A finished silver print is an image made of fine metallic silver, and metallic silver is chemically vulnerable: it reacts slowly with atmospheric sulphur compounds and oxidants, which is one route to the yellowing and fading seen in old prints. Sepia toning by the bleach-and-redevelop process replaces that metallic silver with silver sulphide, a more stable compound, and in doing so shifts the image from neutral black to a warm brown. The colour is the visible result; the change in chemistry is the substantive one.

The two baths and their chemistry

The process is indirect, working in two separate steps rather than acting on the metallic silver directly. The first bath is a bleach built around potassium ferricyanide and a soluble bromide. The ferricyanide oxidises the black metallic silver, and in the presence of the bromide it is reconverted to silver bromide, a pale, faintly cream-coloured halide. As the bleach acts the image appears to fade almost to nothing, leaving a ghost of the densest tones.

The print is then washed and transferred to the redeveloper, a sulphiding bath that converts the silver bromide to brown silver sulphide. Kodak’s Sepia Toner T-7a, published in the company’s G-23 toning data, uses a bleach stock of 75 g potassium ferricyanide, 75 g potassium bromide and 195 g potassium oxalate per two litres, used diluted with an equal part of water, and a redeveloper stock of 45 g sodium sulphide in 500 mL of water, further diluted for the working bath. Redevelopment is fast, typically completing in under a minute once detail returns.

Sulphide and thiourea redevelopers

Two families of redeveloper are in common use. A 1–2% solution of sodium sulphide is the traditional choice and gives a strong, slightly cold brown, but it releases hydrogen sulphide and carries the characteristic odour of the process. The alternative is an alkaline thiourea bath. Ilford’s toning information notes that the darkening solution in this indirect process can be either sodium sulphide or an alkaline thiourea solution, the latter being odourless. Both demand careful handling, and thiourea in particular is a well-known fogging agent that should be kept clear of unexposed materials. The practical attraction of thiourea is control: the final image colour can be shifted from yellow-brown toward a redder brown by altering the pH of the toning bath, which a fixed sulphide formula cannot do.

Dilution, partial bleaching and split tones

The bleach governs how much of the image is committed to the toner. Bleaching to completion converts the whole tonal scale, producing a uniform sepia. Stopping the bleach early leaves the deepest shadows as untoned metallic silver while the highlights and mid-tones bleach and redevelop to brown. The result is a split-toned print with warm highlights and cool, near-neutral shadows.

A dilute bleach makes this controllable. A working bleach of around 1% potassium ferricyanide with a small proportion of bromide acts slowly enough that the moment to interrupt it can be judged by eye, where a strong bleach would race past the desired point. Thorough washing between the baths is not optional: residual fixer carried into the bleach turns it into a reducer and prevents clean redevelopment.