Immediately on coming in contact with the Hyposulphite of Soda in the fixing and toning Bath, the chocolate brown or violet tint of the Positive disappears, and leaves the image of a red tone. Albumen proofs become brick-red; Ammonio-Nitrate a sepia or brown-black. If the colour is unusually pale at this stage, probably the Silver Bath is too weak, or the quantity of Chloride of Ammonium or Sodium insufficient.
After the print has been thoroughly reddened, the toning action begins, and must be continued until the desired effect is obtained. This may happen in from ten minutes to a quarter of an hour, if the solution is in good working order and the thermometer at 60°; but much depends upon the temperature, and the activity of the Bath. English papers, and especially the same prepared with Albumen, tone more slowly than foreign papers plain salted.
The brown and purple tints are an earlier stage of coloration than the black tones, and therefore the latter require more time. It must be borne in mind however that prolonged immersion in the Bath is favourable to sulphuration and yellowness; tending also to render the image unstable and liable to fade in the half-tones. This fading may not be seen decidedly whilst the print is in the Bath, but will show itself in the after-processes of washing and drying.
The ultimate colour of the Print will vary much with the density of the Negative and the character of the subject; copies of line engravings, having but little half-tone, are easily obtained of a dark shade resembling the original impression.
Some advise that on removal from the toning Bath the Print should be soaked in new Hyposulphite for ten minutes, to complete the fixation; but this precaution is not required with a Bath of the strength given in the formula. An analysis of an old Bath which had been extensively used, indicated only ten grains of Hyposulphite of Silver to the ounce, so that it was far from saturated.
The occasional addition of fresh crystals of Hyposulphite of Soda to keep up the strength of the Bath, is useful, the exact quantity added not being material.
The washing, drying, and mounting of the Positive Proofs.—It is essential to wash out every trace of Hyposulphite of Soda from the Print if it is to be preserved from fading, and to do this properly requires considerable care.
Always wash with running water when it can be obtained, and choose a large shallow vessel exposing a considerable surface in preference to one of lesser diameter. A constant dribbling of water must be maintained for four or five hours, and the prints should not lie together too closely, or the water does not find its way between them, (see the remarks at [p. 162]).
When running water cannot be obtained, proceed as follows:—first wash the Prints gently, to remove the greater part of the Hyposulphite solution. Then transfer them to a large shallow pan, in which may be placed as many Prints as it will conveniently hold. Leave them in for about a quarter of an hour, with occasional movement, and then pour off the water quite dry. This point is important, viz. to drain off the last portion of liquid completely before adding fresh water. Repeat the process of changing at least five or six times, or more, according to the bulk of water, number of Prints, and degree of attention paid to them.
Lastly, proceed to remove the size from the Print by immersion in boiling water.[48] This process will give some idea of the permanency of the tints, since, if they become dull and red, and do not darken on drying, the Print is probably toned without Gold. Ammonio-Nitrate and plain paper Prints prepared on foreign papers by the modes described in this Work, may be expected to stand the test of boiling water; Albumen Prints and Positives on English paper are a little reddened, although not to an objectionable degree.