Gaslight papers can also be used if their gelatine films satisfy the above mentioned requirements. Therefore we have the widest possible choice in the printing materials for bromoil.
A great number of bromide papers of different manufacturers are well suited for bromoil printing; it is, however, advisable to make a preliminary investigation as to the amount of hardening they have undergone, for it occasionally happens that different emulsions of the same brand show quite different grades of hardening, so that on one occasion it is possible to make bromoil prints on them without the least difficulty, while the same paper at another time may absolutely refuse to take the ink. On account of the great popularity of the bromoil process in recent years, it can be easily understood that some manufacturers might seek a wider sale for their products by claiming for them a special suitability for this process. It is therefore a wise precaution to previously test even those brands which are advertised as specially adapted for bromoil printing, and not to depend too much on such claims.
Development.—The processes of tanning in the film of a bromide print, produced by the bleaching of the silver image, which will be described later, are of an extremely subtle nature. We must therefore endeavor to avoid all causes for damage in this process and especially everything which tends to harden the whole film even to the slightest degree. Any tanning, which affects the whole gelatine film, has the same effect as general fog in a negative. It is well known that almost all the developers used in photography have more or less tendency to harden the gelatine film. A very considerable damage to the bromoil print through the use of a tanning developer might naturally be imperceptible to the eye. Yet this may at times manifest itself in a very undesirable and disturbing form, especially when the bromide paper has been so much hardened in manufacture that it possesses only just the necessary qualification for bromoil printing. It may then happen that the last remainder of swelling capacity can be taken from the paper by the use of a tanning developer. However desirable it might be and however it might simplify the process to be able to use any desired developer in producing the bromide print, to avoid trouble it must be observed that the use of developers which tan the film may seriously influence the result, even though it is possible to get some kind of prints in many cases. If the worker is absolutely sure that the bromide paper which he is using is not strongly hardened and is therefore well suited for bromoil printing, he may undertake development with any one of the ordinary developers which he prefers.
The developers, which do not exercise a hardening influence on the gelatine, are the iron developer and amidol (diamidophenol hydrochloride). As the iron developer is not really suited to this purpose on account of certain unpleasant qualities inherent in it, it is advisable to use amidol for the development of bromide paper for bromoil printing whenever possible, and the best developer is composed as follows:
| Amidol | 1.7 | g | 12.3 | gr. |
| Sodium sulphite, dry | 10 | g | 77 | gr. |
| Water | 1000 | ccm | 16 | oz. |
The sodium sulphite is first dissolved in water, and the easiest way is to pour the necessary quantity of water into a developing dish and sprinkle the pulverized or granular dry sodium sulphite into it while the dish is constantly rocked; solution takes place almost instantly under these conditions. Larger lumps, which would stick to the bottom of the dish, must be immediately stirred up. As soon as the sodium sulphite is dissolved, the amidol should be added and this will also dissolve immediately. The addition should be made in the order described, for, if the amidol is dissolved first, the solution is often turbid. If dry sodium sulphite is not available, double the quantity of crystallized sulphite may be used.
The amidol developer should be freshly prepared each time that it is used, as it does not keep in solution. The measurement of the quantities of amidol and sulphite given above does not need to be made with the most painstaking care, as small variations in the quantities are unimportant.
In using amidol developer the greatest care must be taken to avoid allowing amidol powder, in even the smallest quantity, to come into contact with the bleached print ready for bromoil printing. Even the finest particles of amidol, although invisible to the naked eye, will produce yellowish brown spots on the gelatine which penetrate through the film and into the paper itself. These dots and spots, especially if, as is usual, they occur in large numbers, will make the print completely useless, and it is impossible to remove them.
If amidol developer is not available, any other developer which is desired may be used. As we have already stated, however, certain possibilities of failure are to be anticipated, but will not necessarily occur.