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.
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