... I wonder whether Canson Baryta Photographique would be similar in this regard.
Yes, I have confirmed that CIBP shares the exact same LILIS behavior as IGFS. A couple of weeks ago, I got access to some portable XRF (x-ray flourescence) equipment at the nearby Williamstown Art Conservation Center, and I examined several papers for inorganic material composition. Among various papers I tested CIPB and IGFS, and their XRF "signatures" were also identical. Both papers use TiO2 as a major whitening agent, some BaSO4 as well. While the BaSO4 signal did qualify them as Baryta papers, it should be understood they aren't anywhere near a traditional full Baryta photo paper, IMHO, but they do look good initially, so I can appreciate their popularity. Because neither paper is an RC photo paper, it's the presence of the TiO2 which is an intriguing aspect of these products.
We've discussed CIPB and IGFS similarities several times before in this forum. My best guess is that the image receptor coatings including all subbing layers and layers on the verso are the same chemistry being coated in the same coating plant. The base sheet, however, may come from different sources, i.e., CIBP paper base being supplied by Canson, IGFS using a paper supplied from a different mill. The base sheet differences would add very minor differences to the two products because most of the visual appearance and printing properties will be determined by the coatings.
I've been giving a lot of thought, actually rethinking the whole "color of white" idea for modern media, and also the notion that an OBA containing paper merely returns to its "natural" color when the fluorescence quits. First, the enduser doesn't know what that underlying color actually is without actually placing the paper under full UV block illumination to shut down the OBA fluorescence, but it is likely to land in a "still looks nominally white" media color range. Second, the LILIS effect and other yellowing/discoloration reactions can take the paper to even higher yellowed states over time which is definitely not the original "natural" underlying state. Whether the viewer can detect a problem in a "single stimulus" viewing environment (i.e., there is no unchanged reference print to compare to) appears to lie in the viewer's knowledge and sophistication, but in my research I"m beginning to focus on this perceived "color of white". For nominally "white" appearing papers, modern media seem to go from a b* value of -10 (yup, that's blue) to a very warm white state with b* = +4 or 5 (think Hahnemuhle Bamboo). Any b* values higher than approximately +5 then begin to bring us to creamy or tan colored papers that most people would not judge as white. Likewise, the vast majority of cool white papers that serious printmakers typically prefer will further be constrained on the cool white end of the scale to about -5. So, the b* scale for "white" papers appears to be about -5 to +5. Now here's where accurate versus plausible color gets interesting. if a paper with initial b* at -5 lost fluorescence and perhaps further discolored over time bringing its final b* state to +5, the I* metric would dutifully report that the media white color accuracy had reached zero, but the unsuspecting viewer might still accept the paper as a satisfactorily "white" paper based on the fact that it would still classify as a "plausible" warm white paper just within the -5 to +5 b* range. The printmaker's very carefull initial highlight color image edits would also be shifted by the change in paper color, but that's another story and also one dependent on viewer sophistication as well.
The LIILS effect for the IGFS is not as severe as what I'm seeing in many RC photo type papers, so it looks like the IGFS may end up in the +5 to +7 b* zone,, i.e., significantly changed but still a plausible white color. That said, I need to do more research on the reaction rates. The LIIS effect in IGFS and CIPB can be detected in very short dark storage time intervals (just a few weeks), but I don't know how far it has gone towards completion. My discontinued samples are only a few years old now and have been kept at ordinary room temperature, so where they are now in color isn't necessarily where they are going to remain as more dark storage time accumulates. Some more yellowing may still take place over more time. That's where more study is needed, but one thing is certain, some papers have already shown discoloration in my tests to +10, +15, even up to +25 b* values. Some of these discontinued test samples now look like the color of a creamy manilla folder.
It's hard for me to accept the notion that such poor white point stability constitutes an "archival" paper, yet some of these bad apples are very popular papers today and enjoying those "archival" claims. IMHO, these poorly performing papers really need to be flagged and perhaps even taken off the market or reformulated, so that printmakers who care about print longevity can more easily avoid them.
I think IGFS is sitting right on the debatable threshold of acceptability. I personally don't use it because there are better papers with similar initial whitepoint color that is retained much better over time. Canson Platine and HN PHoto Rag Pearl (my personal favorite in the glossy/luster FA category) come to mind, and there are other FA papers as well not exhibiting any serious yellowing problems in my tests.
best,
Mark
http://www.aardenburg-imaging.com
