Tuesday, June 3, 2008

The Gum Bichromate Process With A New Colloid.

[The following is the extended account, as published in the Journal of the Royal Photographic Society, of the experiments made by H. S. Starnes and the subject of the paper read by him some time ago before the Society, Mr. Starnes, it will be seen, employs, instead of gum arabic, gum Senegal and prepares this latter gum upon the paper in an acid condition. In this respect the process bears some resemblance to that of Mr. Nelson K. Cherrill published some years ago in which arabinic acid of gum arabic was previously separated by treatment o! the gum with acid. EDS., " B.J."]

           He considered that the bichromate printing process was an ideal one, for the following reasons:-1st. There is an almost unlimited range of colours; 2nd, if suitable pigments are used there is no doubt about their permanence; 3rd, there are no such things as double tones; and, 4th, there can be the same surface of paper as in an engraving. On the other hand, the gum-bichromate process will not give the same fineness of grain that a silver print does, but except for small portraits the grain will probably be fine enough.
           In all types of bichromate printing the principal difficulty is in removing only the soluble parts not acted upon by light. In Sir Joseph Swan's original carbon process the film of gelatine and pigment was transferred to a temporary support and the soluble matter was washed away from the back quite satisfactorily, but it required double transfer and warm water. Then came the Artigue process, which did away with the double transfer, but had to be developed with wet sawdust, and frequently the lighter detail would not stand the friction, and was washed away.
           Shortly after the advent of the Artigue process the lecturer worked out a process which Sir Joseph Swan told him was the first real advance in bichromate printing since the original patent. The exposed print was soaked in water for a minute or two and laid face upward on a piece of glass, a piece of dry blotting paper was laid over it, and a soft clothes brush was brushed over the back. The soluble part of the film was taken up by the blotting paper, and the insoluble portions forming the image, especially the lighter tints, were pressed down into closer contact with the paper support. Sir William Abney had suggested to him that possibly that method of development might give a suitable grain for process work. The lecturer had no experience of process work, but was pleased to pass on the suggestion to anyone who could make use of it. Although the blotting paper preserved the light detail, still the lecturer had the same trouble as those who tried to revive Pouncey's method of printing very deeply and removing the soluble portions with a wet brash - there was no dependence on the condition and solubility of the gum arabic.
           When the bichromate printer was an artist he could remove what he liked and leave what he pleased, but the ordinary worker lacking that ability was likely to give the process up in despair. He felt, however, that there was something in the process if he could get a more suitable colloid than gum arabic. He had gradually worked his formula down to the point that to get the best results the paper must be coated so thinly that one minim of the combined mixture of water, gum, pigment, and bichromate must cover four square inches of surface. The brush, the sawdust, and the nozzle of the garden hose were all too brutal in removing the pigment, and blotting paper was too expensive. After experiments, he came to the conclusion that a straight tube about 3 in. in length, with a bore about the same as in a tobacco pipe, and fitted to the domestic water tap, answered well, as the force of the abrasion could be controlled by varying the supply of water. When he wished to concentrate the action of the water upon small areas he used smaller nozzles, which fitted over the first one. There was then pleasure and interest in working on every square inch of the picture.
           In searching for a suitable paper he discarded most of those used by the bichromate printers for one reason or another. Cartridge paper and the cheaper drawing papers allowed the coating to sink in unequally. Whatman's paper, in addition to being expensive, allowed the pigment to sink in to some extent, which, although just what the water-colour painter wanted to prevent washing up, was not the property wanted when using the bichromated solution. Some pre-war note-papers were better, and he found a paper used in collotype printing which was at first excellent, but later samples were of inferior quality. He was now using a foreign paper obtained from Spicer Bros., and would send a sample to anyone who sent a stamped directed envelope to him at King Henry's Road, Lewes.
           Some of the pigments he had tried contained a proportion of dye, which stained the paper. Messrs. Brooke, Simpson, and Spiller had made him a stock of suitable pigments. He used a carbon black modified with blues, browns, or reds, as required.
           He found that refined sodium bichromate worked better than the other bichromates, and got a good sample from J. J. Griffin, Ltd. The stock he has was made in Germany, but he hoped that English firms would now make it.
           Nine-tenths of the trouble in bichromate printing arose from the varying characteristics and conditions of the colloids used. The conditions in which gum arabic is collected and stored make it hope less for the purpose. He, found that he might get passable results with it from one negative, yet could do nothing at all when working from another, even though the paper in use was cut from the same coated sheet. The problem had bothered him for years.
           He had to make a rather startling statement, which wag that wider certain conditions the action of light makes a bichromated colloid soluble instead of insoluble. This would explain why the readings of the actinometer were not always reliable, and why one f negative would give better results than another. The action seemed to be as follows: - When the bichromate is added to the colloid it renders the latter more or less insoluble at once. On exposure to light it slowly becomes quite soluble, and after it has reached that stage it begins to get insoluble as under normal conditions. Different samples of gum work differently with regard to the length of time for those two actions to take place. Freshly-made solutions are more prone to act so than solutions that have been kept for some time.
           At first he thought that different samples differed as slow and rapid plates do; then he found that there must be two actions going on simultaneously, because the parts of the print under the densest parts of the negative were darker than under the half-tones, while under the lightest (or clear glass) parts of the negative the prints seemed to print normally, but not with the density that the amount of pigment ought to have given; so evidently some of the gum was not holding the pigment on to the paper. One day he got a print that had black sky. The trees in the distance looked as though they were covered with snow, and the shadows, which ought to have been the darkest parts of the print, were simply Half tones. That print gave him a clue to the mystery. He showed a print which at the first stage of exposure was a negative from a negative; another showed the two actions going on simultaneously during the same exposure to tight, and another which had first gradually been rendered soluble until, by the time it was soluble under the densest part of the negative, the other parts had again become insoluble in their proper sequence, and it was becoming a practically normal print. He was inclined to think that the first stage was purely a physical one, that the bichromate had bound up the colloid too tightly, and that the first thing the light had to do was to unfasten the straps, if he might use that phrase.
           About thirty years ago he 1had written an article for THE BRITISH JOURNAL OF PHOTOGRAPHY, in which be gave a somewhat similar explanation of the action of light on a dry plate.
           He had searched through many books on bichromate printing to see if a double light action of this kind had bam observed, bat no one seemed to nave noted the phenomenon The matter was one of considerable importance, as it accounted for a very common fault in prints. It was often found that bichromate prints break down in the rendering of the lightest tints, because under the densest part of the negative the film, instead of being made insoluble for the lightest tints of the print, would be undergoing just the reverie action, and would be made even more soluble, and would wash away more easily, so that it appeared as though the print had been under exposed. The lecturer had made many experiments in the hope of finding some method of keeping the film of bichromated pigment soluble, so that the light action should be restricted to its legitimate function of insolubilising it according to the gradation of the negative. It was not until be had worked out the following formula that he began to see daylight

