Alright Niranjan, that's very interesting for sure, but I do have trouble equating fixers to toners. This of course says very little about the interesting effect of apparently stronger bleaching in the highlights than in the shadows; I've never done any measurements on this, but of course overall bleaching is something we're all familiar with in some cases, I guess. I sure am in the case of selenium toning VdB prints, which did indeed severely degrade shadows - but highlights just as well, in my experience.
I only brought up toning because you made a general comment about bleaching:
If silver bleaching were an issue, it would be the shadow detail that would etch away first. Not the 'delicate highlights' as you said, because those aren't delicate, at least not relative to the shadows. So I pointed out that in case of silver bleaching that occurs in toning, the experience is quite the opposite. On the basis of kinetics of dissolution, one can explain why that would be so. In the highlights, particularly in warm papers as Ian pointed out, the particles are much smaller and far apart while in the shadows, they tend to be larger and packed closer together. Because of larger surface-to-volume ratios and faster diffusion, bleaching action is favored in the highlights than in the shadows.
With gold toner (I generally use a gold thiourea toner) I don't recognize this at all; I mostly see a very pronounced contrast boost as toning runs to completion, with an initial contrast decrease but no clear evidence of disproportionally severe bleaching in the shadows.
Yes, gold is quite different. There is only replacement toning and no pronounced competing bleaching action that seems to be present in both selenium and sulfide toners (so far the ones I have tried.) That's why people prefer it for silver based prints. So as you start toning, the highlights get toned first increasing their density and
decreasing the apparent contrast as a result. As toning is continued to completion, more of the shadows come in and the contrast is boosted back up. Here your observation seems to be in agreement with the same controlling mechanism that also explains the bleaching action.
Mind you, if you have a high and a low density patch, I think it's kind of obvious that in any bleaching situation, the high density patch will lose a lot more density in an absolute sense. But that would still happen if the bleaching is proportionate to the initial density, see what I mean?
Not always. As I explained above, kinetics of bleaching action can create disproportional outcome between the shadows and the highlights. When we do bleaching of cyanotypes with an alkali, the highlights start disappearing quite rapidly while the shadows lag behind significantly (particularly if using a weaker base) - something that is exploited by many to get a split toning effect when followed up with a polyphenol toner.
In any case, this is how I try to understand, which I am sure is far more simplified/simplistic(?) than it actually is - obviously subject to change with any new information....
Also, this is far OT from OP's original query. So I will rest at that.
Cheers!
:Niranjan.
