Alum was once a very valuable commodity because of its many uses.
Its main use was as mordant, a fixative when dying fabrics. Derived from the French – ‘to bite’, mordants are added during the dyeing process to help the colours stick. Once treated with alum, the fabric is colourfast and ready to use.
From pre-Roman times the woollen industry was particularly important in Britain. So important in fact that, by the end of the 15th century, the wool trade was described as ” … the jewel in the Realm”.
Much of the tax revenue of England was associated with the woollen industry. The value of wool and cloth was higher if it had been dyed. Having a cheap and regular supply of alum was therefore a matter of national importance.
Today, the Lord High Chancellor in the House of Lords sits on the woolsack as a reminder of the former trade.
Leather and paper
Leather has been preserved using alum since Egyptian times. Animal skins need to be preserved to stop them rotting and to make them supple and water-resistant.
Preserving leather using alum is known as tawing and produces a white leather which can then be dyed any colour. The alternative process of preserving leather uses tannins which dye leather various shades of brown.
Tawed leather can be put to many uses but was traditionally used for book-binding and making decorative leather items.
Alum is also used in paper-making. In order to stop the paper absorbing the ink and creating a blotting paper effect, paper is treated with a product known as size. Size coats the paper and causes the ink to dry on the surface remaining clear and legible. One of the earliest types of size was a mixture of alum and water. It was commonly used during the medieval period and many books survive from that time, which were made using alum-treated paper.
Alum has had many other uses including candle-making, as a fire-retardant and for hardening the nibs of quill pens. Today it is still used in a number of products.
Alum is one of the main ingredients in natural deodorants. It stops the growth of bacteria, which stops the smell associated with sweat. It is also used in styptic pencils. These are used by men, when shaving, to stop small cuts from bleeding. The alum helps close the pores and stops bacteria growing in the cuts.
Alum is also an ingredient in baking powder and can be used to help keep pickles crisp when pickling.