Hypo-Alum Sepia Toner

[Raghu Kuvempunagar]

Some variants of hypo-alum toner have potassium iodide in them. The toner with iodide is said to give warmer tones than without it. How does it do this? I mean, chemically what happens when iodide is present in the toner that results in warmer tones? Would potassium bromide or chloride work in place of iodide?

 

[Raghu Kuvempunagar]

Potassium iodide is a very strong restrainer. Could the reason for its use in hypo-alum toner be to slow down toning and thereby get warmer tones? I recall seeing a thiourea toner formula (thanks to @Ian Grant) that used potassium bromide for the same purpose. But thiourea toners (without restrainer) are fast acting whereas hypo-alum toner is slow. Wouldn't iodide make the latter even more slow?

@Ian Grant: any thoughts on why bromide shouldn't be used in place of iodide?

 

[Ian Grant]

There's no explanation given by Mason on why Potassium Iodide increases warmth, the toner works by forming Silver Polysulphides directly, it's a Toner that matures and improves with age and use, and lasts for years. I made mine in the 1970s

The Iodide will have an effect on the grain size of the Silver Polysulphide, it's possibly inhibiting this slightly as warmer tones are finer grained.

With Sulphide or Thiourea toners the choice of halide in the bleach affects the colour by controlling the grain size, Iodide is the coldest, rarely used, Bromide is the norm, but for the warmest tones Chloride is the best.

Ian