Why do some dyes ‘run’ when washed?

You may be familiar with this. You bought a very attractive looking dress, but it lost some of its colour when you washed it. Ever wondered what caused it to lose colour? Let’s have a look.

Dye cloth at school

This is an experiment you can do in your school lab. Take a small square of cotton cloth, and dip it in a dye solution. You can use a very dilute solution of crystal violet; it is a common dye used in labs for preparing biological samples. When the cloth has soaked in the colour, squeeze out the excess dye (remember to use rubber or plastic gloves so your hand is not stained) and hang the cloth to dry.

When it has dried, wash the cloth with soap and water. Alternatively you can use very dilute alcohol. Did the cloth lose colour? (That is called ‘running’ of the dye.) Is there something you could have done to stop the ‘running’?

Dye the cloth again

Repeat the experiment again, with the same piece of cloth. But now, after you have squeezed the dye out, dip the cloth for 2-3 minutes in Gram’s iodine. (It is commonly found in labs; else you can make it by dissolving 2 ml of iodine in 98 ml of 70% alcohol.) Rinse the cloth quickly under tap water and let it dry. Try to wash the cloth after it has dried. Did the dye ‘run’ now?

A beautiful kind of art called ‘indienne’ is made by printing patterns on cloth or paper using dyes. These dyes are fixed permanently onto the paper or cloth by using mordants. The picture you see is an indienne from France, probably made in the 18th century.

The world of mordants

The iodine served as a ‘mordant’. A mordant is a chemical that acts as glue between the dye and the cloth. Apart from iodine, many other kinds of chemicals are used. Tannic acid, alum and salts of aluminium, chromium and copper are used more commonly. An unpleasant-smelling but very effective mordant used in dyeing wool is dog urine!

Depending on the cloth and the dye used, the mordant may be added to cloth along with the dye, or after the dyeing has been completed (like you used iodine). Sometimes you may have to dip the cloth in mordant first, and then apply the dye. You can learn more about dyes and dyeing here.