What it shows
Many natural dyes wash out of cotton unless a mordant is used. A mordant such as alum forms a link between the dye and the fibre, making the colour stronger and more lasting.
The science
Cotton is cellulose with many OH groups. A dye sticks to the fibre only weakly, so soap and sunlight can remove it. Alum is a source of aluminium ions, Al3+, which bind to the fibre and also to the dye molecules, forming a larger and more stable complex. This is why mordanted cloth keeps its colour longer. Turmeric turns reddish brown in soap, which is slightly basic, so it also acts as an indicator.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 4 pieces of white cotton cloth, 10 cm by 10 cm each
- Turmeric powder, 2 teaspoons, and dry onion skins, a handful
- Alum (phitkari), 3 g
- A steel pan, a spoon and a stove (adult)
- Water
- Mild soap or detergent
- A clothes line and sunny place
- Labels and a notebook
Safety
For this project: An adult must supervise all heating. Use tongs, because the cloth and water are hot. Do not use cooking pans afterwards for food. Do not taste the dye or alum.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Divide the four cloth squares into two for turmeric and two for onion skin. Label them with a pencil.
Dissolve 3 g of alum in 200 ml of hot water and soak one turmeric piece and one onion piece for 30 minutes. This is the mordanted set. The other two are the plain set.
Boil a teaspoon of turmeric in 300 ml of water for 15 minutes and strain. In another pan boil the onion skins in 300 ml of water for 20 minutes and strain.
Put the plain and the mordanted turmeric pieces in the turmeric dye, and the two onion pieces in the onion dye. Simmer for 20 minutes with occasional stirring.
Let the cloth cool in the dye for an hour, then rinse until the water runs clear and hang to dry in the shade.
Cut each dyed piece in half. Keep one half as a reference and put the other half to wash for five minutes in soap water.
Hang the washed halves and the references together on a line in strong sun for two days. A common mistake is to mix up the labels, so write them in pencil on a corner.
Compare the colour depth of mordanted and plain pieces after washing and sunlight, and rank them by eye or with a colour card.
What you should see
The mordanted pieces should be deeper in colour and should fade less after soap and sunlight than the plain pieces. Turmeric should look more reddish after soap.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a mordant?
A substance that fixes a dye to the fibre.
2Why does turmeric colour change in soap?
Soap is slightly basic, and turmeric changes colour in a base, like an indicator.
3Why keep a reference half?
It shows what the colour was before washing so the change can be judged.
4Why is alum considered a safer mordant?
It is much less toxic than heavy metal mordants such as chrome and copper.
Take it further
Try tea, marigold petals and beetroot and record which dyes are fast to washing and to light.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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