If a slow bromide emulsion is coated upon paper, the material is known as bromide paper and is used for printing, and especially for making enlargements. The less sensitive papers which are commonly used for contact printing by artificial light contain silver chloride in the place of silver bromide.

In order to obtain silver nitrate the first step is to dissolve metallic silver in nitric acid. The silver replaces the hydrogen of the acid and forms silver nitrate, the hydrogen liberated decomposing a further portion of the nitric acid. The silver nitrate is crystallized out of the solution and obtained in colorless, transparent flakes.

Silver Nitrate for photographic use has to be extremely pure, and the metallic silver contains a small quantity of other metals such as copper and lead, from which it is necessary to free it. This is accomplished by recrystallization so that the silver nitrate is finally obtained in a perfectly pure form.

In order to ensure the purity of the silver nitrate which it uses, the Eastman Kodak Company prepares its own, and is the largest maker of silver nitrate in the world, using about one-twenty fifth of all the silver mined in the United States. In order to be perfectly certain of the purity of this vital material, the company even manufactures the nitric acid used for dissolving the silver.

Silver nitrate is very soluble in water, the solution being strongly caustic so that it attacks organic material. Blackening of the skin, wood, cloth, and other similar substances, follows on exposure to light.

When a solution of silver nitrate is added to a solution of a bromide or chloride, reaction occurs and the insoluble silver bromide or chloride is precipitated. Thus, if we add silver nitrate to potassium bromide, the reaction occurs according to the following equation:

Ag NO3 + K Br = Ag Br + KNO3 Silver Potassium Silver Potassium Nitrate Bromide Bromide Nitrate.

The potassium nitrate formed remains in solution, but if the solution is at all concentrated, the silver bromide is thrown down to the bottom of the vessel as a thick, curdy precipitate.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

The bromides and chlorides used in photography are chiefly the salts of potassium and sodium. Both the bromides and the chlorides are obtained from naturally occurring salt deposits, but whereas these deposits consist chiefly of chlorides, they contain only a very small quantity of bromide, and bromide is therefore a very much more expensive material than chloride.

The elements chlorine, bromine and iodine are all obtained from natural salt or from the sea, iodine being usually derived from certain sea weeds which extract it from the sea water and thus make it available in a concentrated form. Chlorine is a yellowish-green gas, very suffocating and poisonous; bromine gives dark red fumes which are even more noxious than chlorine and condense to a liquid, and iodine forms shining, black crystalline flakes which on heating give a violet vapor. The chief chlorides, bromides and iodides used in photography are the following:

Ammonium Chloride

Made from ammonia and hydrochloric acid, should have no smell, and when evaporated by heat should leave no residue behind. White crystals soluble in water.

Ammonium Bromide

Very similar to the chloride, which is the only impurity likely to be present.

Ammonium Iodide

Should consist of colorless crystals. Decomposes in light and is stained yellow by the iodine liberated. Very soluble in water and deliquescent. Soluble in alcohol.

Sodium Chloride

Ordinary table salt is fairly pure sodium chloride and a very pure salt is easily obtained. The pure salt is stable and not deliquescent. Soluble in cold water to the extent of 35 %. Solubility increases very little on heating.

Sodium Bromide

Is a white salt, similar to the chloride but more soluble. Is generally pure but may contain chloride.

Potassium Chloride

White salt, very similar to sodium chloride.

Potassium Bromide

Occurs as colorless cubical crystals and is generally pure. Very soluble in water.

Potassium Iodide

Similar to bromide. Very soluble. May contain as impurities carbonate, sulphate and iodate, but is usually pure. Potassium iodide dissolves iodine, which is insoluble in water, and is therefore used to prepare a solution of iodine.

The gelatine which is used to hold the sensitive silver compound is a very complex substance which is obtained from the bones and skins of animals, and it has some curious and valuable properties. In cold water it does not dissolve but it swells as if, instead of the gelatine dissolving in the water, the water dissolves in the gelatine. If the water is heated, the gelatine will dissolve, and it will dissolve to any extent. It cannot be said that there is a definite solubility of gelatine in water in the same sense as salts may be considered to have a definite solubility. As more gelatine is added, the thicker the solution becomes. If the gelatine solution is heated, it will become thinner and less viscous when hot, and will thicken again when cool, but it will not recover completely. It will remain thinner than if it had not been heated, so that the heating of the gelatine solution produces a permanent change in its properties. If a gelatine solution is cooled, the gelatine will not separate from the solution in a dry state, but the whole solution will set to a jelly. If the jelly is heated again, it will melt, and a jelly can be melted and reset many times. During the treatment there will be produced a progressive change in the jelly, and if the process is continued too long, sooner or later the solution will refuse to set and will remain as a thick liquid.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Eastman Portrait Film Negative, Artura Print By W. O. Breckon Pittsburgh, Pa.

Gelatine belongs to the class of substances which are called colloids, the name being derived from a Greek word meaning "gummy." When a gelatine jelly is dried it shrinks down and forms a horny or glassy layer of the gelatine itself, smooth and rather brittle. This dry gelatine when placed in water will at once absorb the water and swell up again to form a jelly. This swelling of gelatine when wet and shrinking when dry is of great importance in photography. When a photographic material with an emulsion made of gelatine is placed in water, the film will swell up and continue to absorb more water and swell for a long time, finally becoming soft and even dissolving, the extent to which this occurs depending on the temperature and nature of the solution in which it is placed. A small amount of either an acid or alkali will produce a considerable increase in the swelling, and since the developer is alkaline and the fixing bath is acid, both these solutions have a great tendency to swell the gelatine, especially when they are warm. In order to avoid difficulty from this course, gelatine emulsions have a hardener added before they are coated, gelatine being hardened and made more resistant to swelling by the addition of alum. Under ordinary circumstances no difficulty is experienced by the photographer owing to the softening of the gelatine, but when photographic materials are exposed to extreme temperatures, care must be taken in handling them. Hardening agents such as alum must be added to the fixing bath, and all solutions must be kept at the same temperature in order to avoid sudden contractions or expansions of the gelatine, which may result in detaching the film from its support or in the production of reticulation.

Until his ship returns - keep him happy with pictures from home.

Potassium Iodide StudioLightMagazine1919 21Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.