QUALITY TESTING OF DYES
Objectives
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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
Approximate time required: 1 - 2 hours (plus 5 mins observation per day for 2 weeks)
Resources needed
Per group of 5 children:
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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:
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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:
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What do you think these pictures show?
How are the tests the same as/different from the tests we have done in school?
Plenary
Children lay out their results in and then visit each other's displays followed by a whole class
feedback session, making evaluations of their own and others work, making
recommendations for improvements.
Background information for adults
Natural dyes can be difficult to extract.
Synthetic dyes have many advantages over natural dyes:
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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 all 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 with 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.
Names:
Fading in sunlight
1. Use the template to cut a strip from both a naturally dyed and a
synthetically dyed piece of fabric.
2. Stick your piece of unfaded fabric in the correct spaces on your record
sheet.
3. Pin the rest of each square sample to a piece of paper:
4. Stick the paper in a sunny window. Make sure your samples face out of the
window.
5. Use the template to cut a strip from both samples every 2-3 days, and stick
these to the record sheet.
How useful was the test?
Write about what your results tell you:
Which dyes would you recommend, and for which fabric types?
Names:
Fading with washing
1. Use one synthetically and one naturally dyed square of fabric.
2. Use the template to cut a strip from each sample.
3. Make a record sheet to stick the two samples on, and to stick more samples
on after washing. Allow a space for sticking a piece of undyed fabric.
4. Think about how you will wash the fabric. What will you keep the same each
time?
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5. List the things you will use for your washing test:
6. Now wash the samples, adding a small piece of undyed fabric to each wash.
What do you think will happen to the undyed fabric?
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7. After each wash, cut another strip from each sample and stick them onto
your record sheet.
8. Stick the sample that was undyed on the record sheet too.
9. Write about your results on your record sheet (think about things like what
the results tell you about each dye, how washing affects each fabric type,
and what happened to the undyed fabric).
Names:
Results of fading in sunlight
Naturally dyed Fabric
Fabric before
placing in sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight
Synthetically dyed fabric
Fabric before
placing in sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight
Fabric after
days of sunlight