A Manual of Photographic Chemistry, Including the Practice of the Collodion Process

by T. Frederick Hardwich

THE COLOUR OF THE IMAGE INFLUENCED BY THE PREPARATION OF THE SENSITIVE PAPER.

This subject should be studied by those who desire to print with taste. By introducing a few simple modifications into the mode of preparing the sensitive paper, almost any variety of tint may be obtained.

The tendency of the "toning" process, to which the print is afterwards to be submitted, is to darken the colour, and, if gold be used, to give a shade of blue. Hence, if the Positive be printed of a red tone, it will change in the gold Bath to a purple; whereas if left, after exposure to light and fixing, of a dark brown or sepia tint, it passes by toning into a pure black.

The Positive should look warm and bright on its removal from the printing frame; but the tint which remains after immersion in Hyposulphite of Soda is the proper colour of the simply fixed print.

The following points may be mentioned as affecting the colour and general appearance of the picture.

a. The proportions of Salt and Nitrate of Silver.—Highly salted and sensitized papers give a darker image than those which, containing a small proportion of Chloride of Silver, are less sensitive to light. Hence in printing upon paper weakly sensitized, in order to bring out the finer details of a highly intense negative, we find the image unusually red after fixing, and of a brown or mulberry colour when toned. The above remarks apply also in some degree to the strength of the Nitrate Bath, and especially so when no organic matter excepting Gelatine is employed,—in such a case the image will be darker after fixing, if the proportion of free Nitrate of Silver be large.

b. Effect of Oxide of Silver on the colour.—Prints formed upon Ammonio-Nitrate papers highly salted are of a sepia colour after fixing, and usually of a pure black or a purple-black when toned. With the increased facility of reduction by light afforded by use of Oxide of Silver, there is also less redness in the print. But if the quantity of salt used in preparing the paper be reduced to a minimum (one grain to the ounce or less), for the sake of economy or to improve the half-tone, then the usual red colour returns, and the Positive is brown or purple after toning, in place of black. Thus by employing a solution of Oxide of Silver, the operator is enabled, without the addition of organic matter, to print Positives of a pleasing variety of tint, combined with a peculiar softness and delicacy, which cannot easily be obtained with the simple Nitrate of Silver.

c. The colour affected by organic matter.—Albumen is coagulated by Nitrate of Silver, and forms a permanent gloss upon the paper. The sensitive albuminized paper darkens in the sun to a chocolate-brown colour, which becomes very red on immersion in the Hyposulphite. The finished prints are clear and transparent; usually of a brown tone, or with a shade of purple when the gold Bath is newly made and active; pure blacks are not easily obtained.

Iceland Moss affects the colour of the proof to a certain extent, but less than Albumen; the finished prints are nearly black if the paper is highly salted.