           A. – Sodium bichromate ……………………………… 1 oz.
                  Water ……………………………………………… 2 oz.
           B. – Alum, saturated solution ………………………… 4 oz.
                  Hydrochloric acid …………………………… 2 drams.

           Take one pert of A to three parts of B. If with a certain sample of gum this makes the film too soluble, then reduce the amount of B.
           Having found gum arabic to be unsuitable, he had tried a number of other colloid, and finally hit upon gum senegal. As an adhesive it was probably not satisfactory, but for printing purposes it works much better than gum arabic, being softer, less brittle, and more under control. It contains 81 per cent, of arabin, as against 70 percent, in gum arabic.
           To make the gun solution a quantity of it is suspended in a bag or ware cage is pint water, and as it dissolves more is added until one fluid once weights 9 drachms. To measure the pigment a salt spoon is used which will hold just 20 minims of water, and four spoonfuls of the pigment (strike measure) are taken the precise quantity nay vary according to the covering power of the colour, and is ground up with 1 drachm of methylated spirit. This is added to 1*(1/2) drachms of water, mixed thoroughly, and placed in a test-tube to settle for a few minutes. One and a-half drachms of the gum solution is taken, and to it is added about three-fourths of the pigment solution, care being taken not to shake up the coarse sediment at the bottom of the test-tube, and the whole is mixed well together. The pigment that had settled at the bottom of the test tube should be re-ground and added to the mixture. To that is added 1 drachm of the sensitizing solution, consisting of 15 minims of solution A and 45 minims of solution B. In winter, solution A may be increased to 20 minims. The quantities given will coat about twelve pieces of paper 10 in. x 8 in.
           To coat the paper it should be fastened by one corner to a sheet of glass or zinc, supported if necessary on a wooden board, which in turn rests upon a penny laid on the table, so that the device may be revolved easily in any direction. A spoonful (20 minims) of the mixture is poured along the length of the paper from left to right at about 1/2 in. from the edge. The colour should be spread evenly over the paper with a 2-in. varnish brush with light strokes, turning the turntable as required, but always keeping the brush flatwise on the paper. If the brush is turned edgewise streaks will appear. With a little practice it will be found quite easy to coat 4 square inches of paper with 1 minim of the mixture.
           The coated paper should be placed about 2 ft. from a fire or gas stove, and by the time the second piece of paper is coated the first ill be dry. After use the brush should be cleansed with water and a nail-brush and dried thoroughly before it is again used.
           The exposure is about one-eighth of that required by P.O. P., and is gauged by an actinometer.
           For development the print is soaked in water for abort a minute and then flooded with solution B (acid alum) 1 drachm, water 2 oz. If the exposure has been correct the colour in the high lights will be sera to float away in a few seconds. The print is then put on a glass easel in the sink and development completed with the aid of the rubber tube and nozzle device previously described. More control over the print is obtained by giving a longer exposure and by using the acid alum solution in a stronger condition.

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