Most of the heathers make a yellow dye, but the one chiefly used is
the Ling, Calluna vulgaris. The tips are gathered just before
flowering. They are boiled in water for about half-an-hour. The wool,
previously mordanted with alum or chrome according to the shade of
yellow wanted, is put into the dye bath with the boiling liquor, which
has been strained. It is then covered up closely and left till the
morning. Or the wool can be boiled in the heather liquor till the
desired colour is obtained.