Well, this begs the question, so let me ask you specifically - do you know whether Ilford Gold Fibre Silk and Epson Legacy Baryta contain TiO2, and if so, have you been able to test them for determining whether degradation creates uneven yellowing over a given number of equivalent years of say dark storage?
I evaluated the original IGFS and the newer Canson Baryta Photographic amongst other papers a couple of years ago when better funding enabled access to a portable XRF (X-ray Fluoresence) unit at a local conservation lab. I had picked these two media among others for XRF analyses because they are media "look-alikes" (as is Epson's Legacy Baryta), because both papers contain low amounts of OBA, and because they were some of the first non RC media in the Aardenburg database to exhibit this light-induced low intensity exposure problem (.e.g, the post exposure yellowing I have encountered on virtually all RC media). XRF detected TiO2 in both media, and the finding supports my hypothesis that it's the combination of OBA and TiO2 which leads to this post exposure yellowing/discoloration issue.
I'm also trying to research what light intensity level on display suppresses the yellowing by light bleaching. I haven't got a firm number yet, and it may ultimately be somewhat media dependent, but it appears that lux levels below 500 lux on display are low enough that the rate of staining proceeds faster than the rate of light bleaching. So, dark storage is simply the boundary condition for not enough light to suppress the extra yellowing (above and beyond the loss of OBA fluorescence) caused by OBA-TiO2 interactions for prints on long term display.
For more info, check out the notes, whitepoint stability measurements, and the I* curves in the PDF report for ID #223 in the Aardenburg Database. HP Z3200 inks on Canson Baryta Photographique. There are several other samples in the database that documented this problem during testing for those instances where I didn't get to measure the samples soon enough after removing them from the light fade units, and thus the staining and subsequent bleaching effects can be observed in the data. As I mentioned earlier, I'm trying to implement a more formal testing protocol that will routinely check for this problem and correctly factor it into the final test scores. Screening studies to check media whitepoint stability before conducting a full test is also a good approach.
kind regards,
Mark
http://www.aardenburg-imaging.com