Non-silver Printing Processes.—Robert Hunt was the first to utilize the light-sensitiveness of ferric oxalate in combination with a platinum salt in the hopes of obtaining images in that metal. But the process was a failure because he did not recog nize that the ferrous salt, formed by the action of light, must form a complex salt in order to reduce the platinum salt to the metallic state. W. Willis in introduced the first workable platinum process. The great advantage of this process is that the pictures are permanent, as platinum is one of the most re sistant of all metals. A modification of the process enables a platinum deposit of a sepia colour to be obtained, and for many years sepia platinotype was the favourite printing process for re sults of the highest quality. The high cost of platinum and the ease with which development papers are worked has recently de creased the use of the process.
Vauquelin in 1798 had discovered the light-sensitiveness of sil ver chromate; Suckow, in 1832, observed that the bichromates were reduced by light in the presence of organic matter. Mungo Ponton, 1839, used paper treated with potassium bichromate. Becquerel, 1840, and Hunt, 1843, made improvements; but Fox Talbot first pointed out, 1852, that bichromated gelatine became insoluble in light, and this was patented as a resist for photo gravure, for which it is still used.
Pretsch, Poitevin, Testud de Beauregard, Garnier and Salmon, and Pouncy, 1859, devised carbon processes, in which carbon itself in a finely divided state was used as the pigment in bichro mated gelatine. But they failed to recognize the important point that, as the light acted first on the outer surface of the gelatine, those parts underlying the half tones and highlights would still be soluble ; hence, on developing with warm water, they would have no anchorage to the support and would be washed away. J. W. Swan in 1864 patented the production of carbon or pigment prints by the transfer of the exposed pigmented tissue to a tem porary support, on which it was developed from the back. Prac tically, his process is in use to-day, and its long scale of gradation, the possibility of using pigments of any colour, and the great stability of the pictures are much in its favour, though the neces sity of day or arc-light for printing is against it.
T. Manly, 1905, patented a pigment process in which a de veloped silver print was squeegeed into contact with pigmented tissue saturated with potassium ferricyanide and bichromate. These salts migrated to the silver image, bleached it, and the reduction products wandered back to the tissue, rendering it in soluble in ratio to the amount of silver in the picture, as though the tissue had been exposed to light under the negative. A modi fication of this, known as "Carbro," was introduced some years ago.
In 1905, G. E. Rawlins reverted to an old process suggested by Mariot in 1866. Gelatinized paper is sensitized with bichro mate, dried, exposed under a negative, freed from excess salt, and inked up with a greasy ink, which adheres to only the light affected parts. Somewhat similar is the bromoil process first sug gested in 1907 by E. J. Wall and worked out by Welborne Piper. This utilizes the principle stated by Howard Farmer in 1889 that finely divided silver imbedded in gelatine reacts with bichromates and renders the contiguous colloid insoluble. A bromide print is treated with a hardening and bleaching solution, fixed, washed and dried, then soaked in water, and inked up with a greasy ink, which takes only on those parts where there was silver. These processes have been used particularly by artistic workers, because there is complete control over the gradations and tones of the pictures, and any coloured ink may be used.
There is practically no advance to be recorded in the iron printing processes, which are chiefly used for architects' and draughtsmen's plans, since Herschel's introduction of cyanotype in 1842.
A process in considerable use for copying engineering drawings, etc., depends on the fact that certain diazo compounds are de stroyed by light. Paper is coated with them and developed by a "coupler" which combines with the diazo compound not destroyed by exposure, forming a positive dye image.