"QUOT homines tot sententiae" 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 10 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 sensitising. Mr. A. W. 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 Pigment

This is usually compounded of 1 oz. of pure gum arabic in tears, and 2 1/2 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 1 oz. gum solution and 1 oz. sensitiser. 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 10 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 actinometer 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 hypochlorite.

Development

Lay the print face downwards in a wide dish of cold water and leave it to soak for 10 min., when it may be lifted out while the water is changed, and examined to see if any portion of the pigment is dissolving out with the bichromate. After the second change of water, it may be dealt with face uppermost, unless these operations are taking place immediately under a brightly lighted window. Gently rocking the dish will probably remove most of the soluble colour, but if portions of the high lights prove obstinate in unveiling they may be delicately persuaded with a soft camel-hair mop brush. If the image is well exposed and fully set, the brush will come in useful in freeing other parts of the picture, as the worker gains more confidence and skill. Frequent changes of water, followed by an alum bath, will remove all traces of the bichromate stain. Warmer water is sometimes employed when the weather is cold, or when portions of the picture cannot be cleared by any other means. For delicate manipulations the print may be laid on a glass slab.

Multiple Gum

For the higher Art purposes of the gum process more than one printing, either in the same, or in several different colours, is desirable. It is of the first importance that the paper selected for double printing be one which will not shrink or stretch seriously after prolonged treatment under water, and, before attempting any ambitious essay, a sample of paper should be tested by pricking distances on it and examining after soaking and drying. A coat of size, made by dissolving 1 oz. of gelatine in 20 oz. of water, should be brushed over the selected sheet and then hardened with chrome alum, or, better still, formalin.

The simplest application of multiple gum is the printing in of clouds, which may be done by spreading a very thin, weak coating of pigment in the upper portion, printing under a cloud negative, and developing. When dry, the pigment can be laid on in its full strength, and the foreground and other features of the landscape printed and developed. Figures can be printed into a scene which lacks them; overlapping is no serious matter to the gum artist, who, with his brush, can correct any anomaly on the wet print.

Where several colours are to follow one another by successive printings and developments a special printing frame will be required in order to secure exact register. Or, instead of the printing frame, the sensitised paper may be laid on an ordinary drawing board, then the negative, face downwards, secured in position by four pins, one on each edge of the glass, and the exposure decided by the actino-meter. If the pins are invariably stuck through the same holes in the paper there need be no fear of any serious irregularity. To enable the paper to withstand repeated sensitising and soaking there must be a good substratum of size.

The Colour Scheme

For a few suggestions on this matter we are indebted to a very successful worker, Mr. M. Richard Witt, who recently read a paper on "Gum Bichromate in Colour" before the Photographic Society of Philadelphia. Mr. Witt suggests three colours: