a red colour will be
produced.

LECTURE VII

SHELLAC, WOOD SPIRIT, AND THE STIFFENING AND PROOFING PROCESS

_Shellac._--The resin tribe, of which shellac is a member, comprises
vegetable products of a certain degree of similarity. They are mostly
solid, glassy-looking substances insoluble in water, but soluble in
alcohol and wood spirit. In many cases the alcoholic solutions show an
acid reaction. The resins are partly soluble in alkalis, with formation
of a kind of alkali salts which we may call resin-soaps.

Shellac is obtained from the resinous incrustation produced on the bark
of the twigs and branches of various tropical trees by the puncture of
the female "lac insect" (_Taccardia lacca_). The lac is removed from the
twigs by "beating" in water; the woody matter floats to the surface, and
the resin sinks to the bottom, and when removed forms what is known as
"seed-lac." Formerly, the solution, which contains the colouring matter
dissolved from the crude "stick-lac," was evaporated for recovery of the
so-called "lac-dye," but the latter is no longer used technically. The
seed-lac is bleached by boiling with sodium or potassium carbonate,
alum, or borax, and then, if it is not pale enough, is further bleached
by exposure to sunlight. It is now dried, melted, and mixed with a
certain proportion of rosin or of orpiment (a sulphide of arsenic)
according to the purpose for which it is desired. After further
operations of melting and straining, the lac is melted and spread into
thin sheets to form ordinary shellac, or is melted and dropped on to a
smooth surface to form "button-lac." Ordinary shellac almost invariably
contains some rosin, but good button-lac is free from this substance.
The presence of 5 per cent. of rosin in shellac can be detected by
dissolving in a little alcohol, pouring the solution into water, and
drying the fine impalpable powder which separates. This powder is
extracted with petroleum spirit, and the solution shaken with water
containing a trace of copper acetate. If rosin be present, the petroleum
spirit will be coloured emerald-green.

Borax, soda crystals, and ammonia are all used to dissolve shellac, and
it may be asked: Which of these is least injurious to wool? and why? How
is their action modified by the presence of dilute sulphuric acid in the
wool? I would say that soda crystals and ammonia are alkalis, and if
used strong, are sure to do a certain amount of injury to the fibr