possible answer to archivalness comparison between carbon gum and pt/pd
I was at the library today and came across Reilly's book Care and Identification of 19th century photographs. It was published in 1986, so the information could be outdated, and I bet Gawain Weaver on the list could say what is and isn't still true.
What I found in there was surprising. It said that platinum was "exceptionally" archival, however, the agents we use to develop and clear wreak havoc with the paper and thus the support turns yellow or whatnot even though the pt/pd layer itself is archival. The image is permanent and unchanging even though ghosting occurs--the ghosting is a catalytic action that affects the paper in contact with the print but not the pt/pd layer itself. This catalytic action does, though, affect the paper support itself as well, embrittling it or discoloring it. In other words, the pt/pd metallic layer is not the issue, but the paper it is on is and that is affected by developers and clears as well as the hastened catalytic action of the pt/pd.
Now, carbon (this is what blew me away) is exceptionally archival also, but the main problem is cracking of the gelatin layers where they are thickest--in the shadow areas of the print. Reilly says since the gum layer in the shadows is not as thick as the carbon/gelatin layer, this is seldom an issue with gum prints. And gum prints have "excellent image stability".
So one strike against pt/pd, one against carbon, none against gum...
That's all for now, going to go read my brand new copy that just arrived from Amazon of Christopher James' new book the Book of Alternative Photographic Processes. I see a lot of names in there I recognize!!! :)
Christina Z. Anderson
Photo Option Coordinator
Montana State University