Hey Bob,
Great, thanks for the follow-up. Yes, many colorants would work and there are many ways to create a workable gamut. See e.g. Calvin's more recent work using natural pigments; it's a much more subdued, neutral palette than you'd get with high-chroma industrial pigments, but still (or perhaps, even more so) offers a very natural looking outcome. The creative possibilities are infinite. This is one of the things that has always drawn me to pigment processes.
As to the dichromate remnants: I've done some testing lately with carbon and my results were somewhat disconcerting. I noticed some color and chroma problems in my test prints, reached out to Calvin, who was extremely helpful, and he put me on the right track by pointing out dichromate stain might be the issue. I looked into it further using pointers he gave and did some testing of pure gelatin prints (without any pigment) at low and high dichromate concentrations to get an impression of the stain and how to manage it. Turns out that the stain is virtually impossible to remove, at least from my test prints onto Yupo. With just a wash, the stain remains of a yellow-orange hue, which implies it's still chromium-VI based and quite probably detrimental to print stability (not just the pigment, but possibly the gelatin and the support as well!) in the long term. Extended washing of several hours removes some of it, but not all...Reduction with bisulfite or ascorbic acid does effectively reduce the chromium-VI to chromium-III as evidenced by the hue shift towards a near neutral grey with a hint of lime. But there's still chromium metal left in the print and I'm not sure how problematic it is. The one thing that really 'helped' was to just use as low a dichromate concentration as possible...but evidently this has implications for tonal scale, pigment loading etc.
Here's the blog post I wrote about it a few days ago:
https://tinker.koraks.nl/photography...rbon-printing/
This is an illustration of the dichromate stain on some carbon-without-carbon (gelatin only) prints:
From top to bottom:
Print with 1% dichromate sensitizer; no substantial washing or clearing of the print. Note low level of stain; there's just not much chromium in there to begin with
Print with 16% sensitizer; note heavy staining. Unwashed and uncleared print.
Print with 16% sensitizer after soaking in water for a few hours. Note reduced but still present stain.
Print with 16% sensitizer after vitamin-C clearing bath (bisulfite does the same thing). Note reduced stain that has shifted to neutral hue.
When doing these tests my interests were more in the hue & chroma problems that come with stain and not so much the print longevity issue, so that's what I focused on.
PS: take he Lab* measurements on that blog page with a huge grain of salt as they were made with uncalibrated equipment. If you want I can give you some more accurate measurements made with the i1Pro I received recently.
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