Re: possible answer to archivalness comparison between carbon gum andpt/pd
There are several reasons for this. One is, of course, that cracking occurs over time, particularly in prints that have been repeatedly cycled between high and low humidity, so it is most prevalent in old images. Newer images (i.e., most alt-process work) may not have been subject to cracking yet. Additionally, commercial carbon tissue from the 19th and early 20th centuries was typically made with thicker layers of gelatin with lower concentrations of pigment than is common among alt printers of the late 20th and 21st centuries, so for prints with equal D-Max the shadow areas are much thicker and more prone to cracking.The most amazing thing to me is that Sandy and others, who have no doubt seen far more carbon prints than I, have never seen cracking in the dmax areas. This has been so common in my experience that it often useful for identification. It doesn't always happen by any means, but I have seen more prints with cracks than I could count.
The thicker prints have an interesting look. It does give the impression of depth (of the medium, not the subject), and you can clearly see the relief, most particularly if you look at a specular reflection. The visible relief can be distracting, IMO, unless the display lighting is carefully done. I believe the reason for the change to thinner tissue is mostly because it prints with more contrast, and folks using negative materials from the 1970s and on later cannot easily make negatives with as much density as was commonly done before 1950. Perhaps because of this, I find fewer modern carbon prints with the breathtaking smoothness of tonality that the best 19th and early 20th century examples possess.
BTW, some early workers applied clear gelatin over their carbon prints to fill in the relief, and some also put a gelatin layer on the back of the support to reduce curling.