SECTION I.. _chemistry of the Various Substances Employed As Developers._

Chemistry of the various Substances employed as Developers.

Development is essentially a process of reduction, or, in other words, of deoxidation. If we take a certain metal, we can, by means of Nitric Acid, impart Oxygen to it, so that it becomes first an Oxide, and afterwards, by solution of the Oxide in the excess of acid, a salt. When this salt is formed, by a series of chemical operations the reverse of the former it may be deprived of all its Oxygen, and the metallic element again isolated.

The degree of facility with which oxidation as well as reduction is performed, depends upon the affinity for Oxygen which the particular metal under treatment possesses. In this respect there is considerable difference, as may be shown by a reference to the two well-known metals, Iron and Gold. How speedily does the first become tarnished and covered with rust, whilst the other remains bright even in the fire! It is indeed possible, by a careful process, to form Oxide of Gold; but it retains its Oxygen so loosely that the mere application of heat is sufficient to drive it off, and leave the metal in a pure state.

Silver, Gold, and Platinum all belong to the class of noble metals, having the least affinity for Oxygen: hence their Oxides are unstable, and any body tending strongly to absorb Oxygen will reduce them to the metallic state.

Observe, therefore, that the substances employed by the Photographer to assist the action of the light, and to develope the picture, act by removing Oxygen. The sensitive Salt of Silver is thus reduced, more or less completely, in the parts touched by light, and an opaque deposit results which forms the image.

These remarks do not apply to the vapour of Mercury employed as a developing agent in the Daguerreotype. The chemistry of that process will be explained in a separate Chapter.

The most important of the developers are as follows:-- Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron.

CHEMISTRY OF GALLIC AND PYROGALLIC ACIDS.

a. Of Gallic Acid.--Gallic Acid is obtained from Gall Nuts, which are peculiar excrescences formed upon the branches and shoots of the Quercus infectoria by the puncture of a species of insect. The best kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall Nuts do not contain Gallic Acid ready formed, but an analogous chemical principle termed Tannic Acid, well known for its astringent properties and employment in the process of tanning raw hides.

Gallic Acid is produced by the decomposition and oxidation of Tannic Acid when powdered galls are exposed for a long time in a moist state to the action of the air. By boiling the mass with water and filtering whilst hot, the acid is extracted, and crystallizes on cooling, on account of its sparing solubility in cold water.

Gallic Acid occurs in the form of long silky needles, soluble in 100 parts of cold and 3 of boiling water; they are also readily soluble in Alcohol, but sparingly in Ether. The aqueous solution becomes mouldy on keeping, to obviate which, the addition of Acetic Acid or a drop or two of Oil of Cloves is recommended.

Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts with the alkaline and earthy bases, such as Potash, Lime, etc., but not with the oxides of the noble metals. When added to Oxide of Silver the metallic element is separated and the Oxygen absorbed.

b. Pyrogallic Acid.--The term pyro prefixed to Gallic Acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a white sublimate forms, which condenses in lamellar crystals; this is Pyrogallic Acid.

Pyrogallic Acid is very soluble in cold water, and in Alcohol and Ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue colour with Protosulphate of Iron, which changes to dark green if any Persulphate be present.

Although termed an acid, this substance is strictly neutral; it does not redden litmus-paper, and forms no salts. The addition of Potash or Soda decomposes Pyrogallic Acid, at the same time increasing the attraction for Oxygen; hence this mixture may conveniently be employed for absorbing the Oxygen contained in atmospheric air. The compounds of Silver and Gold are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the reducing agent absorbing the Oxygen, and becoming converted into Carbonic Acid and a brown matter insoluble in water.

Commercial Pyrogallic Acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as Metagallic Acid, which is formed when the heat is raised above the proper temperature in the process of manufacture.

CHEMISTRY OF THE PROTOSALTS OF IRON.

The combinations of Iron with Oxygen are somewhat numerous. There are two distinct Oxides which form Salts, viz. the Protoxide of Iron, containing an atom of Oxygen to one of metal; and the Peroxide, with an atom and a half of Oxygen to one of metal. As half atoms however are not allowed in chemical language, it is usual to say that the Peroxide of Iron contains three equivalents of Oxygen to two of metallic Iron.

Expressed in symbols, the composition is as follows:--

Protoxide of Iron, Fe O. Peroxide of Iron, Fe{2}O{3}.

The Proto- and Persalts of Iron do not resemble each other in their physical and chemical properties. The former are usually of an apple-green colour, and the aqueous solutions almost colourless, if not highly concentrated. The latter, on the other hand, are dark, and give a yellow or even blood-red solution.

The Protosalts of Iron are alone useful in Photography; but the following experiment will serve to illustrate the properties of both classes of salts:--Take a crystal of Protosulphate of Iron, and, having reduced it to powder, pour a little Nitric Acid upon it in a test-tube. On the application of heat, abundance of fumes will be given off, and a red solution obtained. The Nitric Acid in this reaction imparts Oxygen, and converts the Protosulphate entirely into a Persulphate of Iron. It is this feature, viz. the tendency to absorb Oxygen, and to pass into the state of Persalts, which makes the Protosalts of Iron useful as developers.

There are two Protosalts of Iron commonly employed by Photographers: the Protosulphate and the Protonitrate of Iron.

a. Protosulphate of Iron.--This salt, often termed Copperas or Green Vitriol, is an abundant substance, and used for a variety of purposes in the arts. Commercial Sulphate of Iron however, being prepared on a large scale, requires re-crystallization to render it sufficiently pure for Photographic purposes.

Pure Sulphate of Iron occurs in the form of large transparent, prismatic crystals, of a delicate green colour: by exposure to the air they gradually absorb Oxygen and become rusty on the surface. Solution of Sulphate of Iron, colourless at first, afterwards changes to a red tint, and deposits a brown powder; this powder is a basic Persulphate of Iron, that is, a Persulphate containing an excess of the oxide or base. By the addition of Sulphuric or Acetic Acid to the solution, the formation of a deposit is prevented, the brown powder being soluble in acid liquids.

The Crystals of Sulphate of Iron include a large quantity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly anhydrous, in which state it forms a white powder.

b. Protonitrate of Iron.--This salt is prepared by double decomposition between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an unstable substance and crystallizes with great difficulty; its aqueous solution is pale green at first, but very prone to decomposition, even more so than the corresponding Sulphate of Iron.