A surface of Chloride of Silver with a bare excess of Nitrate, darkens on exposure, but it does not reach the bronzed stage; the action appearing to stop at a certain point. On placing the print in Hyposulphite of Soda, it becomes very red and pale, and when tinted, looks cold and slaty, without depth or intensity.
c. The sensitiveness and intensity affected by substituting the Oxide of Silver for the Nitrate.—Many operators employ a solution of Oxide of Silver in Ammonia[17] or Nitrate of Ammonia, in preparing Chloride of Silver paper. By doing so, a great increase of sensitiveness, and also of intensity of image, is obtained. This will be understood if we remember that the action of light in producing the print is of a reducing nature. Hence the substitution of Oxide for Nitrate of Silver facilitates the decomposition; just as Ammonio-Nitrate of Silver is more readily reduced by Gallic or Pyrogallic Acid than the simple Nitrate (see p. 31).
[17] The chemistry of Ammonio-Nitrate of Silver is explained in the Vocabulary, Part III.
Ammonio-Nitrate paper has the disadvantage of soon discolouring when kept; but it is very serviceable in printing during the winter months. The proportion of Chloride in the salting Bath may, if desired, be considerably reduced; the intensity of action being greatly exalted by the use of the Oxide of Silver.
d. Employment of organic matters.—Those recommended in this work are—Albumen, Gelatine, and Iceland Moss. Albumen adds much to the sensibility of the paper, and gives very fine surface definition. A less amount of Chloride is required than in the case of plain paper simply salted, the glutinous character of Albuminous liquids causing more of the fluid to be retained upon the surface of the paper, and the animal matter assisting the reduction. By varying the proportion of salt, both feeble and intense Negatives may be printed successfully upon albuminized paper. No process gives better results, either as regards sensitiveness, or in faithfully rendering all the finer details of the Negative, than the process with Albumen.
Iceland Moss, when boiled in water, yields a mucilaginous liquid which is conveniently employed as a vehicle for Chloride of Silver; it increases the sensitiveness of the paper and gives additional power of bronzing, by assisting to reduce the free Nitrate of Silver. Many other organic matters, tending to absorb oxygen, would act in the same way.
Gelatine is used in positive printing; it is analogous to Albumen in composition, and, like it, forms a red compound with Suboxide of Silver. It is serviceable in keeping the print at the surface of the paper, but does not alter the sensibility or the general appearance of the finished picture so greatly as Albumen.
e. Impurities in Nitrate of Silver.—Nitrate of Silver used for Photographic printing should be free from even a trace of Protonitrate of Mercury, since it is known that the precipitation of Chloride of Mercury prevents the darkening of Chloride of Silver by light.
The peculiar condition of Nitrate of Silver spoken of at [page 101], in which it is thought to contain Oxides of Nitrogen, is likely to interfere with Photographic printing. This is probably the explanation of a faulty state of the Nitrate solution, in which it yields red and feeble positives, and does not darken in colour in exciting albuminized paper. The remedy will be, to fuse the Nitrate of Silver at a moderate heat before dissolving it.