Another process, which is quite as simple as that already described, and which is actually simpler if the original negatives are used as the printing matrices, is the so-called etching process. In this the gelatine in contact with the metallic silver of the image, instead of being insolubilized, is chemically etched or dissolved away.

This process is founded on an observation made originally by Liesegang in 1897, and was improved upon by Andresen in the following year. The former noticed that when a negative was immersed in a solution of ammonium persulphate, the gelatine surrounding the silver image was so softened that it became soluble in hot water. Andresen improved the idea by using hydrogen peroxide, and this is actually the etching agent in this process, that is, it gives up oxygen when reacting with the silver and the nascent oxygen attacks the colloid.

The etching fluid is:

Hydrogen peroxide 30.0 ccm.

Cupric sulphate 20.0 g.

Nitric acid 5.0 ccm.

Potassium bromide 0.5 g.

Water to 1000.0 ccm.

This is used at normal or room temperature, preferably 180 C. (700 F.) If the temperature rises, the action is apt to be too violent in the deep shadows.

If one elects to use the original negatives, it is apparent that it obviates the making of a set of transparencies and second negatives. There is no real objection to this use of the primary negatives, except that the gelatine may become damaged in the course of time and one has then no reserve negatives to fall back upon, from which to make further matrices. It is as well, therefore, to make transparencies from the original negatives and keep these in reserve. It may be emphasized here (because, although we are considering one special process, this recommendation is applicable to every case in which transparencies are made for the purpose of reproduction of negatives) that they should not be like lantern slides, that is, brilliant and hard, but rather of a soft character, fully exposed without dense shadows. In fact the best type of reproduction transparency is that in which there are no patches of clear glass and no dense shadows that one cannot easily read print through. One can, in making the second negative from such plates, obtain any desired degree of contrast by the use of contrast plates, slight underexposure or the liberal use of bromide in the developer. Whereas, if the transparency is wanting in detail, there is no method of obtaining it.

If one elects not to use the original negatives, one must make transparencies from them and duplicate negatives from these. Obviously with all these processes, in which either negatives or positives are used as print-plates, one is not tied to the original size of the negatives, as they may be enlarged or reduced in the camera, and thus any size of picture secured.

It is advisable to soak the negatives to be used in five per cent chrome alum solution for about fifteen minutes, and then wash and dry; but if films are used, and these are the most satisfactory because of their pliability, it is not necessary to use the alum bath.

The negatives should be immersed in the etching solution and allowed to soak for about five minutes, and then the dish gently rocked, and it will be seen that the image will begin to gradually disappear and dissolve off. If the solution is properly made up, the image does not change color at all, but comes away with the gelatine in its original black color. The solution becomes opaque from this suspended silver and turns milky, because the bromide gradually converts it into silver bromide. If, after use, the solution is allowed to stand, the bromide settles down to the bottom of the dish and the supernatant liquid can be poured off and used again if thought necessary; but as the cost of the solution is really very small, this is hardly worth while.

The action of the bath is allowed to continue until, after rinsing the plate or film with water, no black image is seen, which is best observed against a white surface. The image will not be quite invisible, but when there is no black silver visible the action is complete. It is always advisable to manipulate all three negatives at once, or each one with fresh solution, as this means regularity of results. As the etching action is due to the evolution of oxygen from the peroxide, and commercial peroxide may be used, it is clear that the more the solution is used the longer it will take to etch. If the solution has 6een used and allowed to stand, and it is thought worth while to use it again, it will probably be unnecessary to add more peroxide; but more bromide should be added, because really the quantity of bromide controls the rapidity of the solvent action, and if it be entirely used up, the silver may be dissolved without the gelatine, though this is not likely if the above ratios are adhered to. Naturally, also, increase in temperature increases the rapidity of the action, but there is no particular advantage in rapid action, and if the rise in temperature is about ten degrees, some of the gelatine not in contact with the silver may dissolve; of course, a rise of one or two degrees is of no moment.

As soon as all the black silver image has dissolved, the plate should be washed for about fifteen minutes and preferably fixed in a chrome alum fixing bath; this is not actually necessary, but as there is always a slight residue of silver bromide left in the image, the hypo removes it. Then, after washing out the hypo, which takes but a short time, the plates should be immersed in five per cent solution of formaldehyde for fifteen minutes, rinsed, and dried. This drying hardens the gelatine so that the image is less liable to damage in the subsequent operations.

The dyes that can be used and the various manipulations are exactly as described in the previous processes, so that there is no necessity to repeat the directions. The print-plates may be repeatedly used, staining up afresh each time.