So is the baryta coating then coated with the emulsion, or is the emulsion included in the coating? Or maye some papers one, some papers the other? I reading things both ways.
Depending on the answer, it would give rise to some previous posts saying it's a whitener, some saying its the emulsion surface regardless of whitening, etc.
My assumption has always been it's a base to the emulsion and it just happens to have perfect qualities insofar as costs, lifespan, durability, etc.
The baryta layer was not coated with the emulsion nor was the emulsion included in the baryta layer.
A Kodak made baryta coated fiber based paper in cross-section looked (past tense since they are no longer manufactured) like this:
A raw paper base - fibers with chemical addenda, made to be smooth or with a texture, may or may not have been tinted and/or optically brightened. The raw base was wound into rolls and sent on to the next operation (baryta coating).
Baryta coating -- in the early 1900's up to 6 layers, each coated separately, from the 60's or so, usually one or two layers. Baryta layer composed of barium sulfate, gel (a binder), and additives including dyes and/or optical brighteners if used. The coated paper was usually calendered after baryta coating (a separate operation) to improve the smoothness, sometimes was calendered between baryta coatings. Rolls of coated, calendered baryta paper were sent to the next operation (emuslion coating -- Kodak called it sensitizing).
The light sensitive layer (emulsion) was coated on top of the baryta coating.
Sometimes an overcoat (gel) was applied over the emulsion.
If matting agent was used (to decrease gloss), it was added to the emulsion formulation prior to coating (so it would be incorporated in the emulsion layer). Barium sulfate (the precipitated pigment), ground glass, glass beads, starches, etc were used as matting agents.
So it is correct that the baryta layer was a "base" for the emulsion. However, it didn't just happen to have the properties mentioned but was the pigment of choice since it had those properties - white, chemically inert (non-reactive with the emulsion), resistant to heat and light fading.
I hope this isn't too confounded - let me know if there are still questions.