The Powder Process is one of importance for reproducing and, at the same time and at one operation, reversing negatives, but for some reason is far more resorted to upon the Continent than in England, where few only have been able to thoroughly master it. Failures are usually attributed to changeability in the climate, but, although success is by no means uniform, it is a thoroughly workable process. The basis is a chromated solution of gum arabic, with which a plate is coated, dried, and in a thoroughly dry condition exposed to the light under a negative. The greater the extent to which the light acts, the smaller will be the amount of moisture the coating of the plate will absorb when exposed to a damp atmosphere. If the surface of the plate be now dusted over with a fine powder, as drop black, or graphite, the greater the amount of moisture certain parts of the film have absorbed, the greater will be the amount of colour such parts will retain, and so exactly reproduce the light and shade of the original negative. Simple as this process reads, the difficulties must not be underrated, and the operator will probably find plenty of opportunity for the exercise of his patience and ingenuity. A consideration of the process as just described will show that a plate which has only been subjected to a short exposure will, on dusting over and development, be found to exhibit greater density than one that has received a longer exposure, a careful adjustment of which will give to the operator great control over the density and characteristics of the reproduced negative, a power certainly too important to be overlooked. A reliable formula for the preparation of a gum-chromate solution for this process is due to Dr. Leisegang:—

Rain water1litre.
Dextrin or glucose50gr.
Gum arabic50gr.
Honey10gr.
White sugar20gr.
Glycerine5gr.
Cold saturate solution of bichromate of ammonia100gr.

which, after careful filtration, is poured over a well cleaned and dusted glass plate. After draining a little by standing on blotting paper, dry over a spirit lamp, and place, while still warm, under a negative in the printing frame—one minute in the sun, 10 to 20 minutes in the shade, is about the necessary exposure. As previously mentioned, the shorter the exposure, the denser will be the resulting negative, a result at variance to any other photographic printing operation. To develop the picture the glass plate is placed, coated side upwards, upon a sheet of clean white paper, then, with a soft camel-hair brush, dust over the picture some very finely-ground Siberian graphite, working the same backwards and forwards over the plate until the necessary strength is attained. If the picture should develop too quickly, and take the colour all over, the cause is under exposure; should the powder adhere unevenly, or the picture present a smudgy appearance, the coating must have absorbed too much moisture, and must be again warmed before further development. This precaution is particularly necessary during the winter months. If a prolonged development be necessary, and after continuous dusting a picture lacking in contrast be the result, it is occasioned by over exposure. On completion of the development the picture should be finally and carefully dusted with a soft brush to remove the superfluous powder, coated with a thin plain collodion, and placed in water, which should be frequently changed, until it fails to show any trace of yellowness. The plate should be afterwards retouched and varnished.

Negative Films have now been before the public some time, and have been considerably used as reversed negatives by printing with the back of the film in contact with the plate. The four principal varieties are, in the order of their introduction—1st, Eastman’s; 2nd, Froedman’s; 3rd, Pumphrey’s.

Eastman’s Stripping Film consists of a substantial paper coated with a layer of soluble gelatine, upon which is spread a coating of insoluble gelatine emulsion; they may be exposed in contact with a sheet of glass in an ordinary dark slide in the film carriers or on the roller slide. There are no special precautions necessary in the development, except that no alum may be used previous to stripping. For use as reversed negatives the ordinary operations are greatly shortened and simplified. A sheet of plate glass is coated with collodion, allowed to set, and then placed in a dish of cold water until all greasiness has disappeared. After fixing and washing, the negative, still supported upon the paper, is placed over the collodionised plate in the same dish, and the two brought out face to face. Upon the back of the negative paper place a piece of blotting paper, cover it with a sheet of oiled paper—as used in copying letters—and apply the squeegee; place a dry sheet of blotting paper upon the back of the film, cover it with a sheet of glass or other flat surface, upon which place a weight. In fifteen minutes the plate may be placed in warm water—about 130° F.—and in a short time the paper backing will float off. By gentle washing with hot water remove the soluble gelatine from the film resting upon the glass, and place the negative aside to dry. It is then complete, and reversed.

Froedman’s Films consist entirely of gelatine, require no stripping, and may be printed from either side. The film is somewhat difficult to handle in the developer, and has to be dried by spirit. One feature against their employment in Collotype is a yellow stain, which probably arises from the bichromate with which the film is hardened, and although more apparent where the negative is protected by the rebate of the dark slide, it undoubtedly pervades the whole picture.

Pumphrey’s Lifting Films are the latest and best of a series of films introduced by the same maker. They consist of a basis of paper coated with a waterproof varnish on one side, upon the other being spread an indiarubber coating possessing considerable adhesive power; to this is attached the gelatine film proper, which consists of two layers, one of plain gelatine, and the other the sensitive emulsion. The films should be exposed in the camera between two glass plates, and the spring of the dark slide should be of considerable strength to insure flatness of the film, and consequent sharpness of the image. The films during development may be treated exactly as an ordinary plate, the manufacturer recommending Beach’s developer. When in the bath they seem little liable to damage, and may be handled with great freedom and facility. Upon completion of the development, fixing, and washing, the paper backing or support is attached by means of paste to a substantial and smooth card. As a precaution, not always necessary, but usually advisable, against the film prematurely separating from its support, around the edges gummed paper strips may be attached to the card, and allowed to extend over the edges of the gelatine film to the extent of 3 mm. When dry the film may be varnished, and afterwards lifted from its support by first cutting through the gelatine and passing the blade of the knife round the edges between the film and the temporary paper backing, the latter remaining attached to the card. The two are by these means readily separated, and the resulting negative film may be printed from either side without disadvantage, and is of sufficient strength to withstand a considerable amount of rough handling.

Other Methods of Reversal.—There are processes of copying, usually by first producing a transparency and from that again a negative. If the wet process is used throughout, a copying camera is essential in both stages. Should dry plates, however, be used, the transparency may be produced in the camera and the negative by contact in a printing frame, or vice versa, or both may be produced by means of the camera. If the transparency be produced by the carbon process a negative may then be made from it by contact, and the necessary reversal will be brought about without the use of the copying camera.