PROCESS Familiarly known as " bi-gum," this process depends on principles first laid down by Poitevin in x855. Briefly, it consists in coating paper with a mixture of gum and pigment sensitised with potassium bichromate solution. This paper is printed under a negative, the bichrom ated colloid becoming more or less insoluble in proportion to the light action. In this way a print may be obtained with a single coating, but it is usual to re-coat the print thus made and again print and develop. This may be repeated almost indefinitely, either for the purpose of reinforcing certain parts of the image, or for producing prints in more than one colour. The paper used must be well sized, in order that the pigment may lie on the surface and not sink into the substance of the paper so as to stain and degrade the high lights. If the paper is not already sufficiently sized a formula suggested by Mummery is 3 to 5 per cent. of gelatine in water with 5 drops of formaline to the ounce. This is brushed evenly over the paper.
The experienced gum worker frequently evolves his own formula for coating the paper. It is best to arrive at this experimentally by proceeding in some such manner as the follow ing : 2 oz. of good dean gum arabic in tears is enclosed in a muslin bag and suspended in 6 oz. of cold water for about two days. This provides the gum solution. Next, a saturated solution of ammonium or potassium bichromate is made. Lastly, the pigment may take the convenient form of moist water colours in tubes. A mix ture may then be made of io parts gum solution, 5 parts bichromate solution, and a quantity of the pigment to be judged always by the length of the " worm " of colour squeezed from the tube. Less bichromate will make the paper less sensitive. The ingredients of the mixture must be thoroughly incorporated by rubbing down on a slab or sheet of glass with a palette knife.
The sized paper is coated with this mixture by means of a camel-hair mop, a hog-hair softener being passed over the surface afterwards in both directions to make it smooth and even. All this must be done quickly before the coating hardens. After about half an hour the paper should be thoroughly dried by heat and placed in a calcium tube if it is to be kept, although it is best to use it as fresh as possible. It must be borne in
mind that the paper is very sensitive to light.
Duration of printing depends to some extent on the composition of the coating. As the image is not visible an actinometer must be used as in carbon printing, or a piece of P.O.P. exposed simultaneously under a negative similar in print ing speed to the one in use. As a rule, the bichromate paper will be sufficiently exposed when the P.O.P. image looks of the right density.
The gum print is now placed face downwards in a dish of cold water. The pigmented gum will soon begin to leave the paper slowly. It is here that the worker begins to exercise that control over the development that constitutes the chief value of the process. He may employ cold or tepid water by laving, spraying, spong ing, or brushing. By such means he retains only such of the pigmented gum as he requires for the rendering of his idea of the subject.
If further printing is contemplated the print is dried and the processes of coating, printing, and developing repeated as before. Here comes in the difficulty of obtaining perfect registration of the second or subsequent images, and some device is necessary for securing that the print shall be replaced exactly in its original position on the negative. Even then there is the expan sion and contraction of the paper to be reckoned with.
When the final development is complete the print is soaked in a 5 per cent. solution of potash alum to remove the bichromate stain, and then rinsed in water.
The use of various papers, the number of pig ments available, the different effects resulting from modifications in coating and development, the power of multiple printings in one or more colours—all these afford opportunity for con siderable exercise of control over the final result. But the very existence of these variable elements precludes the possibility, even if it were desir able, of laying down any hard and fast rules for working the process. The individual worker must gradually formulate his methods by careful experiment and observation, in which case he will ultimately find the process most plastic, interesting, and valuable. (See also " Arabin Gum-bichromate Process.")