_By John Bostock, M.D._[2]
When the sulphate of iron and the infusion of galls are added together,
for the purpose of forming ink, we may presume that the metallic salt
or oxide enters into combination with at least four proximate vegetable
principles--gallic acid, tan, mucilage, and extractive matter--all of
which appear to enter into the composition of the soluble parts of the
gall-nut. It has been generally supposed, that two of these, gallic
acid and the tan, are more especially necessary to the constitution of
ink; and hence it is considered, by our best systematic writers, to be
essentially a tanno-gallate of iron. It has been also supposed that
the peroxide of iron alone possesses the property of forming the black
compound which constitutes ink, and that the substance of ink is rather
mechanically suspended in the fluid than dissolved in it.
Ink, as it is usually prepared, is disposed to undergo certain changes,
which considerably impair its value. Of these the three following are
the most important: its tendency to moulding, the liability of the black
matter to separate from the fluid, the ink then becoming what is termed
ropy, and its loss of colour, the black first changing to brown, and, at
length, almost entirely disappearing.
Besides these, there are objects of minor importance to be attended to
in the formation of ink. Its consistence should be such as to enable
it to flow easily from the pen, without, on the one hand, its being so
liquid as to blur the paper, or, on the other, so adhesive as to clog
the pen, and to be long in drying. The shade of colour is also not to be
disregarded: a black, approaching to blue, is more agreeable to the eye
than a browner ink; and a degree of lustre, or glossiness, if compatible
with the due consistence of the fluid, tends to render the characters
more legible and beautiful. With respect to the chemical constitution
of ink, I may remark, that although, as usually prepared, it is a
combination of the metallic salt or oxide, with all the four vegetable
principles mentioned above; yet I am inclined to believe that the last
three of them, so far from being essential, are the principal cause
of the difficulty which we meet with in the formation of a perfect
and durable ink. I endeavoured to prove this point by a series of
experiments, of which the following is a brief abstract:--Having
prepared a cold infusion of galls, I allowed a portion of it to remain
exposed to the atmosphere, in a shallow capsule, until it was covered
with a thick stratum of mould; the mould was removed by filtration,
and the proper proportion of sulphate of iron being added to the clear
fluid, a compound was formed of a deep black colour, which showed no
farther tendency to mould, and which remained for a long time without
experiencing any alteration.
Another portion of the same infusion of galls had solution of isinglass
added to it until it no longer produced a precipitate; by employing the
sulphate of iron, a black compound was produced, which, although paler
than that formed from the entire fluid, appeared to be a perfect and
durable ink. Lastly, a portion of the infusion of galls was kept for
some time at the boiling temperature, by means of which a part of its
contents became insoluble; this was removed by filtration, when, by the
addition of the sulphate of iron, a very perfect and durable ink was
produced. In the above three processes I conceive that a considerable
part of the mucilage, the tan, and the extract, were respectively
removed from the infusion, while the greater part of the gallic acid
would be left in solution.
The three causes of deterioration in ink, the moulding, the precipitation
of the black matter, and the loss of colour, as they are distinct
operations, so we may presume that they depend on the operation of
different proximate principles. It is probable that the moulding more
particularly depends on the mucilage; and the precipitation on the
extract, from the property which extractive matter possesses of forming
insoluble compounds with metallic oxides. As to the operation of the tan,
from its affinity for metallic salts, we may conjecture, that, in the
first instance, it forms a triple compound with the gallic acid and
the iron; and that, in consequence of the decomposition of the tan,
this compound is afterwards destroyed. Owing to the difficulty, if not
impossibility, of entirely depriving the infusion of galls of any one
of its ingredients, without, in some degree, affecting the others, I was
not able to obtain any results which can be regarded as decisive; but
the general result of my experiments favours the above opinion, and
leads me to conclude, that, in proportion as ink consists merely of the
gallate of iron, it is less liable to decomposition, or to experience
any kind of change.
The experiments to which I have alluded above, consisted in forming a
standard infusion by macerating the powder of galls in five times its
weight in water, and comparing this with other infusions, which had
either been suffered to mould, from which the tan had been extracted
by gelatine, or which had been kept for some time at the boiling
temperature; and by adding to each of these respectively, both the
recent solution of the sulphate of iron, and a solution of it, which
had been exposed for some time to the atmosphere. The nature of the
black compound produced was examined by putting portions of it into
cylindrical jars, and observing the changes which they experienced
with respect either to the formation of mould, the deposition of their
contents, or any change of colour. The fluids were also compared by
dropping portions of them upon white tissue paper, in which way both
their colour and their consistence might be minutely ascertained. A
third method was, to add together the respective infusions, and the
solutions of the sulphate of iron, in a very diluted state, by which I
was enabled to form a more correct comparison of the quantity, and of
the state of the colouring matter, and of the degree of its solubility.
The practical conclusions that I think myself warranted in drawing
from these experiments, are as follow:--In order to procure an ink
which may be little disposed either to mould or to deposit its contents,
and which, at the same time, may possess a deep black colour, not liable
to fade, the galls should be macerated for some hours in hot water, and
the fluid be filtered; it should then be exposed for about fourteen days
to a warm atmosphere, when any mould which may have been produced must
be removed. A solution of sulphate of iron is to be employed, which
has also been exposed for some time to the atmosphere, and which,
consequently, contains a certain quantity of the red oxide of iron
diffused through it. I should recommend the infusion of galls to be
made of considerably greater strength than is generally directed; and
I believe that an ink, formed in this manner, will not necessarily
require the addition of any mucilaginous substance to render it of
a proper consistence.
I have only further to add, that one of the best substances for diluting
ink, if it be, in the first instance, too thick for use, or afterwards
become so by evaporation, is a strong decoction of coffee, which appears
in no respect to promote the decomposition of the ink, while it improves
its colour, and gives it an additional lustre.
[2] Chairman of the Committee of Chemistry, in the Society for
the encouragement of Arts, Manufactures, and Commerce. This
valuable article is extracted from the 47th Vol. of its
Transactions.
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