After washing in several changes of water, the proofs may be toned in any toning bath, and then fixed with sulphocyanide of potassium, washing afterwards in the usual manner.
THE PLATINOTYPE.
This process, discovered by William Willis,[16] yields very fine impressions which wholly consists of platinum and are, therefore, chemically permanent. It has been described theoretically and practically by Pizzighelli and Kübl in a paper for which the Vienna Photographic Society has awarded the Voightlander prize.[17] The following is an abridgment of this important process, as described by the authors:
The paper, calendered or not,[18] is sized with gelatine or arrowroot. The color of the proof with the latter size is brownish black, and bluish black with the former.
To prepare the gelatine solution 10 parts of gelatine are soaked in 800 parts of water and then dissolved at a temperature of 60 deg. C. (140 deg. Fahr.), when 200 parts of alcohol and 3 parts of alum are added and the solution filtered.
To prepare the arrowroot solution 10 parts of the substance are powdered in a mortar with a little water and mixed to 800 parts of boiling water, added gradually in stirring. After boiling for a few minutes 200 parts of alcohol are added and the mixture filtered.
These solutions are employed warm. The paper is immersed for two or three minutes and hung up to dry in a heated room, then immersed a second time and dried by hanging it up in the opposite direction, in order to obtain an even coating.
The potassic platinic chloride is an article of commerce. It should be soluble without residue in 6 parts of water and without acid reaction. In this proportion it constitutes the normal stock solution employed in the various formulas.