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Use of alum as a mordant

Alum, to make a mordant, is dissolved in water, and very frequently, a quantity of tartrate of potass is dissolved with it. Into this solution woolen cloth is put, and kept in it till it has absorbed as much alumine as is necessary. It is then taken out, and for the most part washed and dried. It is now a good deal heavier than it was before, owing to the alum that has combined with it.