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.