THE GUM-BICHROMATE PROCESS Quot homines tot sententi " is apparently the motto of all who adopt and recommend this very beautiful process. It is almost outside the range of ordinary photography, beyond the fact that it commences with a negative from which an image is printed in bichromated gum or fish glue. The after manipulations are at the particular artist's will. We might term it a " go as you please " process, so diverse are the directions given by its numerous devotees.
Preparing the Paper.—Almost any substantial kind of paper is suitable as the foundation of a gum print provided it is not too heavily sized. An even grain is necessary unless the pictures are large and not likely to suffer by the straw lines and other watermarks. Whatman's and other drawing papers in different qualities, rough or smooth, or the grained papers used for water colours, will be the most serviceable. A paper that will not expand and contract in an irregular manner is also an important consideration. Mr. James Packham commences by sensitising his paper with a m per cent. solution of bichromate of potassium, immersing it for about 2 min., after which it is dried. The subsequent gum pigment is then applied in unsensitised state. Other workers prefer to mix the bichromate with the pigment, while yet a third-class coat the paper with gum pigment before sensi tising. Mr. A. \V. Hill sizes the paper first of all with Le Page's liquid glue, watered down with five times its bulk of water, and applied with a broad varnish brush.
The Gum is usually compounded of r oz. of pure gum arabic in tears, and 22 oz. of water. To avoid impurities the gum should either hang in the water in a muslin bag or be strained from woody impurities when dissolved, which will probably be at the end of twenty-four hours. Most readers are well aware that it is not possible to mix colours directly with any vehicle. A quantity of dry colour in powder must be laid on a slab and carefully ground up with a palette knife into close association with a few drops of the gum, after which it is worked up with a brush into a mixture with the remainder of the gum solution. The
proportion of pigment to be used in each case varies with the worker and also with the colours employed. Mr. J. C. S. Mummery, one of the most successful and eminent exponents of the process, takes 40 gr. ivory black and 8 gr. burnt sienna to r oz. gum solution and r oz. sensitises. Others are content with as little as 12 gr. to the ounce of gum solution. Until the beginner has mastered the elements, and can strike out his own independent line, we should advise about 36 gr. to the ounce of solution. A great range of colours is available for practice, but the easiest are vegetable black, burnt sienna, Vandyke brown, and the ochres.
Coating the Paper.—Lay the paper on a drawing-board, pin down the corners, and spread the coloured gum evenly over the surface of the paper. If the paper expands from moisture unfasten the pins and stretch it flat before refixing. The gummy surface must be still further levelled, either by going over it with a very broad brush, a leather lithographic roller, or, as some find better still, rubbed in with a piece of soft muslin rolled up into a ball. If the sensitising solution has not been mixed up with the gum this must be applied as soon as the surface is dry. It will consist of a io per cent. solution of potassium bichromate with about a dram of ammonia. But if more advanced work is likely to be aimed at, and especially multiple printing, the sensitising solution should always be mixed with the gum pigment.
Exposure.—This must ultimately be by sensitometer, and will vary considerably, not only according to the actual actinic power of the light, but according to the colour of the pigment. An actinorneter may be used ; but the image ought to be visible to some extent, at any rate in the shadows, unless the pigment has been laid on too thickly. Under-exposures are practically worthless ; over-exposures may to a certain extent be reduced by a weak solution of sodium hypo chlorite.