Having made our prints, we proceed to tone them. A word or two as to how many should be manipulated at a time. Whilst I do not advocate single prints or even couples, preferring fairly large batches, there is a limit in this direction. I have toned as many as 450 cabinets and C.D.V.s. mixed in one batch in a large dish under the pressure of business circumstances, but I cannot recommend the practice, nor should I like to be called upon to swear that the whole of that 450 were alike in colour, or even that each print was free from patchiness. Fifty cabinets, seventy or eighty quarter-plates, or about a hundred C.D.V.s., make a very comfortable lot for an experienced hand, whilst a beginner will find twenty prints at a time quite as much as he can manage.
Before toning P.O.P. prints it is necessary to get rid of certain free silver and other soluble salts that are in the paper. It is usual to recommend that the prints be well washed for this purpose. Herein I differ from the usual printed instructions issued with P.O.P.s. If the washing method is adopted the presence of free nitrate of silver is instantly detected by the milkiness caused in the wash water, and unless the first washing is very expeditiously performed and a fresh water bath substituted, this free silver has a great tendency to make the whites of the photograph turn yellow. Again, the water that we use is almost invariably conveyed through metal pipes—sometimes made of iron; in fact, I believe all the larger mains are iron. Water coming through such pipes nearly always carries particles of the metal with it in suspension, and woe betide a print in the first wash water that encounters any of these floating particles ! Black spots invariably result, and very often they take the shape of comets with long tails, which are particularly annoying. Now these troubles can be avoided if, instead of trying to wash out the soluble salts, we proceed to convert them into insoluble chloride of silver—that is to say, insoluble in water, but soluble in hypo, and therefore discharged by-and-by in the fixing bath. For this purpose a bath is prepared composed of ordinary table salt, one ounce dissolved in a pint of water, using larger proportionate quantities for larger batches of prints. With a paper containing a large proportion of free acid as well as silver, it may be advisable to add to this salt bath a crystal or two of washing soda, but I have not found it necessary when using Barnet P.O.P. In the salt bath there is no milkiness, because the free silver is immediately converted into a chloride, and in addition to that all prints are brought to one uniform red colour, which enables the progress of toning to be very easily judged. Care should be taken when inserting the dry prints into this bath that no bubbles of air adhere to either front or back of the prints, otherwise the conversion into chloride will be unequal, and patches of a different colour will show after toning. These patches will be found to correspond with any bubbles that have not been wiped away in the salt bath. If a batch of twenty prints are inserted singly, and each one freed from bubbles as immersed, the first (or bottom print) will be ready to come out by the time the last one is put in.
The extraneous salt now needs washing away, but as table salt is so easily soluble in water this need not be a long job. At the same time it should be carefully performed, otherwise the salt may upset the toning bath, retarding its action, or even stopping it entirely for a while. If two dishes are employed and the prints passed singly from one into clean water in the other, draining each print between the dishes until all are in the second dish, it will readily be understood that a very large proportion of the salt is left behind in dish No. 1, probably nine-tenths of it. Throw the contents of dish No. 1 away, replace with clean water, pass the prints back again in the same manner, and a large proportion of the small quantity of salt carried into dish No. 2 must again be left behind. If the process be repeated four or five times—say for five minutes—there will be not much salt left in the prints—at all events, not enough to affect the toning bath.
Some toning baths should be made up at the time of using, whilst others are improved by making up an hour or so in advance. Sulphocyanide baths are ready for use immediately the red colouration (which shows on first mixing) has disappeared. This is usually but a minute or two, and, on the other hand, the bath seems to work just as well if prepared some hours in advance. Formate, bicarbonate, and borax baths should only be mixed immediately before use, while phosphate and tungstate baths are better for keeping about an hour before use. An acetate bath is an extreme case, and is not in full ripe working order until a week old. Platinum baths are best directly they are mixed. Sulphocyanide has by some means or other become the standard bath, although it does not strike me as an ideal toning compound. Yet it is hard to name a better. Probably if the formate bath was as much studied and experimented with it would prove a better toning agent; but sulphocyanide is always given a first place on instruction sheets, and is almost invariably the one used by beginners. If the bath is made up from weighed-out chemicals, great care should be taken to ensure absolute cleanliness in the bottles in which stock solutions are kept, especially gold. A bottle intended . for storing gold should be washed out with ammonia and water or hot strong soda water, and after rinsing should be again washed with dilute hydrochloric acid, followed by washing in plain water. As gold solutions are somewhat liable to the action of light, it is a good plan to paste brown paper around the outside of the bottle. A deep amber-coloured glass bottle will have the same effect, but blue glass, such as pyro bottles are made of, has practically no protective action.