" 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 fibre of
wool, and more if used hot than cold. Of the two, the ammonia will have
the least effect, especially if dilute, but borax is better than either.
The influence of a little sulphuric acid in the wool would be in the
direction of neutralising some of the ammonia or soda, and shellac, if
dissolved in the alkalis, would be to some extent precipitated on the
fibre, unless the alkali, soda or ammonia, were present in sufficient
excess to neutralise that sulphuric acid and to leave a sufficient
balance to keep the shellac in solution.
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