Page 30 - Curriculum Visions Dynamic Book

Dyeing natural fibres
Dyeing is different for natural and for artificial fibres. Dyes are held on the surface of natural fibres. A spun fibre may be made of hundreds of strands, but within these strands can be tens of millions of tiny spaces called pores. They dramatically increase the surface area onto which a dye can hold. As a result of these pores, cotton and wool have real surface areas of 45,000 square metres per kilogramme.
When a colour is applied to this kind of fibre, it is necessary to build up a layer of dye molecules up to ten thousand molecules thick. The thickness of dyes is trapped and stored
in the pores rather than on the fibre surfaces. That is the reason the colour is not easily rubbed off during use.
Modern dyes – called reactive dyes – are designed so that the dye molecules also fix themselves into the pores by bonding with molecules on the surface of the pores. Older dyes simply rely on being trapped in the pores.
Most dyeing of natural fibres can be done simply by dipping the fibre in the dye solution. This is called direct dyeing. However, many fibres do not absorb dyes well, and some intermediate chemical, called a mordant, is needed to fasten the dyes to the fibre.
A common mordant is alum (potassium aluminium sulphate). The fibres are first
dipped in the mordant, and the potassium and aluminium in the mordant fix themselves onto the fibre. When the dye is applied in a separate dipping, the dyes are attracted to the potassium and aluminium on the fabric surface. This makes a soluble dye insoluble, so that the dye will not be removed when the fabric is washed.
(Right) Naturally dyed fibres hanging out to dry. 30
See Vol. 6: Dyes, paints, and adhesives to find out more about dyeing.