Sulphuric, Acetic Acids, etc.—Acids of all kinds appear to exert an injurious influence upon Positive prints, and especially so upon the half-tones of the image, the effect varying with the strength of the acid and the degree of dilution with water. Even a vegetable acid like Acetic gradually darkens the colour and destroys partially or entirely the faint outlines of the picture.
Bichloride of Mercury.—The most important particulars relating to the action of this test upon Photographs are well known. The image is ultimately converted into a white powder, and hence, in the case of a Positive print, it becomes invisible; immersion in Ammonia or Hyposulphite of Soda however restores it in a form often resembling in tint the original impression. A point worthy of note is the protective effect of a deposit of Gold, which is very marked, the proof, after toning, resisting the action of the Bichloride for comparatively a long time.
Ammonia.—The effect of Ammonia upon a print is rather to redden the image than to destroy it; the half-tones become pale and faint, but they do not disappear. Toning with Gold enables the proof to resist the action of the strongest solution of Ammonia, and hence Ammonia may safely be employed as a fixing agent after the use of the Sel d'or Bath.
Hyposulphite of Soda.—A concentrated solution of Hyposulphite of Soda exercises a gradual solvent action upon the image of Photographic Prints, at the same time tending to communicate Sulphur and to darken the colour of the impression. A faint yellow outline of Sulphuret of Silver usually remains after the solution of the image is completed.
Developed prints of all kinds, but in particular the Talbotype proofs upon Iodide of Silver, are less readily dissolved by Hyposulphite of Soda than those obtained by the direct action of light. There is also a slight difference between plain and Albuminized prints, which is in favour of the former, the albuminized paper always losing somewhat more by immersion in the Hyposulphite Bath than plain Chloride paper sensitized by Nitrate of Silver.
Cyanide of Potassium.—The solvent action of Cyanide of Potassium is most energetic upon Photographs formed on paper. These images, whether developed or not, do not withstand the test so well as the impressions on Collodion. Albuminized proofs are also somewhat more easily affected than prints on simple Chloride paper sensitized with Nitrate or Ammonio-Nitrate of Silver.
Heat, moist and dry.—Long-continued boiling in distilled water has a reddening action upon Positive Prints. The image becomes at length pale and faint, resembling a print treated with Ammonia before toning. A deposit of Gold upon the image lessens, but does not altogether neutralize, the effect of the hot water. If the boiling be long continued, the violet-purple tone often imparted by the Gold invariably gives place to a chocolate-brown, which appears to be the most permanent colour. Prints developed by Gallic Acid upon paper prepared with Serum of Milk or with a Citrate, suffer as much as others obtained by direct action of light. Ammonio-Nitrate prints on highly salted paper, which become nearly black when toned with Gold, retain their original appearance the most perfectly; a slight diminution of brightness being the only observable difference after long boiling in water. Albumen proofs, and prints on English papers, or foreign papers prepared with Serum of Milk, Citrates, Tartrates, or any of those bodies which redden the reduced Salt, are, as a rule, rendered lighter in colour, and pass from purple to brown when boiled in water.
Dry heat has an opposite effect to that of hot water, usually darkening the colour of the image. On exposing a plain paper print simply fixed, and thoroughly freed from Hyposulphite of Soda by washing, to a current of heated air, it changes gradually from red to dark brown, in which state it continues until the temperature rises to the point at which the paper begins to char, when it resumes its original red tone, becoming at the same time faint and indistinct.