Of
all the aspects of alternative printmaking, the least familiar is the
hand coating of the light sensitive sensitiser onto plain drawing paper.
The nearest most photographers will have been to this operation is if
they have used liquid emulsion. This involves applying to the paper a
thin layer of gelatin containing silver bromide. This has to be done in
orange safelight conditions. Exposure is the same as when using conventional
paper, using an enlarger, and processing manually. A large proportion
of the sensitisers used in this manual have the appearance of no more
than coloured water - usually yellow or orange, regardless of the final
colour of the print. You may find it difficult to comprehend how a piece
of drawing or cartridge paper stained with yellow liquid could possibly
produce the rich chocolate brown of a kallitype or the cool grey tone
of a platinum print.
The answer is simple. The action of light on the various metallic salts, in conjunction with the appropriate chemistry reduces them to microscopically small particles of metal, which lodge in the fibres of the paper. In modern photographic papers, the salts of silver are suspended in a layer of gelatin coated onto paper by the manufacturer. In some cases, as in R.C. papers, this gelatin is itself coated onto an intermediate layer of poly-ethylene. Although modern papers are made in glossy, lustre or matt surfaces, none of them can really compare with the smooth velvety sheen of hand-coated prints.