Care of the paper is another point that is well worth attention, and all Bar net P.O. P. is sent out carefully packed, first in white paper, then in waterproof paper, and finally in a non-actinic envelope. When a packet is opened and a sheet or two removed for immediate use, the remainder should be wrapped again in its original casings, and the packet kept between the leaves of a fairly heavy book. Prints made and awaiting toning should be treated in exactly the same manner and not left lying loosely in a drawer. Metal boxes for storing papers and prints are somewhat dangerous, but if the P.O.P. is kept wrapped in its original damp-proof-paper packing no harm should arise. In the printing frame also some precaution should be taken to prevent damp getting to the paper, or it will cause unequal expansion and cockled paper. Printing frames which have not been in use for a day or two pick up a lot of dirt and moisture on the cloth backs. The best preventives of trouble arising from this cause are rubber sheeting as used in platinotype printing, or two or three sheets of celluloid. These also help to ensure perfect contact between negative and paper at all points.
The temperature of photographic solutions is a subject which is very much neglected in this country. We hear and read of workers in other climates who have to wait until the cool of evening before commencing chemical operations, and even then they use cool well-water or adopt the method of adding ice to their solutions.
The reader usually feels thankful that he does not need to resort to such practices in this country, and then promptly forgets all about it. In reality there is just as much need here in England to study temperature as anywhere else, although, judging by the recent samples of summer weather that we have experienced, there is very little necessity for ice-water. But in the reverse direction a great deal may be done. Almost every kind of chemical action may be accelerated by heat, and whether we are developing, toning, fixing, or washing, this rule holds good. Plate development is outside the province of this article, but toning is a big part of our subject. If the toning bath is made up from ordinary tap-water drawn from the mains during any but the hottest months of the year, difficulty will be experienced in getting good tones with ease and regularity. On the other hand, excessive heat has its dangers as well.
In the case of P.O.P. the most evident danger is that of melting or even softening the gelatine coating of the paper. A simple plan is to purchase a small thermometer, preferably made entirely of glass (mine cost is. 6d.), and to accept as a general rule the dictum that no bath should be below 6oc F. or above 700 F.
In the case of the toning bath this heat may be attained first of all by making part of the solution with warm water, and the prescribed temperature can be maintained by having an outer and larger dish containing hot or warm water. In this larger dish the actual toning dish may lie or even float, and the contents of the inner dish can be kept at any required heat by additions of hot or cold water to the outer dish; and this without dilution of the toning or other solution. To a P.O.P. worker struggling with a batch of prints in a cold dark room or domestic kitchen or bath-room the adoption of the above method will probably be a revelation. At 650 F. toning becomes a pleasant and expeditious operation, and, still more, the tones obtained in a bath at this temperature are, to my mind, finer in quality than those reached by slower or quicker baths at lower or higher temperatures.
With the fixing bath the effect is not so apparent to the naked eye, but a little careful reasoning will prove that the same rules hold good. The fixation of a paper print is not a thing that can be readily judged by the eye at any time. Some old-fashioned printers with twenty-five to forty years' constant every-day experience will tell you that they can see when a print is fixed. Perhaps they can. But never a one has been able to tell me exactly what the appearance of a properly fixed print should be, and I much prefer to work by a more scientific rule, which allows sufficient time for the complex chemical operation of fixation to be complete. Suppose, for a moment, that a quarter of an hour is allowed for this purpose, and that the prints are kept moving during the whole time in a solution at about 6o° F. (either natural or induced by other means as suggested), there is every probability, nay more, a certainty that such prints are thoroughly fixed. Now take the opposite and, unfortunately, more usual course of paying no regard to temperature. A number of prints have been toned and await the fixing bath. The requisite quantity of hypo crystals are weighed out (sometimes guessed) and water added from the tap. The immediate effect is a lowering of the temperature 150 or 200, sometimes even to freezing-point, and the only natural inference is that the necessary chemical action cannot take place in the same time as when normal temperature is maintained. In this connection a word of warning may be given. On no account should boiling water be added to hypo crystals. If this is done a very peculiar effect may be observed. Some of the crystals appear to be slightly melted by the heat and will be found to be sticking tightly to the bottom of the dish, and if not removed will cause trouble afterwards. A good plan is to put the hypo crystals into about half the necessary bulk of water, which should have a temperature about 8o° F. Solutionis thus quickly obtained, and the total quantity is then made up with hot or cold water as needed until the whole is at normal heat, and finally this temperature is kept up by means of the outer dish. The final washing of prints will also proceed more effectively if the water is not too cold, and I shall show later that this not a difficult matter to manage.