Nitric acid acts like sulphuric and hydrochloric
acids, but it gives a yellow colour to the fibre. You see this clearly
enough in the fur that comes from your furriers after the treatment they
subject it to with nitric acid and nitrate of mercury. There is a
process known called the stripping of wool, and it consists in
destroying the colour of wool and woollen goods already dyed, in order
that they may be re-dyed. Listen, however, to the important precautions
followed: A nitric acid not stronger than from 3 deg. to 4 deg. Twaddell is
used, and care is taken not to prolong the action more than three or
four minutes.
_Action of Alkalis._--Alkalis have a very considerable action on fur and
wool, but the effects vary a good deal according to the kind of alkali
used, the strength and the temperature of the solution, as also, of
course, the length of period of contact. The caustic alkalis, potash and
soda, under all conditions affect wool and fur injuriously. In fact, we
have a method of recovering indigo from indigo-dyed woollen rags, based
on the solubility of the wool in hot caustic soda.
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