The Gelatinous sizing used for the English papers, and obtained by boiling hides in water, and hardening the product by an admixture of Alum, has a reddening influence upon reduced Silver salts, analogous to that of Albumen, or of Caseine, the characteristic animal principle of milk. Positives printed upon English paper, commonly assume some shade of brown more or less removed from black; the darker tones being more readily obtained upon the foreign papers.
Citrates and Tartrates have a marked effect upon the colour of prints. Paper prepared with Citrate, in addition to Chloride of Silver, darkens to a fine purple colour which changes to brick-red in the fixing Bath. The Positives, when toned, are usually of a violet-purple or of a bistre tint, with a general aspect of warmth and transparency.
SECTION II.
The Processes for Fixing and Toning the Proof.
This part of the operation is one to which great attention should be paid, in order to secure bright and lasting colours: it involves more of delicate chemical change than perhaps any other department of the Art.
The first point requiring explanation is the process of fixing; to which ([p. 41]) brief reference has already been made. The methods adopted to improve the tint of the finished picture will then be described.
CONDITIONS OF A PROPER FIXING OF THE PROOF.
This subject is not always understood by operators, and consequently they have no certain guide as to how long the prints should remain in the fixing Bath.
The time occupied in fixing will of course vary with the strength of the solution employed; but there are simple rules which may be usefully followed. In the act of dissolving the unaltered Chloride of Silver in the proof, the fixing solution of Hyposulphite of Soda converts it into Hyposulphite of Silver ([p. 43]), which is soluble in an excess of Hyposulphite of Soda. But if there be an insufficient excess,—that is, if the Bath be too weak, or the print removed from it too speedily,—then the Hyposulphite of Silver is not perfectly dissolved, and begins by degrees to decompose, producing a brown deposit in the tissue of the paper. This deposit, which has the appearance of yellow spots and patches, is not usually seen upon the surface of the print, but becomes very evident when it is held up to the light, or if it be split in half, which can be readily done by gluing it between two flat surfaces of deal, and then forcing them asunder.