I hear what makes a true baryta paper is something called Barium Sulfate. That's great and all but what is that?

This is how barium sulfate is traditionally used in terms of photographic paper making. For silver halide grades a thin layer of barium sulfate called baryta is first coated on the base surface to increase the reflectivity of the image. The light-sensitive silver halide emulsion is then coated over the baryta layer The baryta coating limits the penetration of the emulsion into the fibers of the paper and makes the emulsion more even, resulting in more uniform blacks. More recently, baryta has been used to brighten papers intended for ink-jet printing. More specific to Legion, the barium sulfate on our Juniper Baryta Rag is ‘embedded’ into the coating layer of the paper