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DRY PLATES, FILMS AND PAPERS 197
platinum salt. When the paper is exposed to light, which should be done under a negative in the shade, the ferric salt is changed to a ferrous salt and a slight change in colour results. When it is subsequently acted upon by a solution of potassium oxalate, the ferrous oxalate is dissolved, and at the same time the platinum salt is reduced and metallic platinum deposited where the ferrous oxalate was previously situated. There is absolutely no control with the platinum paper, and therefore it is well to lower the paper face downwards into the developer, as this gives the iron salts a better chance to escape.
After development in this way the print must at once be placed in a clearing bath of dilute pure hydrochloric acid (1 in 60), and it should be passed in succession through at least three different baths of this clearing solution ; the last bath should be quite free from any yellow tinge due to the presence of any platinum or iron salts which have been removed.
The prints must then be carefully washed, and may be dried between blotting paper if desired.
Fine black tones are obtained by means of this paper, and sepia tones can also be produced by several methods — e.g. the introduction of chloride of mercury into the oxalate developer. By such means a fine range of colours is obtainable from a dark rich sepia to a golden brown, but at the same time it should be noted that the permanence of such prints is not so great as those of the black tone. In gold toning, a combination takes place between the gold and the metal present forming the image, but the platinum actually replaces the other metal, so that the image is rendered absolutely permanent.
Cyanotype. — There are some less commonly used processes for obtaining prints which depend upon the action of light upon iron salts. As a rule they depend upon the conversion of a ferric salt into a ferrous salt by the action of light in the presence of organic matter such as paper.