Re: possible answer to archivalness comparison between carbon gum andpt/pd
I am currently making a very thick carbon tissue (weight height of about 1mm) for negatives with a log density range of about 2.7. With the correct sensitizerSome of the 19th century examples I have, and others of that era I have seen, have deepest shadows around 0.075" (nearly 2 mm) above the substrate. Around 0.035" to 0.050" seems more typical in that era, and some are only 0.020" or 0.030". The alt-carbon prints I've seen in the last 15 years or so appear to have less than 0.010" of relief, but I haven't been in a position to measure one accurately. Some commercial tissue I was shown recently looked to have less than 0.020" of gelatin (again, eyeballed, not measured).
I'm ambivalent about the relief -- as I mentioned, I find it distracting if the lighting allows for any specular reflections or shadows of the relief from low-incidence light. I have always believed that the best tonality is achieved by mapping the scene values to the most extended negative values you can obtain, then using a low-contrast printing medium to suit the negative. (I think this is true for pretty much any full-tonal-range medium -- certainly carbon, Pt, kalli, and cyano in my experience). So, I favor fairly thick tissue, but not because it makes prints with lots of relief. I actually stopped making carbon prints for a time after I started making monochrome dye transfer prints, which have a very similar tonality but no relief in the finished print.