The fixing-bath for P.O.P. should not be so strong as for negatives; three ounces of hypo to each pint of water is sufficient for most papers. A small quantity of ammonia facilitates not only the fixing process, but also the subsequent removal of the hypo from the paper. Keep the hypo dish or tank well away from the toning-bath, or yellow stains and untoned patches are sure to result. Any alterations of colour while the print is fixing need occasion no anxiety, as the proper tint will generally return after the paper has been washed and dried.
There are good reasons for not combining these two operations, if prints to last for any length of time are required. In the long run it does not save much time or trouble, and the operations are far less under control. Usually, to make sure that the prints are properly fixed, the ordinary fixing bath must be used, either before or after the combined bath. We give Mr. W. K. Burton's formula.
Chloride of Gold....... 6 gr.
Nitrate of Lead....... 3 „
Hyposulphite of Soda...... 3 oz.
Water......... 20 „
Many combined baths are made up with alum and other substances, which tend to decompose the hypo, and may produce sulphide toning. Of course, toning by means of mono-sulphide of silver would be permanent, but whether this would be the sulphide produced is questionable. M. Lumiere suggests the following formula, in which the presence of sodium bisulphite lye will prevent the decomposition of the hypo.
Hyposulphite of Soda......5 oz.
Sodium Bisulphite lye......100 minims.
Lead Acetate........20 gr.
Alum . .......400 „
Gold Chloride........6 „
Dissolve the hypo in the greater part of the water, add the bisulphite and lead, and then the alum dissolved separately in a little water, and lastly the gold in a solution.
These prints may be very easily developed, and a few years ago the process was very commonly practised. The paper must be kept as far as possible from daylight before exposure, and the subsequent procedure should take place by artificial light. Print only until faint detail is visible in the half-tones, or, as some suggest, the faint image given by two feet of magnesium ribbon burnt close to the printing frame. Wash well before placing in the following developer:
Hydroquinone...... . . 15 gr.
Sodium Sulphite....... 60 „
Caustic Soda........ 30 „
Water......... 10 oz.
Mr. Stanley C. Johnson recommends placing the print direct from the frame for five minutes in a 6 gr. per ounce solution of potassium iodide, and for chestnut tones advises:
Pyro.......... 15 gr.
Glacial Acetic Acid....... 15 minims.
Alcohol (90 per cent.)...... 1/4 oz.
Water......... 9 ,,
For violet tones.
Sodium Acetate . . . . . . 10 ,,
Followed by immersion in any ordinary combined bath before washing. Care must be taken not to carry development too far, as the print will darken considerably while in the toning bath.
When fixation is complete, which except in cold weather will be in about fifteen minutes, the prints must be washed for at least an hour. It is not sufficient to lay them at the bottom of a dish, even if water is slowly percolating through. The hypo salts are heavy, and tend to lie at the bottom, and if the prints are left there they will simply continue to retain these salts, one of which is only slightly soluble. Some of the patent washers are excellent, and keep the prints in constant motion. A simple device is to transfer the prints every few minutes to a second dish, changing the water on each occasion.
An alum or formaline bath is important for gelatine prints, especially if they are desired with a fine glaze. This glaze is obtained by squeegeeing them face downwards on a pulp or ferrotype plate. Lay the prints in position on the prepared surface, cover them with two thicknesses of hard blotting-paper, and roll them well. No attempt must be made to remove the prints until they are quite dry. If plate-glass is used as a substitute for ferrotype, the surface must be waxed with a solution of any fine wax (composite candle will do) in benzole. Rub a little carefully on the surface of the glass and polish it off with a fine duster. Prints that do not require a glazed finish may be blotted off in clean hard blotting-paper, to remove the superfluous moisture, and then hung up to dry.
The utmost cleanliness and care is necessary in using printing-out papers. Stains, yellow high lights, and other blemishes may generally be traced to impure chemicals or imperfect fixing. Finger marks on the sensitized surface will betray themselves by uneven toning.