ARABIN GUM•BICHROMATE PROCESS A gum-bichromate printing process, worked out by Nelson K. Cherril, and published in June, 19°9. To prepare the arabin, a quantity of best Soudan gum arabic is sifted through a 40-mesh sieve. Place in a quart earthenware jar 15o ccs. of water, 7 ccs. of pure hydrochloric acid, and then sift into the mixture, stirring the while, ioo g. of the powdered gum. Keep the whole at about Iz0° F. (about 49° C.), and stir frequently until solution is complete. Cool and add 600 ccs. of the best methylated alcohol, free from petroleum, and stir for half an hour or so, or until the arabin is thrown down as a white precipitate and has lost all stickiness or gum miness. Filter through two thicknesses of cheese-cloth, gather the arabin in the cloth into a ball, and squeeze it well, place it in a small jar, cover with new spirit, stirring it and break ing it up well, and leave for several hours until the spirit has absorbed all the water. Squeeze again in cheese cloth, then put the arabin in a towel and squeeze it in a press, with as heavy pressure as possible. Break up the cake formed, to allow the remaining alcohol to evaporate, with gentle heat ; then break the remaining lumps in a mortar and dry until all is a dry, gritty powder. The formula in the English system would be roughly as follows :— Water . . . . . 5f oz.
Hydrochloric acid . . . 127 mins.
Powdered gum . . 1540 grs.
Methylated alcohol . . 21 oz.
To prepare synthetic gum, take zo g. of arabin, 20 g. of heavy magnesium carbonate, and from 4o ccs. to 75 ccs. of water, according to the thickness of the solution preferred for coating. This formula in English is — Arabin . . . . 308 grs.
Magnesium carbonate . . 308 „ Water . . . . . ri to zi oz.
When mixed, stir occasionally until the froth subsides, then filter through muslin.
To prepare the pigment, lampblack is used ; wash it with repeated doses of mixed ether and acetone until all fatty, gummy matters are removed ; or, preferably, burn small pieces of camphor slowly under a piece of porcelain—say the bottom of a porcelain developing dish. Scrape off the soot with a palette knife into a test-tube and wash with mixed ether and acetone until these solvents come away with only slight discoloration. Pour off as much as possible without losing the black, and dry by stirring the test-tube in hot water, keeping the water from the pigment. When dry, the
tube is inverted, and the black will fall out freely. A special lampblack, known as No. 4, has been prepared for this particular process. To mix the gum and pigment for coating upon paper, it is necessary to experiment with the particular paper to be used, taking a normal temperature —say 95° P.—for the developing water and a normal time—say forty-five minutes—for devel opment. The mixture must be such as will just soak clean from the paper in the development time. With too little gum the pigment soaks into the paper ; with too much, it washes away before development is complete. In practice it is best to make up an under-gummed and an over-gummed ink, and experiment with these will show the proportions for any paper. For instance, Cherril recommends the making of one ink containing gum in the proportion of 20 arabin to 75 water, and the other in the proportion of 20 to 45 of water. If both these are pigmented in the same proportion as to quantity—that is to say, about 40o to 5oo mm. of lampblack to each to ccs., the one will be found to give too much penetration to Joynson's or Rive's paper, and the other too little ; a mixture of the two will be found to give a good result. The mixture is sensitised just before use by an addition of an equal volume of bichromate solution made by adding i5 g. (23o grs.) of ammonium bichromate to ioo ccs. (3i oz.) of water ; dissolve by heat, and neutralise by stir ring in a little chalk, decanting when effer vescence ceases and the solution settles. The paper to be used is brushed over thinly with the freshly-mixed gum and bichromate, the brush marks being obliterated by crossing and recross ing the strokes. After drying, the paper is ready to be exposed. Exposure should be by actinometer after the manner of carbon, and the paper is much more sensitive than the average gum-bichromate paper prepared by other processes. If the development of a print from an ordinary negative is complete in about forty-five minutes in water at a temperature of 95° F. (35° C.), the result will be perfect. Develop ment may be performed in a vertical tank by floating face downwards on the water, only "controlling" in the usual manner. (For par ticulars of gum work in general, see " Gum bichromate Process.")