The Darkening of the Sensitive Paper by Light.—The operator should be familiar with the changes of colour which indicate the progress of the reduction of the sensitive layer. Much in this respect depends upon the kind of organic matter used, but there is always a regular sequence of tints; in the case of a paper prepared simply with Chloride of Ammonium and Nitrate of Silver, it is as follows: pale violet, violet-blue, slate-blue, bronze or copper-colour. When the bronzed stage is reached, there is no further change. On immersion in the fixing bath of Hyposulphite, the violet tones due to Subchloride of Silver are destroyed, and the print assumes a red or brown colour, which is deepest and most intense in the parts where the light has acted longest.
Hence we see, that, to produce a good print, it is essential that the Negative should possess considerable intensity in the dark parts. Pale and feeble Negatives yield proofs which are wanting in vigour, and have a flat and indistinct appearance. The combination cannot be exposed to light for a sufficient length of time to bring about the requisite degree of reduction of the Chloride of Silver; and hence the deepest shadows of the resulting Positive are not sufficiently dark, and there is a want of contrast which is fatal to the effect.
A good Negative should be so opaque as to preserve the lights of the printed image beneath clear, until the darkest shades are about to pass into the bronze or coppery condition. If the amount of intensity be less than this, the finest effect cannot be obtained.
CONDITIONS AFFECTING THE SENSITIVENESS OF THE PAPER AND THE INTENSITY OF THE IMAGE.
Some of the principal of these are as follows:—
a. The Strength of the Salting Bath.—The sensibility of the paper is regulated up to a certain point by the amount of salt[16] used in the preparation. The quantity of alkaline Chloride determines the amount of Chloride of Silver; and with a proper excess of Nitrate of Silver, papers are to a certain point more sensitive in proportion as they contain more of the Chloride.
[16] The difference in the atomic weights of the various soluble Chlorides used in salting must be borne in mind. Ten grains of Chloride of Ammonium contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two grains of Chloride of Barium. (See the Vocabulary, Part III.)
Highly sensitized papers darken rapidly, and pass very completely into the bronze stage. Those containing less Chloride darken more slowly, and do not become bronzed with the same intensity of light. A Photographic print, formed upon paper highly salted and sensitized, is usually vigorous, with great contrast of light and shade; particularly so when the printing is conducted in a strong light. Hence it will be an advantage, with a feeble Negative, and in dull weather, to double the ordinary quantity of Salt, whereas in the case of an intense Negative, and with direct sunlight, the deep shadows will be too much bronzed unless the quantity of Chloride and Nitrate of Silver in the paper be kept low.
In proportion as Photographic papers are highly salted and sensitized, they become more prone to change colour spontaneously in the dark.
b. Proportion of Nitrate of Silver.—The compound on which a positive print is formed is a Chloride, or an organic Salt of Silver, with an excess of Nitrate of Silver. Nothing is gained by increasing the proportion of Chloride of Sodium, unless at the same time an addition be made to the quantity of free Nitrate in the sensitizing Bath.