Ag,S,0,, 2Na,S0,.
If the hypo is not in excess another double salt is formed.
Ag,S,0,,Na,0,.
This salt is almost insoluble, and is with difficulty eliminated from the film. The importance of having plenty of hypo in the bath will be at once obvious.
After fixing, the plate is thoroughly well washed to eliminate all soluble matter. A clear ing bath of acid and alum is sometimes applied. Before this application, however, it is absolutely necessary that all the hyposulphite be thoroughly eliminated from the film, otherwise it will be decomposed, the following reaction taking place: Na,S,0,+2HC1=11,0+SO„+2NaCI.
If the negative image be too thin for the subsequent manipulation, it is intensified. This is accomplished either by increasing the deposit of silver, or by partial substitution of another metal for it. The chemistry of intensification is given under Intensification (g v.).
If, on the contrary, the negative image is too dense for the subsequent process, it is reduced by the application of a solution of potassium ferricyanide and hypo. The action that takes place here is in all probability the reduction of the ferricyanide (IC,Fe,Cy, ,) to ferrocyanide Fe the oxygen formed, combining with the silver of the image, would, as silver oxide be removed by the hypo.
The negative completed, we have next to consider the methods of making the positive print. The most generally employed method is that known as albumenized paper printing. Ordinary paper is coated over with albumen containing ammonium chloride. When dry this is floated face downwards on a solution of silver nitrate. The reaction that takes place is usually described thus: In addition to the silver chloride formed, free silver nitrate is also present. Further, a combination of the silver with the albumen also takes place, forming albuminate of silver. The principle of the process is this—silver chloride, if exposed to the light, in contact with organic matter, is darkened to sub-chloride, 2AgC1=Ag,C1+Cl.
Chlorine is liberated, which, if silver nitrate be present, combines with its silver, forming silver chloride again, and free nitric acid.
The silver albuminate present is reddened on exposure to the light to a sub-oxide This when acted upon by the hypo is converted into metallic silver and silver oxide, which is to a great extent dissolved. The hypo also removes the silver sub-chloride, leaving a certain amount of metallic silver from it.
Positive prints upon albumenized paper have usually a disagreeable color. For this reason toning is resorted to. It consists essentially in the deposition of gold or other metal upon the silver of the print. The free nitrate of silver is first removed from the paper by washing, other wise gold nitrate and silver chloride would be formed by double decomposition. The toning bath usually consists of a gold chloride solution, and any salt in which a fixed alkali is combined with a weak acid. Acetates and alkaline carbonates are capable of precipitating gold from neu
tral solution in the presence of organic matters. With the acetate bath the following represents the change that takes place: 2AuC1, + NaC,H,0,=Au, +NaC,C1,0,+3HCI.
Phosphate, biborate, bicarbonate, and many other salts of sodium and potassium may be used, the action being similar in each case, i.e., to deposit the gold on the silver particles of the print.
Besides gold, other metals, as uranium, platinum, etc., may be used.
In printing with gelatino-chloride papers the chemical action is somewhat similar, except that a quantity of a citrate is added to a film, forming a compound of silver citrate, and doing away with the free nitrate. Further, gelatine is the vehicle used for holding the sensitive salt in suspension, so that the silver albuminate is not formed. Prints upon gelatino-chloride paper are considered to be more permanent than those made upon albumenized paper.
In fixing prints the chemical change is very similar to that which takes place in fixing the dry plate, and which has already been described. The soluble double salts are formed Owing to the decomposing action of acids upon hypo it is advisable that the fixing bath be kept alkaline.
It is very important that after fixing, the hyposulphite of sodium be thoroughly dissolved from the film. Potassium sulphocyanide may also be used as a fixing agent. It is not superior to hypo, however, and is a violent poison.
The chemical action of bromide paper is the same as with bromide plates. The same kind of emulsion is usually employed, it being spread on to paper instead of glass.
Alpha paper is similar to bromide paper, except that a chloride is used instead of a bro mide, chloride of silver being formed.* In platinum printing the salts of platinum are reduced, that is to say, the metal is set free by ferrous salts, and ferric salts when exposed to light become reduced to ferrous.
The reactions that may be stated to take place are thus represented: + + 6KC1+ 3Pt. Pe, + The carbon process, and the basis of the majority of the photo-mechanical processes, depends upon the sensitiveness of the dichromates to light. In the carbon process the paper is first coated with a solution of pigmented gelatine. This is floated on a solution of potassium or ammonium dichromate. In presence of gelatine these dichromates become reduced by the action of the light, a portion of their oxygen combines with the gelatine, causing it to become quite insoluble. Thus: + CrO, + When hot water is applied the portions unacted upon by the light which remain soluble are washed away, leaving the image in pigmented gelatine.