Alum is a complex double salt of aluminium sulphate and ammonia or potassium sulphate.
It is a clear crystal that solidifies easily out of solution and forms large regular crystals.
The large alum crystal shows the type of alum that would grow in the barrels at the alum house
These would then be ground into a powder and put in sacks to take to London to sell.
Alum was mainly used in the dyeing industry, as a mordant to fix dyes to fabric, but it was also used to preserve leather, in paper making, as a fire retardant and for making quill pens.
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