Sepia Toning.
Sepia toning is one of the oldest methods used for toning black and white paper prints and was widely used for toning professionally produced portraits at the time that the motion picture industry was developing at the turn of the century.
In addition, lantern slides were frequently sepia toned although whether this was to improve their stability to strong carbon arc light sources or to produce a warmer brown image is not clear. Silver sulphide, the image resulting from toning was certainly more stable than a poorly washed conventional silver image.
The procedure uses two wet solution stages:
1. Bleaching
A solution of Potassium ferricyanide [20 gm], and Potassium bromide [5 gm] in 1 Litre of water was used to bleach the silver image back to silver bromide. At 200C this usually takes about 3 minutes. This is followed by a wash in running water at 20C for about 3 minutes.
2. Sulphiding
A solution of 5 gm per litre of Sodium sulphide was used to convert the silver bromide to silver sulphide. Silver sulphide is normally dark brown or black but, perhaps due to other reactions taking place or to impurities the image is often a strong red-brown. The process is easily carried out using modern release stock but is unpleasant and unsafe unless great care is taken to prevent the formation of Hydrogen sulphide gas, which is readily produced if the sulphide solution becomes acid. Attention to safety precautions is essential and it should only be done in areas of good extraction. Hydrogen sulphide is toxic and levels of its concentration in the air must be kept at levels lower than the current TLV. The process takes about 2 minutes and is usually followed by a water wash of about 5 minutes.
The procedure was originally carried out in large tanks with the film wound round wooden racks holding from 100 to 300 ft of film. It would be possible to carry out the procedure in a modern processor if the tanks were made of rigid PVC, or titanium, and no iron was present anywhere in the equipment. Prussian blue staining occurs where ferricyanide bleaches are used in conjunction with iron parts and this was a common problem in the past. Tanks were made of ceramics to avoid the problem. However since it requires about nine different processes to produce a similar result to most original films it is rarely practical to convert a processing machine to tint or tone, and instead, batch processing using rolls of film in short lengths in small tanks is more realistic.
Sepia or Sulphide Toning must be carried out as a two-stage process with washing between and after. Sometimes a "clearing" stage to eliminate some of the hydrogen sulphide released was inserted halfway through the last wash. This was a dilute Potassium permanganate solution.
Many of the other metallic toning processes could be carried out in two stages - a bleach as before followed by the toning solution containing the metal salt to replace the silver, or, more commonly, in a single solution consisting of potassium ferricyanide and the replacement salt in a suitable solution. The results are not quite the same in each case, either in terms of colour or chemical composition. The single solutions are not as stable or long lasting as two separate solutions with a rinse or wash between them. Some of the single solutions probably could not be kept for more than a few hours. However, although the textbooks and manufacturer's manuals list both, sometimes as alternatives, sometimes as recommendations, it is not really known which were used most. Probably most toning was done in batches using single solutions discarded after use, so that a long life was not required. Most of the manuals of the 1920's imply that single solution toning was almost universal.