493. Ferric Salts, more particularly the organic salts (tartrate, citrate, oxalate, iron carbonyl, etc.), are converted into ferrous salts by the action of light. This property may be taken advantage of in various ways by reason of the very different reactions of these two classes of compounds.
Ferrous salts give a blue precipitate with potassium ferricyanide, whilst no precipitate is formed with ferric salts, and it is possible to obtain a blue image which can be fixed by simply washing in water. Ferro-prussiate papers, based on this reaction, are used industrially in large quantities for making copies of tracings, the " blue-prints " thus obtained consisting of white lines on a blue ground.
Ferrous compounds are capable of reducing the salts of the precious metals to the metallic state, and this property forms the basis of the kallitype process and the platinum process. Such processes give grey or black tones by simple washing in water or by treatment with a solvent capable of dissolving the ferrous salt formed and also the silver or platinum salts.
494. Various animal and vegetable colloids (gelatine, albumen, gum, etc.), when impreg nated with a bichromate and exposed to light, are tanned by the chromium oxide which is formed during the photo-chemical decomposition of the bichromate. Differences of solubility and permeability between the parts exposed to light and those protected from its action may be used to obtain photographs in a large number of ways which have the following characteristic in common : In all cases the image consists of a pigment (colour in powder-form, or ground with oil) or a colouring matter which has taken no part in the reaction (pigment prints).
For example, suppose a finely powdered pig ment is incorporated in gelatine, and the mixture coated on paper ; after sensitizing with bi chromate it is exposed to light under a negative (neglecting for the time being the transfers usually necessary). It is then treated with warm water, which dissolves the unhardened gelatine, gradually revealing the picture, which now consists of pigment incorporated in insoluble gelatine. Whatever the chosen pigment, such papers are known as carbon tissues, since lamp black was the pigment with which they were first made.
A film of bichromated gelatine, which has been washed with water after exposure to light under a negative, is not wetted in those parts which have been protected from light-action. Now greasy inks are repelled by a wet surface, and can adhere only to a dry one, so that inking with a brush or roller will thus form an image consisting of printing ink, which can either be kept as such, or transferred to plain paper (oil prints, transfer prints in greasy ink).
A layer of bichromated gelatine which has been exposed to light under a positive is capable of absorbing dye from a dye-bath in the parts which have been protected from the action of light, as the dye is not able to penetrate those parts which have been rendered impermeable. In this way it is possible to obtain a perfectly transparent picture consisting entirely of stained gelatine (hydrotype).
Another process is as follows : Glass is coated with a thin layer of a mixture of gum-arabic, some hygroscopic substance (honey, glucose, sugar), and a bichromate, and the whole dried by gentle heat. This is then exposed under a positive, and a finely-powdered pigment spread with a brush over the surface. The moisture of the atmosphere, which is absorbed more rapidly on the parts protected from light than on those which were exposed, renders the gum-arabic adhesive again and capable of fixing the powder which has been dusted on, thus forming a positive picture when the excess of powder has been dusted off. This can be permanently fixed by varnishing if an inert powder has been used (e.g. plumbago), or by baking if the colour is vitrifiable (powder or dusting-on process; photo graphic enamels, and photo-ceramics).
495. An image consisting of metallic silver can be converted into a pigment one, or it can be utilized to obtain a pigment image in another layer of gelatine. For this purpose, the silver is caused to bring about a reaction, giving rise to tanning products which can render insoluble the gelatine in immediate contact with the original silver image.