This is another inverse process, that is, one in which a positive is used for the print-plate, which must be exposed through the back. The directions already given for the bichromate relief process also apply to this, and, as in that, warm water is used to dissolve the gelatine. The basis of the process is the tanning action of oxidized pyrogallol on gelatine, and this is so great that the latter actually becomes insoluble in warm water, as was pointed out by Wamerke in 1881.

It is a well-known fact that the oxidation of pyrogallol, and all developing agents, is prevented by the addition of sodium sulphite and, therefore, by the addition of more or less of this the height of the relief obtained, or the quantity of gelatine rendered insoluble, can be varied. It is possible to use a plain solution of pyro with soda or ammonia, but there is danger of superficial tanning, and as one also wants freedom from fog, it is advisable to use some sulphite and some bromide. A suitable developer may be made on the following lines:

Pyrogallol 5 g.

Sodium sulphite, dry 10 to 20 g Sodium carbonate, dry 25 g.

Potassium bromide 1 g.

Distilled water to 1000 ccm.

The sulphite should be dissolved in 100 ccm of the water, the pyro added, and the solution at once bottled. The carbonate and the bromide should be dissolved in 900 ccm water, and when required for use the two solutions should be mixed. Obviously, any quantity of stock solution of the carbonate can be made up; but it is not advisable to make up any quantity of the pyrogallol in stock, as every time the bottle is opened the pyro becomes more or less oxidized, as the amount of the sulphite is so small that it is not an efficient preservative, and consequently tanning of the gelatine, where it is not wanted, might take place. Unless the three transparencies are developed together, fresh solution must be used for each one, as otherwise if the developer be used for two or three plates in succession, one meets with precisely the same trouble again, because the developer oxidizes much more rapidly in an open dish in the presence of the carbonate than in the bottle with the plain sulphite solution.

It seems unnecessary to repeat what has already been said when describing the other processes, as to the necessity of avoiding underexposure, but as this developer contains a fair proportion of bromide and this delays the appearance of fine detail, it is as well to give a generous exposure so as to avoid too bare high-lights.

As soon as development is complete, the plates should be at once washed in water to stop development. There is no necessity to fix. They can then be treated with warm water at about 380 to 400 C. (100° to 1050 F.), in which the gelatine not in contact with the silver will dissolve, leaving a relief containing silver. It is advisable to remove this, as it renders the later registration more difficult, and naturally any silver solvent may be used. While thoroughly efficient, the least satisfactory from a practical point of view is Howard Farmer's hypo and ferricyanide, as it will not keep. The following reducer, known as Belitski's, is just as efficient, keeps well in the dark, and may be used repeatedly:

Potassium ferric oxalate 50 g.

Distilled water 200 ccm.

Dissolve and add:

Sodium sulphite, dry 20 g.

Water 100 ccm.

This at once forms a blood-red solution and to it should be added:

Oxalic acid, dry crystals 15 g.

Shake the solution until it turns a fine green, without any tinge of yellow; if this latter persists, and all the oxalic acid has dissolved, a few more crystals of the acid must be added, and the solution well shaken; but if the solution is green and there is some undissolved acid, the mixture should be poured off from the crystals. To this solution should be added:

Hypo 250 g.

Water to make 1000 ccm.

When the hypo is dissolved the solution is ready for use. Possibly some little difficulty may be experienced in obtaining the potassium ferric oxalate, and it should be noted that this is not ferric oxalate; but there is no difficulty in making it by dissolving 35 g of ferric chloride (not the anhydrous salt) in 50 ccm water and adding 64 g of neutral potassium oxalate dissolved in 150 ccm warm water. This introduces a little potassium chloride into the solution, which does no harm. After fixing, the relief should be washed to remove the hypo and dried. The other steps in the process are as already outlined for previously described methods.