_Blue Prints_.--The best formula for this process, of many that I have
tried, is that furnished by Prof. C.H. Kain, of Camden, N.J., in which
the quantity of ammonio-citrate of iron is exactly double that of the red
prussiate of potash, and the solutions strong. This gives strong prints
of a bright dark blue, and prints very quickly in clear sunlight.
Dissolve six grains of red prussiate of potash in one drm. of distilled
water; in another drm. of distilled water dissolve twelve grains of
ammonio-citrate of iron. Mix the two solutions in a cup or saucer, and at
once brush over the surface of clean strong paper. Cover the surface
thoroughly, but apply no more than the paper will take up at once; it
should become limp and moist, but not wet. The above quantity of
solution, two drms., will suffice to sensitize ten square feet of paper,
or three sheets of the "regular" size of plain paper, 18×22. As fast as
the sheets are washed over with the solution, hang them up to dry by one
corner. The surplus fluid will collect in a drop at the lower corner, and
can be blotted off.
_Black Prints_.--Wash the paper with a saturated solution of bichromate
of potash, made quite acid with acetic acid. After printing, wash the
prints in running water for twenty to thirty minutes, then float them
face down on a weak solution (five to ten per cent.) of protosulphate of
iron for five minutes, and wash as before. If preferred, the iron
solution may be washed over the prints, or they may be immersed in it,
but floating seems preferable. After the second washing, wash the prints
over with a strong solution of pyrogallic acid, when the print will
develop black, and the ground, if the washings were sufficient, will
remain white. A final washing completes the process.
If a solution of yellow prussiate of potash be used in place of the pyro
solution, a blue print is obtained. Bichromate prints can be made on
albumenized paper by floating it on the solution, and by using a
saturated solution of protosulphate of iron and a saturated solution of
gallic acid. Very fine prints can be so produced nearly equal to silver
prints, and at somewhat less cost, but with a little or no saving of time
or labor.
_Chief Proof Solution_.--If old oxalate developer be exposed in a shallow
vessel in a warm place, a deposit of light green crystals will be formed,
composed of an impure oxalate of iron. If these crystals be dissolved in
water, and paper washed with a strong solution, when dry it may be
exposed in the printing-frame, giving full time. The image is very faint,
but on washing in or floating on a moderately strong solution of red
prussiate of potash for a minute or less, a blue positive is produced,
which is washed in water as usual to fix it. The unused developer
produces the best crystals for the purpose, and the pure ammonio-oxalate
is vastly better than either.
All of the above operations, except the printing, should be carried on in
the dark room, or by lamp or gas light only. The solutions and the paper
should also be kept in the dark, and prepared as short a time as possible
before use.