wrote:

I'll check into it. I may
be right, an RC additive.


I've checked into it. A lengthy article by Ctein in
Photo Techniques does include paper coatings.

Apparently I'm correct. The "photoactive" compound
titanium dioxide, "whitener", is used with RC papers. Barium
sulfate, baryta, is used with FB. The sulfate does not work
with polyethylene. All as of 1997.
Does one or the two of those wash out? Or as some
other suggested, is something else going on? Or, as I now
suggest, nothing is going on that need concern us.
Dr. Henry's tests with ultraviolet light may have been
no more than curiosity getting the better of him. Did
he compare baryta with the dioxide? That would be
testing RC vs FB.
And that's it; barium sulfate - FB, titanium dioxide - RC.
Dan