wrote:
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.

Those compounds are not "photoactive" as brighteners. They are merely
highly reflective sizing used in the paper to make it a bit more opaque
and to increase the reflectance. They play little or no part in the effect
under discussion he the brightening of the paper by shifting invisible
(i.e., long wave ultraviolat) light into the visible spectrum.

Does one or the two of those wash out?

TiO2 is insoluble in water, so it does not wash out. BaSO4 is nearly
insoluble and does not wash out to any appreciable extent.

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?

Rather than second guessing Dr. Henry's tests, why not just read them? He
writes very clearly. He did not compare Barium Sulphate vs Titanium
Dioxide. There was no need to. He was testing brighteners, now paper
sizing agents.

That would be
testing RC vs FB.
And that's it; barium sulfate - FB, titanium dioxide - RC.
Dan

Which sizing is used in the paper (BaSO4 vs. TiO2) has nothing to do with
this discussion of brighteners.

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