Digital BW, The Print

There have been numerous threads and discussion about Epson Archival/
Enhanced Matte versus Matte Paper Heavyweight. The consensus is that EAM and 
EEM are identical and contain an optical brightening agent (OBA), a result 
of which these papers undergo some "yellowing" as the OBA degrades with age. 

Matte Paper Heavyweight (HWM) is similar to EAM/EEM, but since the HWM
has a distinctly lower brightness rating than the other two, I have believed that 
it does not contain OBA. But what about its tendency to yellow with age (HWM 
yellowing presumably would be caused by its acidic nature)? Since I can't recall seeing 
anecdotal evidence for yellowing of _any_ of these matte papers, I've assumed the 
process is fairly slow (some period of _years_). 

Well yesterday I had occasion to make a test B&W print using my favorite paper for 
B&W printing, which is Matte Paper Heavyweight. When I compared the fresh HWM 
print with one I had made in August, 2003, I was startled to see that the paper 
(margin) of the 6-month old print was quite noticeably yellowed. The older (August) 
print had been kept in a small pile of loosely stacked prints--shielded from light for 
the vast majority of that brief time. The new print's paper was from a polyethylene 
envelope in a 50-sht box--the same package of paper used for earlier print. When I 
began comparing the new print to other HWM prints made within the past year, it was 
clear that _all_ had lost some "visual" whiteness. I tried to do a side-by-side scan of 
two prints to show the relative change in paper "color", but it was unable to show any 
difference (as measured by Photoshop eyedropper.) But by eye (daylight), the change 
was unmistakable. Since HWM has been in use for several years, I'm surprised I've not 
seen reports of similar (and even worse) changes with this paper. Is this rapid 
yellowing (within few months) actually a well-known aspect of HWM?

Phil