After treatment in any normal bleach the toning solution contains
formamidine sulphinic acid. This is mixed with alkali and left for a
while before use, its activity increasing rapidly over an hour or so.
At the moment a kit usiong this for sepia toning is available. The
exact formula is not published. It is claimed to be a better and safger
alternative to traditional sulphide or thiourea (thiocarbamide) toners.
In the UK this toner is available from Silverprint.
Peter
petermarshall@cix.compulink.co.uk