Bromide Printing and Enlarging / A Practical Guide to the Making of Bromide Prints by Contact and Bromide Enlarging by Daylight and Artificial Light, With the Toning of Bromide Prints and Enlargements

by John A. Tennant

In the sulphide process, the image which, in a black-developed print, consists of metallic silver in fine division, is converted into silver sulphide, a substance which in the ordinary way is also black, but when produced in a fine condition on a photographic print is brown to sepia color. Silver sulphide is a most permanent substance. Therefore a sulphide-toned print should be permanent, too, a conclusion which is fully borne out in practice. A sulphide-toned print is at least as permanent as the bromide from which it is made. The image of the latter is susceptible to practically only one agent likely to come in contact with it, namely, sulphur fumes from burning gas, which partially sulphurize it and give rise to iridescent markings resembling those due to stale paper. Now, as the sulphide-toned print is the result of this sulphurizing process carried out with intention to a state of completeness, the result should be—and proves to be in practice—immune to this one cause of defacement.

In converting the silver image into one of silver sulphide, the method is to first act on (bleach) the silver image with some reagent which will change it into a compound of silver susceptible to the action of sulphide. Iodine has been used for this, giving an image of silver iodide. Bromine gives one of silver bromide. A mixture of potass bichromate and hydrochloric acid gives silver chloride, as does also a solution of chlorine, though in the former case the presence of the chromium compounds affects the color obtained. But the best of the lot is a solution of the two substances potassium ferricyanide and potassium bromide, which forms an image of silver ferrocyanide and silver bromide. Both of these are converted into silver sulphide when treated with a solution of sodium sulphide. In the case of the hypo-alum process, in which the prints are toned direct (without bleaching) in a mixture of hypo and alum, the image is also changed into silver sulphide, but only to a partial extent. Theoretically, the method is not so good as sulphide proper; it is much more inconvenient in practice except on a commercial scale, while the results cannot be said to quite equal those by the sulphide process as regards permanency.

So much by way of theory. We will now give the formulae for the two solutions required in the sulphide process. The first of these is the “bleach,” or oxidizing mixture of bromide and ferricyanide. Within reasonable limits, the proportions of these salts and the quantity of each in the solution does not matter very much. Each chemical can, if desired, be kept in a separate solution if care be taken to keep the mixture in the dark,—that is, in a cupboard where it will not be exposed constantly to daylight. The ferricyanide suffers in time by exposure to daylight; but, as both it and the bromide are comparatively cheap and serve for a large number of prints, there is no need to take excessive care. The ferricyanide-bromide mixture, however, keeps very much better than a plain solution of ferricyanide alone. Formulae which place the salts in separate solutions are a mistake.

As good a formula as any is: Potass ferricyanide, 300 grains; potass bromide, 100 grains, water 20 ounces; Ammonium bromide may be used in place of the potassium salt in the above formula; the difference is not marked, but the ammonium compound tends to give a somewhat warmer brown or sepia. In the case of many formulae, it will be noticed that equal quantities of bromide and ferricyanide are recommended. Although, as just stated, variations in the formula are not at all marked in their effects, a proportion of bromide over one-quarter of the ferricyanide does tend towards the yellowish color of which complaints are now and again heard. I want to make it clear that the opportunities for going wrong with the bleacher are very small indeed. Without encouraging the reader to be careless let it be said that “any old formula” (of ferricyanide and bromide) for the bleacher will prove successful. Not so, however, in the case of the sulphide solution, which requires to be very carefully made up and used.

Sulphide, not sulphite. The material for the toning or darkening of the bleached print is the chemical substance, sodium sulphide, of the formula Na₂S·9H₂O. This is purchased as small crystals which greedily absorb water and rapidly become almost liquid if not properly corked. Not that this totally unfits the sulphide for use. Sulphide which has gone liquid will at all times be found to work perfectly, but it is of course open to suspicion, and, in any case, it is not possible to know what is the strength of a solution made up with such a supply. For this reason, it is best to make up the sulphide into solution of 20 per cent strength, and add this to water to make the toning bath. And it is here that a caution must be noted. The weak working solution, which is only about 1 to 2 per cent strength, keeps very badly indeed, and should be made up fresh from the stock solution at the time of toning each batch of prints. This is one of the most necessary items to bear in mind in using the sulphide process.

