IN our last volume there appeared two very interesting notes—one by Mr.
G. Watmough Webster, F.C.S., and the other by Mr. A. R. Brown—on the
preparation of a shellac varnish of a peculiar kind, and easily miscible
with water. Though we have been long familiar with the mode of preparing
such a varnish—thanks to the kindness of a friend—we have lately gone
over the whole matter, and have been so interested that we now venture
to recall the matter to the consideration of our readers—this week
mentioning the most convenient mode of preparing the so-called varnish,
and reserving to another time an account of several applications of it
which appear to possess some interest.
The most rapid mode of obtaining the shellac varnish conveniently is
certainly that of Mr. Webster. This gentleman dissolves the resin in
spirit. We may remark that strong spirit gives the most satisfactory
results. The solution when now treated with a very small amount of the
ordinary liquid ammonia becomes immediately miscible with water. Care
must be taken, however, only to add sufficient ammonia to accomplish the
desired end, and this can be easily accomplished by testing a little of
the mixture by addition of water after every fresh dose of alkali. In
this way any unnecessary deepening of the colour is avoided, and a
useful liquid obtained.
We now turn to the second plan—that mentioned by Mr. Brown. It has been
long well known that _fused_ shellac dissolves much more easily in
alcohol than the resin which has not been melted. Mr. Brown adopts the
same plan in obtaining solution in ammonia. The resin is melted under a
layer of water, and, when perfectly fused, strong liquor of ammonia is
slowly poured in, the whole being carefully mixed. Gradually the resin
is taken up by the liquid, and a good, brown mixture obtained. If too
much ammonia be used here the colour is greatly deepened, owing to the
action of the alkali on a peculiar impurity in the resin.
Both the above processes work well; but, when small quantities have to
be prepared, Mr. Webster’s plan yields the best product, as a solution
of shellac in spirit can be obtained having a comparatively slight
colour, whereas the direct use of ammonia deepens the tint. Whichever
plan be used a good result can be arrived at, and a liquid obtained
capable of numerous and most useful applications, whether as a species
of varnish, a cement, or material for, in a sense, waterproofing paper.
* * * * *
CHROMATE of potash is used in dyeing in conjunction with dextrine, &c.,
for discharging the colour from various fabrics; it seems, however, that
a difficulty has been noted in using such a mixture, for it is found
that certain of the goods become discoloured by the chromate, and that
this undesired colour is not easily removed. Will our readers be
“surprised to hear” that the cause of this circumstance appears to have
been only just found out, if we may judge from a quotation in the
_Journal of the Chemical Society_? It has been discovered that it is
exposure to light during treatment which prevents the removal of the
offending material. This circumstance indicates how little attention is
in general paid to the phenomena attending the chemical action of light;
and, notwithstanding the fact that the remarkable property possessed by
the chromates of rendering insoluble gelatine, &c., has been utilised to
an extraordinary extent in the various pigment printing processes,
operative chemists seem to have only now become alive to the fact, and
able to apply the obvious remedy in using yellow glass for the rooms in
which the treatment of goods with the chromated dextrine is carried on.
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