U of S | Mailing List Archive | alt-photo-process-l | Re: was Miracle size for gum now tonal range

Re: was Miracle size for gum now tonal range

On Oct 11, 2009, at 12:04 PM, Katharine Thayer wrote

The second misconception is that if you keep adding dichromate, you'll get more hardening, resulting in what, a greater DMax? More steps? Or what? (I'm trying to understand how, in your mind, this argument bears on the question of how dichromate concentration affects tonal range). But the fact is that it's been shown in research studies that beyond the point where the colloid has achieved maximum insolubility (I forget how long the exposure was at that point, but not a long exposure) you can keep adding to the reduced chromium by continuing exposure, but it doesn't add anything to the hardening of the colloid. Besides, there's much more dichromate in most of our mixes than is necessary to completely harden the gum, as can be demonstrated by the fact that you get bright yellow (unreacted) dichromate coming off in the wash water after a gum layer is completely hardened.
A glance at a glancing response to the above suggests that this point was imperfectly understood by some readers, and perhaps imperfectly communicated. Here, I was responding to Marek's argument (below) that the more you increase the dichromate the more hardening you get, which is true, but only to the point that the layer is completely hardened. When I asked for clarification about what he thinks you get with increased dichromate, I meant *beyond* that point. Up to that point, of couse, the answer is clear: with more dichromate you get more speed and more steps printed. But after that point I wasn't sure what you get; I don't think you get anything, but I was trying to understand what Marek thinks you would get beyond that point.

That was my point there, and as I said, perhaps imperfectly communicated. But no, I certainly wasn't arguing that in general, within the range of normal practice, more dichromate doesn't get you definite benefits; after all, I use saturated ammonium dichromate precisely *because* I have determined that for me, the combination of speed and longer tonal range suits my needs exactly. But I wasn't sure what it was that Marek was arguing and was asking questions to try to help me parse his argument.

On Oct 11, 2009, at 10:42 AM, Marek Matusz wrote:

I think I have to disagree with this statement. Here is my rationale. Let's think about the chemistry of the gum printing. A photon of light interacts with chromium (VI) reducing it to Cr (III), which binds to gum hardening it. So the more light you use the more Cr(III) you produce and more hardening of gum. The same goes with dichromate concentration, the more dichromate you have the more cr(III) is produced at the same amount of light.