Ralph:
I have had fully sepia toned (full bleach + redevelop), and almost full selenium toned prints (harder to judge, as there is no bleach step as I used it) subsequently ran through a selenium toner or sepia toner respectively. Toning times for the
second toner were deliberately exaggerated long to ensure any remaining silver would be converted and any effect on existing
toned parts of the images could be judged.
I can assure you:
- a fully sepia toned image (silver sulphide) IS NOT converted to silver selenide by selenium toner, the images remain as they are.
- a fully selenium toned image IS NOT converted to silver sulphide by a sepia toner, the images are again unaffected by the second toner.
I actually ran a whole bunch of tests using different papers (Ilford, Kentmere) and different amounts of double toning of sepia and selenium, going from fully sepia toned, as judged by a full bleach, to almost full selenium toned (judged by eye) with about 6 steps in between, and the results were consistent.
As you also more or less say: it is the
remaining untoned silver and possible remaining bleach products like silver halide or silver ferricyanide that is converted in a subsequent toning bath.
Anyway, this is not a statement about the effects of any other types of toners (gold), I haven't tried it so can't comment, or that silver sulphide and silver selenide are wholly immutable. Like Ian Grant suggests, there might well be some chemical substances capable of attacking both, but another consideration in that respect is if the paper base or gelatine would survive such conditions...
Is it possible to use a thiocarbamide toner as a direct toner?
I see no reason why not, but wouldn't expect much tonal change.
I have attempted this just once in experiments. Any toning effect of a
working strength thiocarbamide / thiourea (same substance, different names), is painfully slow to the point of non-existent without a pre-bleach. With "working strenght", I mean the concentrations as designated by the manufacturers of two bath bleach / redeveloper sepia toners for making the redeveloper solution.
I don't know if a higher concentration solution would possibly make a viable direct toner...