Although the process of printing in platinum is one of the most beautiful that can be imagined, and has the advantage of absolute permanency, yet it is not used to the extent that might reasonably be supposed. It is the simplest, quickest and easiest printing process, providing one has a good negative, but a good negative is essential. Unfortunately, the bulk of negatives taken on films are thin and under-exposed and, as such, are useless for platinotype. When we have a good, well-developed and clean negative then no process is so satisfactory as platinotype, for no paper where salts of silver are used will give such rich blacks as are obtained with metallic platinum.
Platinotype paper is sold in hermetically sealed tins and the contents of these tins when they are opened should at once be transferred to a storage tin or tube sold specially for the purpose. These storage tins contain a supply of chloride of calcium, a salt which absorbs moisture from the atmosphere and keeps the paper in good condition. The chloride of calcium should be occasionally examined, and if found in the slightest degree soft or moist, replace it by a dry supply. It may be dried by heating in a shovel till red hot and replaced in the tin immediately it is cool.
To secure the most brilliant results the sensitized paper before, during and after its exposure to light must be kept as dry as possible.
Care should, therefore, be taken to see that the negatives are " bone dry," and after placing in the printing frame with the yellow or sensitive side of the platinotype paper in contact with the film, a quite dry pad of rubber sheeting should cover the paper, then a felt pad and afterwards the back of the printing frame. The Platinotype Company point out in their instructions : The effect of damp is seen in a want of vigour, a general muddiness of tone, and, where the sensitized paper has been exposed to its influence for some days, in the impaired purity of the whites.
The paper should never be torn, as there is risk of particles of platinum falling on the paper and causing black spots on the print.
The " black " papers keep in good order for a considerable time if kept in a cool dry place in proper calcium tubes or the unopened patent tins in which the paper is supplied.