This is the most generally used of all the mordants, and has been
known as such from early times in many parts of the world. For most
colours a certain proportion of cream of tartar should be added to the
alum bath as it helps to brighten the ultimate colour. The usual
amount of alum is a quarter of a pound to a pound of wool. As a rule,
less mordant is needed for light colours than for dark. Excess of alum
is apt to make the wool sticky. The usual length of time for boiling
is about an hour. Some dyers give as much as 2-1/2 hours.
Example of mordanting with alum--1/4 lb. of Alum and 1 oz. cream of
tartar for every pound of wool. This is dissolved and when the water
is warm the wool is entered. Raise to boiling point and boil for one
hour. The bath is then taken off the fire and allowed to cool over
night. The wool is then wrung out (not washed) and put away in a linen
bag in a cool place for 4 or 5 days, when it is ready for dyeing,
after being thoroughly washed.