U of S | Mailing List Archive | alt-photo-process-l | Re: haunted GUM (update)

Re: haunted GUM (update)

Okay, I'm going to backpedal a little here:

On Oct 7, 2009, at 7:32 AM, Katharine Thayer wrote:

phritz, there's no reason why changing to potassium dichromate from ammonium dichromate should cause tonal inversion, as long as you're adjusting the exposure to accommodate the difference in sensitivity. It's the dichromate ion that participates in the reaction that hardens the gum, and that doesn't change;

In general and for most practical purposes, the dichromates are considered roughly equivalent at somewhat equal concentrations, with some wiggle room; the exactly equivalent concentrations aren't exactly the same concentration in the two dichromates, but close enough that it doesn't matter for this discussion. However, I was technically incorrect in saying that only the dichromate iion is involved in the crosslinking and so the reaction is the same whichever dichromate you use. Fairly recent research published research on the crosslinking of PVA, validated by a chemist I was working with a few years ago, demonstrated that the crosslinking is actually different for the two compounds, at least for PVA; the crosslinking for the ammonium dichromate involves the N in the ammonium ion as well as the Cr in the dichromate ion.

I don't have any idea whether, much less how, that difference in crosslinking might affect the production of a tonal inversion; all I'm saying here is that my first statement that there isn't any reason why changing dichromates would produce tonal inversions was stated with more confidence than was warranted. The fact is that I really can't say with any certainty that the dichromate you use does or doesn't affect tonal inversion.