I've been waiting for someone with some actual facts to answer this one
(Greg? Are you there?). But meanwhile, let me comment on two points:
First, if there IS a difference between fibre & RC in that respect, my
hunch is it would be more theoretical than meaningful (and as we know,
theoretically, the bumblebee can't fly!). Perhaps the fibre paper
wouldn't be in as close contact with the film emulsion (or the part "in"
the paper wouldn't), but I doubt that would make a visible difference in
a same-size application (though maybe one of those 50 or 100 times
enlargements beloved of photo magazines circa 1980 would show it). If
you tear apart RC & fibre paper you don't SEE a difference in emulsion
thickness (or at least I don't).
Note that if you make a duplicate negative from Kodak direct duplicating
SO339 (what Kodak calls a "second original") you have to either expose the
duplicate through the film base or print through the film base to get the
final result right-reading -- and that's a fairly thick base. I did this
for a friend in a jam when the December sun wouldn't print his thinnish
8x10 camera original the way the summer sun had and collectors were
breathing down his neck (honest!). We were delighted with the miraculous
"save" (I boosted contrast by longer developing) and it wasn't until a month
later that it dawned on me that 7 or however many mm or whatchmacallims
of film base had been inserted between the image and its paper. We hadn't
NOTICED a difference.
Now let it be said, that the print was on a softish paper (cyanotype) and
the whole esthetic was "glow" rather than sharpness attack. A rock-hard
paper might well have shown a loss of crisp.
Which brings me to my second point. I wouldn't use a paper print as
interpositive in the first place for platinum or any project in which the
primary raison d'etre was killer sharpness (as it is, apparently, with
platinum these days though it wasn't in the time of Kasebier &
Stieglitz). In fact, starting from a small-format negative (not from a
print already on hand), all other things being equal, I'd make the
positive on film. For one thing I don't like "straight" silver printing &
don't feel very good at it. (The many negs I made from prints were
because I had a huge cache of solarized , so-called "Sabattier effect"
silver prints I wanted to translate.)
The "literature," by the way, makes this point about a transparency being
more delicate and its scale more controllable than paper. I happened to buy
an old Camera Craft last week for an article on paper negatives, and
there it was (1935) and I remember it from one of
McKay's 1940 books on the negative (AnselAdams & Wm Mortensen weren't
the only ones!) and while I've learned by sad experience never to trust
"the literature" I saw it again in some heavy-duty sensitometry
book or other, and that book inflicted such brain damage I believed
anything it said that I could understand. Fact is, sad but true, a positive
transparency is far more beautiful to LOOK at than a print, though
whether that translates into more beauty two generations later, I dunno.
Yours in acutance,
Judy