Has anyone tried diazo for kallitype contrast control?

I’ve read about using dichromates for contrast control with kallitypes, either added to the sensitizer or developer. I’ve been planning on printing a series of photos as kallitypes, but my college would rather we didn’t use dichromates due to the toxicity. I remembered reading a while back about printers in Europe using diazo for gum printing, and it’s got me wondering whether it could also be used for contrast control in kallitypes. It’s not that expensive, but I figured before I buy some I’d see if anyone has any knowledge on the topic already. The first thing I’m wondering is if anyone knows how dichromates function to increase kallitype contrast. I’m assuming when added to the sensitizer they increase the sensitivity of the chemistry, similar to how they are used in gum printing. But how does adding it to the developer help? Is it a different characteristic of dichromates which make them work in that regard?

Ethan Brossard said:

I’m assuming when added to the sensitizer they increase the sensitivity of the chemistry

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Nope; AFAIK dichromate functions as a toe-cutter, so it effectively it cuts out the low densities in the print. If you combine that with overexposure of the print, you get increased contrast (and a somewhat different curve shape). In my experience at least with Van Dyke Brown, adding dichromate always comes at the cost of reduced dmax in the print and hence a reduction in print quality.

As to the chemistry, I don't think azo would necessarily be a substitute for dichromate in this application because they're totally different types of molecules; I'm not sure if they even harden gelatin through the same mechanism. Anyway, I'd actually be surprised if azo sensitizer for screenprinting would work similarly with kallitypes.

Either way, I guess you'll see my next comment coming: if you can, try to evade the issue altogether by matching the negative to the printing process. In my experience, the outcome of trying to print a negative that doesn't really suit the printing process nearly always results in a compromised result. I played with all sorts of contrast modification measures in alt. process printing (as well as regular printing) and with the exception of carbon transfer, I've come to accept alt. processes as having an inherent contrast that can only be marginally adjusted through things like paper choice, slight modifications in sensitizer chemistry (e.g. tartaric acid in Van Dyke) and toning after printing.

Sorry, I'm not of much help. In case you do end up trying, be sure to post about your results. I'm sure many would be interested to learn about them; I certainly would.

Here dichromate is not working in its photosensivity capacity but as an oxidizing agent. At lower exposures, it acts to oxidize the Fe(2) back to Fe(3), cutting the toe part of the curve, as Koraks said. As far as I know the azo moiety is not oxidizing so it would not be expected to work in the same manner as the dichromate. There are other oxidizing agents and I did some inconclusive experiments with a couple of them - potassium permanganate and hydrogen peroxide with the classic cyanotype. There are also sodium/potassium perchlorates. But I have not seen them used successfully in the literature, so makes me wonder why. Nitrate ion is also oxidizing and said to act like dichromate in the Simple cyanotype by Mike Ware. In that case, the nitrate is formed in situ from reaction of ferric nitrate, citric acid, and ammonia.

You could perhaps try another way to clean up the highlights - selenium or sulfide toning. For example, I toned POP with KRST which increased the Dmax as well as bleached the highlights. KRST didn't work well for salt which bleached both ends, but Kala Namak did. The effect is unique for each paper/chemistry/process combo, so you will have to experiment and see if such an approach works in your case.

:Niranjan.

It should also be noted that if diazo is added to the sensitizer it will leave a fairly ugly stain that would require additional baths for removal which would add yet another variable to be taken into consideration.

If you can't meet your requirements by adjusting your negative, you could try the vandyke brown process instead of kallitype. It's possible to increase VDB contrast quite a lot by replacing some of the ferric ammonium citrate with ferric citrate. (The challenge is finding ferric citrate; I had to make my own.) I find that the practical limit for this is an exposure scale of about 1.5, which is roughly equivalent to grade 1 or 0.