That's a good question Jaro, I always thought it was paper.But when you posted the question I googled it.Seems it can be either glass, paper, cotton, or canvas.
" Phenolic Sheet
Available in 4 grades;
XXX NEMA designation for paper based laminates,
“C”designation for canvas based laminates,
LE designation for fine weave cottonbase laminate
and G10/FR4 for glass cloth laminate. "
Ralph
