629. Kallitype and Imitation Platinum Papers. Soluble ferric oxalate is reduced to the insoluble ferrous oxalate by the action of light. The latter salt is a powerful reducing agent but, by reason of its insolubility, it is unable to reduce silver salts unless a solvent of this salt is added in the form, for example, of a concentrated solution of alkali oxalates or tartrates.
This reaction may be used in three different ways '. The paper is sensitized with ferric oxalate ; the image is developed with a solution containing both the silver salt and a solvent of the ferrous oxalate.
2. The paper is sensitized with ferric oxalate to which a silver salt has been added. The image is developed with a solvent of ferrous oxalate (Kallitype paper).
3. The paper is sensitized with a mixture of ferric oxalate, a silver salt, and a large excess of alkali oxalates. The image is developed in plain water by immersion or by steaming (imitation platinum paper ; E. Boivin, 1891).
We will confine ourselves in this instance to indicating the method of preparation and the use of a Kallitype paper, according to a modi fication (N. C. Hawks, 1916) of the original working methods of W. W. J. Nicol (1890).
A good quality drawing paper is given a supplementary sizing of starch, arrowroot, or gelatine (the paper being floated on a 1.5 per cent paste or solution) ; when dry, it is coated with the following sensitizer by means of a soft brush— Ferric oxalate . . oz. (225 grin.) Neutral potassium oxalate . 500 gr. (55 grin.) Silver nitrate . . 500 gr. (55 grin.)
Distilled water, to make . 20 02. (I,000 c.c.) After drying, it is exposed to daylight until the outlines of the shadows are visible.
According to circumstances, a 2 per cent solution of potassium bichromate is added to the developer given below. This permits of the contrast of the print being regulated as required within wide limits, the contrast being increased as the bath is made richer in bichromate (medium quantity ; 30 ininims per I oz. of bath (6 c.c. per ioo c.c.) ).
Borax oz. (50 grin.) Sodium tartrate . 525 gr. (Co grm.) Warm water, to make . 20 oz. (x,000 grin.) The sodium tartrate may be replaced by Rochelle salt.' The developer prepared in this way gives black tones ; warmer tones may be obtained by reducing the quantity of borax ; the addition of a few drops of phosphoric acid produces a purple tone.
Development may be carried out in two baths, one containing the average quantity of bichromate, the other containing none (or very little). Development is begun in the first bath and continued in the second if the contrasts seem excessive.
After development, wash for two minutes in plain water, and fix in a very dilute solution of hypo (about 3 per cent), made alkaline by the addition of a few drops of ammonia (being extremely dilute, the fixing bath should be renewed very frequently). Afterwards, wash in five or six changes of water, press between blotting paper, and put to dry.
The prints obtained in this way may be treated with any of the toning baths which are used for silver print-out papers.