Baryta is Barium Sulfate. It is made as a finely divided powder and when mixed with water makes a thick paste. It is used in medicine for taking high contrast X-Rays of the intestines. However, it will not coat as is. Therefore, this paste is mixed with a fairly substantial quantity of gelatin in order that it then becomes coatable. This gelatin/water/Baryta mix is coated on FB paper and then subjected to pressure and heat to get a smooth surface.
This gelatin/baryta mix swells and shrinks during the process.
So, you have /gelatin overcoat/gelatin + emulsion/gelatin + baryta/paper/ And everything in this happy family swells when wet and shrinks when dry. If the proportion of gelatin to Baryta is incorrect, small flakes of the Baryta comes off at the edges of the paper in a process called skiving, and during coating, bad FB paper causes severe dust in the coating room.
There, more about Baryta than you ever wanted to know.
I did leave out the fact that Baryta is hardened just like the emulsion layer(s) and it contains spreading agents, antioxidants, dyes and etc.. It can get very complex.