As the art of man can unmake whatever his ingenuity can make, we have
no right to expect an indelible ink; however, an approximation to it
may be made as follows: make a saturated solution of indigo and madder
in boiling water, in such proportions as to give a purple tint; add
to it from one-sixth to one-eighth of its weight of sulphuric acid,
according to the thickness and strength of the paper to be used. Write
with this ink, and expose the paper to a gradual heat from the fire,
when the characters will be completely black, the letters being burnt
in and charred by the sulphuric acid. If the acid has not been used in
sufficient quantity to destroy the texture of the paper, and reduce it
to the state of tinder, the colour may be discharged by washing it with
a strong solution of oxalic acid in water. When the full proportion
of acid has been employed, crumple and rub the paper, and the charred
letters will fall out; then by placing a black ground behind the
letters, they may be preserved, and thus a species of indelible writing
may be procured, the letters being, as it were, stamped out of the
paper.