In the earlier printing processes used by photographers - those in which the image was obtained by the continued action of light and which were toned by the deposition of gold from a toning bath - the prints obtained were in various shades of purple and brown, and these shades became so associated with photographs in the minds of the pub-lic that when the black and white prints made on Velox and bromide papers began to displace the earlier Solio and Aris-totype prints, the gen-eral public would scarcely recognize them as "photographs" at all, and a demand soon arose for some method of toning the black images of bromide and Velox prints to a brown or sepia similar to that of the gold toned printing-out papers. It seems to be characteristic of mankind to want what they have not got, and it is interesting to note that with the earlier printing-out processes which easily gave warm tones, chemists were anxiously working to get methods of obtaining black and white prints, while with the developing-out processes, which naturally give good black and white prints, photographers desire to obtain warm sepia and brown tones.
The processes for obtaining sepia prints from the black developed-out images all depend on one chemical reaction; namely, that by which silver bromide is converted into silver sulphide. Silver sulphide is a dark coloured, almost black, substance well known to the housekeeper - if not by name - as the tarnish which appears on silverware after it has been some time in the air, the surface of metallic silver being attacked by sulphur compounds in the air, which generally come from the products of combustion of gas in the cooking range.
Fig. 87. Combined Enlargement from Two Negatives.
Now, when any chemical substances can be produced by the interaction of two other chemical substances in solution the question as to whether it will be produced depends upon whether it is more or less soluble than the substances which can form it. Silver sulphide is less soluble than silver bromide so that when silver bromide is treated with a solution containing sulphur in a free form it is changed into silver sulphide and the silver sulphide is deposited in its place. On the other hand, metallic silver, such as that which forms the image in a developed print, is less soluble than silver sulphide and consequently we cannot change it into silver sulphide by simply treating it with a solution containing free sulphur, but if in this solution we have some substance which will dissolve metallic silver, then we can change the metallic silver itself into silver sulphide. It is on these principles that the sulphur toning processes are based. One toning process depends upon changing the silver image of the print back into silver bromide. Now, we know that silver is obtained from silver bromide by reduction, just as iron is got out of iron ore, and therefore we can get back silver bromide from silver by oxidation, which is the reverse process to reduction. If we use any solution which will oxidize silver and have potassium bromide present in the solution, the silver image will be turned into silver bromide. The usual way to do this is to treat the black print after fixing and washing with a solution containing potassium ferricyanide, which is an oxidizing agent, and potassium bromide, and this turns the black silver image into a yellowish-white image of silver bromide which is scarcely visible, so that the process is called "bleaching" since the black silver turns into white silver bromide, and then after washing, this silver bromide is treated with a solution of sodium sulphide, which turns it into the brown silver sulphide which gives us our sepia toned print. So, to make a sepia Velox print by this method, we treat it with the "bleaching solution," which turns the silver into silver bromide, and then "redevelop" this, as it is called, in a solution of sulphide, which converts the silver bromide into silver sulphide and gives us our sepia print.
Fig. 88. Brownie Enlarging Camera.
There is another method of obtaining sulphide toned prints which is somewhat simpler. We have seen that we cannot turn silver directly into silver sulphide by a solution containing free sulphur unless we have a solvent of silver present in the solution. Now, it so happens that hypo is to some extent a solvent of silver, and also that with a weak acid, hypo gives free sulphur. Alum behaves chemically like a weak acid and it also has the valuable property of hardening the print, so if we put the print which we wish to tone into a solution containing hypo and alum, the silver will slowly be changed into silver sulphide and the print will be toned brown. This change goes on very slowly at ordinary temperatures, but by heating the solution it goes much more rapidly, so that if we heat a bromide or Velox print in a solution containing hypo and alum, we shall get a good sepia tone at the end of ten or twenty minutes without any further difficulty, the only objection being that the bath, like all baths containing free sulphur, and like the sodium sulphide used for redeveloping in the other toning process, smells rather unpleasantly.
Equally good results in sepia toning cannot be got with all papers, but a great deal depends on the development of. the print. To get good sepias, development should be full; an underdeveloped print will always give weak, yellowish tones when compared with one in which development has been carried out thoroughly, which will give a strong, pure sepia. It is important to remember this, as two prints which may look alike as black and white prints will tone differently if they have not been developed to the same extent.