Ammonio-citrate of iron
Gum Arabic, or dextrin
The sensitising solution is spread over well-sized paper with a pad or brush, working in one direction, and then across to even the marks out. The paper is then hung up to dry, and appears of a greenish-yellow colour; and where the light acts on it it turns blue. After exposure it is merely washed in water, when the image becomes bright blue, and the ground, or unexposed portion, should remain quite white. Over-printed proofs may be reduced, after thoroughly washing, by being dipped into a weak solution of ammonia or a 2 per cent, solution of sodium carbonate, well washing, and then dipping into weak hydrochloric or acetic acid and well washing. Under-printed proofs may be intensified by immersion in a solution of ferric chloride, or nitrate or sulphate of iron, 3-5 parts to 1000 parts of water, fill the image appears darker in colour, and then well washing. Corrections, or taking out spots, etc., can be effected by touching the dry prints with a 4 per cent, solution of oxalate of potash, with which also titles may be written; and if red aniline ink be added to the above, or 4 parts of oxalate be dissolved in 100 parts of the red ink, the title will appear red on the blue ground. The blue images thus obtained can be con-verted into ink images, or blackish images, by soaking first in 5 per cent, carbonate of potash solution, washing, and then immersing in a similar strength of tannin solution; a good brownish-black colour is obtained by immersing them direct in a saturated solution of carbonate of soda, mixed with an equal quantity of water, to which has been added as much tannin as it will dissolve. It has also been suggested immersing the prints in weak hydrochloric acid to clear the whites, then in weak ammonia l : 15,000, and finally in a bath of
Alum.................. 10 parts.
Tannin ..........'.. ... 1 part.
Water ............... 130 parts.
exposing the prints to sunshine for ten minutes, and then bathing in dilute ammonia. Cyanotype, or Ferroprussiate prints, may also be converted into silver prints by immersing them in