PDF - CES (IISc)

4 i4
ECONOMIC AND TECHNICAL
An aqueous solution of litmus when exactly neutralized by
an acid is
violet coloured ; it becomes red with the smallest trace of free acid, or blue
with free alkali. Litmus paper is prepared by steeping specially prepared
unsized paper in the dye solution. It is as a ready and sensitive indicator
of acidity or alkalinity that litmus is of so much value. According to Zopf 1
it is also used as a blueing agent in washing and as a colouring of wine.
Litmus is chiefly manufactured in Holland. Still another substance somewhat
differently prepared from the same lichens is sold as French purple,
a more brilliant and durable colour than orchil.
Fig. 134. Lecanora tartarea Ach. (S. H., Photo.).
d. OTHER ORCHIL LICHENS. Though species of Roccella rank first in
importance as dye-plants, purple and blue colours are obtained, as indicated
above, from other very different lichens. Lindsay 2 extracted orchil from
about twenty species. Those most in use in northern countries are on the
whole less rich in colouring substances ; they are : Umbilicaria pustulata,
species of Gyrophora, Parmelia and Pertusaria, and above all Lecanora
tartarea (Fig. 134). The last named, one of the hardiest and most abundant
1
Zopf 1907, p. 393.
2
Lindsay 1855.