This is made by adding potash to a solution of alum, till its earth
begins to be separated, then boiling the mixture to precipitate the
subsulphate of alumina, which is to be strained upon a filter, and
dissolved in acetic acid of moderate strength with the aid of heat. This
mordant is very rich in alumina, and marks 20° B.
5. Aluminous mordant.
Take 12-1/2 gallons of water.
100 lbs. of alum.
150 lbs. of liquid pyrolignite of lime at 11-1/2° Baumé.
This mordant is made with heat like the first; after cooling, some alum
crystallizes, and it marks only 12-1/2° B.