The ferrotype and how to make it - The Grian Press

THE COLLODIO>\ 57
tion to give, in this place, an account of the
process of manufacturing Collodion wool or gun-
cotton, which I have discovered in an English
work on photography ; because the possession
of a reliable process is desirable and quite in
harmony with the purposes of this book.
HOW TO PREPARE GUN-COTTON.
Five parts, by measure, of sulphuric acid and
four of nitric acid of specific gravity, not lower
than 1.4, are mixed in an earthen-ware or thin
glass-vessel capable of standing heat ;
small por-
tions of water are added gradually (by half
drachms at a time, supposing two ounces to have
been mixed), testing after each addition by the
immersion of a small portion of cotton. The
addition of water is continued until a piece of
cotton is found to contract and dissolve on im-
mersion ; when this takes place, add half the
quantity of sulphuric acid previously used and
(if the temperature does not exceed 130°, in
which case, it must be allowed to cool to that
point,) immerse as much cotton, well pulled out
as can be easily and perfectly soaked ; it is left
in for ten minutes, taking care the mixture does
not become cold. It is then transferred to cold
water and thoroughly washed. This is a matter of
much importance, and should be performed at first
by changing the water many times, until it ceases
to taste acid, and treating it then with boiling
rain-water, until the color of blue litmus paper