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gold, color, silver, toning, print, image and chloride

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THIOSINAMINE (Formula CS NH, H,, Synomyns orless and odorless crystals easily soluble in water and alcohol. Liesegang, in 1883, pointed out the value of the substance as a fixing agent. It was, however, very expensive. Recently the price has been very much reduced. It is also valuable as a clearing agent for chloride emulsion papers and plates.

Dr. Bogisch pointed out that the brown stains especially resulting from the use of an organic developer—a pyro-developed one for instance—could be readily removed by immersion for ten minutes or so in a solution composed of a thousand parts of water, twenty of thiosinamine, and ten of citric acid.

It is also claimed that with this substance discolorations produced by the use of certain developers or old fixing solutions can be easily removed. In using the ferrous oxalate developer for bromide paper, it is well known that an acid clearing bath is necessary to prevent yellow dis colorations. A small quantity of thiosinamine and acid has this effect, with the additional advan tage, it is said, of being a permanent one. The directions given for its use are as follows : The negative or paper print (bromide or chloride silver plates and papers) are previously fixed with hypo, and after the hypo has been thoroughly removed by washing, the negative or paper print is put into one of the following solutions until the discoloration or stain has disappeared: process of changing the color of the silver print or other kind of positive from an unpleasant to an agreeable tone. The color of a silver print upon albumenized paper is usually of an offensive red, which it is very necessary to alter. The operation in toning usually consists in changing the color by partial substitution of gold, platinum, or some other metal for the silver of the print.

It is true that if a silver print be merely fixed in an acidified fixing bath of sodium hyposul phite, an agreeable brown tone will be obtained, for the reason that the sulphur precipitated by the action of the acid would adhere to the image, but this method of sulphur toning, although practiced in the earlier days, is now abandoned on account of the inferior permanency of the results.

The principal method of toning is by means of an alkaline solution of gold. Platinum is also finding much favor as a toning agent.

The accepted theory of the gold toning process is that the gold is deposited from an alka line solution of its terchloride in a layer over the reduced silver. Gold is very easily reduced from its salts to the metallic state. When, therefore, the silver image is immersed in the solution of chloride of gold, the silver having a great attraction for the chloride, the gold parts with it willingly, and it goes over to the silver to form silver chloride and the dark sub-chloride where the silver has been reduced by the light. Now the gold having parted with its companion, natu rally falls down as a precipitate of metallic gold depositing itself upon the image. The color of the gold precipitated is regulated by the rapidity of its precipitation. A very slow precipitation gives fine particles of a reddish color, whereas quick precipitation gives coarser particles of a bluish color. The color of these particles gives the color of the image, consequently by regula ting the precipitation we can get a variety of agreeable tones between the two extremes of red and blue. It will be obvious from this that the color of the image is no guide to the amount of gold in it. It is stated upon good authority that the greater the amount of gold contained in the print, the much greater the permanency of the image; it therefore follows that it is not a good plan to be too economical with the gold salt.

It should also be remarked that the variety of color differs very greatly with the kind of bath used with different brands of paper, and also to a certain extent with the quality of the negative.

The prints are prepared for the toning process by well washing in several changes of water to remove all soluble matter, except with toning baths containing chloride of lime and sulphocy anides, as these apparently require a free silver before any action will take place. Some opera tors prefer to place them after washing in a very weak solution of common salt—about a pinch to the pint. This gives the prints a redder color, and makes the color change in toning more obvious. A preliminary bath of sodium carbonate gives more purple tones; it is also very neces sary with " preserved " ready- sensitized papers, which usually contain a large quantity of free acid, and which it is necessary to neutralize.

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