OK, thanks Mark. This is really valuable. Using HN Photo Rag as a reference point for "acceptable" use of OBAs, how would you rate your known samples of Ilford Gold Fibre Silk or Canson Baryta Photographique in relation to that?
I personally have mixed feelings about IGFS and Canson Baryta Photographique. I really love the texture and initial white point, but I don't use it personally because the back coating will not allow me to back print (I'm a big fan of metadata on the verso of my prints). That said, these papers do fit my "yes/low" criterion for OBA content, but that content is likely in one of the subbing layers and not just in the paper core.
One way I can tell these two different brands are more than likely coming from the same coating plant and sharing the same front and back side coating technology (paper base is probably different) is that they have a somewhat unique signature for the light-induced low intensity staining (LILIS) issue I alluded to earlier. The LILIS effect appears rather quickly (in just a few weeks time) after removing the samples from the light exposure unit compared to other media that also exhibit the LILIS effect. Because some of my measurements of the test samples after each exposure interval in AaI&A testing don't always occur as fast as I would like (I can't afford staff to help me always make the report updates in a more timely fashion), some of the tests reports show some dips in the I* curves and then some "recovery" at the next exposure interval when the measurements are made more promptly. The LILIS stain is thus light bleachable, but it returns once the sample is again retired to a dark storage condition. IGFS and CIBP thus show the LILIS effect quickly enough to be detected in my routine light fade testing practices whereas with most other products that exhibit the phenomenon it usually takes much longer (several months as opposed to several weeks) to show up in the retired samples. Consequently, accelerated light fade tests alone don't always give us the full picture of all light induced changes that will occur over time. One has to go back and add in dark storage effects many months after the testing to complete the evaluation (I'm currently working on how best to present these finding of my research, but time is always my worst enemy).
For the CIBP and the IGFS, the visual appearance changes due to the LILIS effect are subtle and not very easily noticed by anyone but the most discriminating viewers. Nevertheless, the additional stain level is measureable, and it causes these products to flirt with the AaI&A Conservation display rating criteria limits in a way where just slightly altering the rating specification would rank this product for better or worse ratings
Anyway, another reason for my mixed feelings about IGFS and CIBP, but I think to keep things in perspective, the LILIS issue is perhaps not very serious with respect to the CIBP and IGFS media for most printmakers, certainly no where near as bad as many RC papers like Epson Premium Luster. EPL is one paper that exhibits the LILIS phenomenon quite strongly in samples I've retired from light fade testing over the past few years...so much so that I would have to put it solidly in the "not recommended" category along with EEF for fine art prints made for "archival" purposes, all things considered.
cheers,
Mark
http://www.aardenburg-imaging.com