Sodium sulphide is sold in various degrees of purity, and the label on the bottle is not always in exact correspondence with the condition of the substance inside, but the two forms which must be adhered to for sulphide toning are the ordinary “pure” and the “pure for analysis.” The former can be obtained from any reliable drug store or photographic dealer. It comes in small lumps, yellowish to greenish in color; when dissolved in water the solution will be yellow, and will usually show a deposit which must be filtered off. This sulphide will give tones which are sepia brown with most papers. In the case of the “pure-for-analysis” sulphide, which is the recrystallized variety, the salt will be pure white and will form a quite colorless and clear solution in water. The tone given by this kind of sulphide is usually of a more purplish color. The distinct difference between the two commercial varieties of sulphide should not be overlooked, as it allows the worker to modify the process usefully when dealing with papers differing (as all papers do) to a slight extent in their adaptability to sulphide toning. The purer form has certainly much better keeping properties than the other, but either, if made up in 20 per cent solution, keeps for a month or two at least—which is enough for all purposes. The chief difference between the two is noticed in the diluted or working solutions. That of the purest sulphide may be kept and used again, though it is not really good policy to do so.

The supply of sulphide should therefore be dissolved as soon as purchased, as follows: Stock sulphide solution—20 per cent; sodium sulphite 4 ounces; water to make 20 ounces. The actual toning solution is made up at the time of treating the prints by mixing the above stock with water, as follows: Sulphide toning bath.—Stock 20 per cent solution 3 ounces; water to make 20 ounces.

This makes a bath which contains about one per cent real sulphide, corresponding with about a 3 per cent solution of the sulphide as purchased. If the bath is much weaker, the tone obtained is usually not quite so good; while, if it is stronger, there is danger of the print’s blistering while toning, or afterward in the washing water. Indeed, some papers need to be toned in a weaker bath, and require also to be fixed in an alum-hypo fixing bath (see later), so that the strength of the toning bath given above may be taken as the maximum, and used at half or one-third strength, as circumstances show to be necessary. And, to repeat the caution once more, the toning bath is to be thrown away as soon as the prints have been passed through it. With these points in our mind as to the making up of the solution, we can come to the process proper.

The prints require to be well washed and free from hypo before being placed in the bromide-ferricyanide bleacher, because any hypo in conjunction with the ferricyanide will form the well-known Farmer’s reducer, and cause patchiness of the prints. It is immaterial whether the prints are taken direct through the toning process or dried in the meantime. Some workers contend that the toning process is more regular if the prints are dried before bleaching. In either case, immersion in the bleacher will cause the fully developed bromide to disappear, leaving only a faint brown image behind. In some cases the image is fainter than in others, the difference appearing to depend chiefly on the developer employed. Developers with a liability to stain will give prints which do not bleach out so completely as those made with cleaner working developers. But, in all cases, two to three minutes’ action of the bleaching solution will be ample; if all pure black is not gone in this time, it is a sign that the bleach is becoming exhausted. The prints should be kept constantly on the move whilst in the solution, and turned over and over to ensure equal action. They are then given quite a brief rinse in running water—half a minute to a minute—and then transferred to the sulphide solution, where they should darken to the full brown or sepia tone in a few seconds. It is well always to leave them here for twice to three times the period required to give the full tone. A wash of half an hour will remove the salts left in the film.

Granted that bleacher and sulphide are in proper working order, there is one further factor in the making of sepia prints which is of vital importance, and that is the proper preparation of the print itself. A good sulphide tone presupposes a good black and white bromide. Not only that, defects in the bromide which may lie latent while the print is untoned come to light in the sulphide bath. This applies to uneven fixation (due to omission to keep prints moving in the hypo bath) and fingering of the surface; while, as regards the original development of the print, making the best of a wrong exposure will not do when sulphide toning is in view. A print that is forced by long development will suffer in tone, the result being colder and less satisfactory as regards vigor. Full exposure, and development which is complete in the normal time for a perfect black print, are the conditions for a good sepia tone, and, when a batch of prints is being put through, it is well to take steps to preserve a uniform time of development in order to secure an identical tone throughout.