black cotton, and no, white is not white. It is not as white, for instance, as the vinyls and polyurethane films I use.
My screen printer's eye can see the black through it, (while not with the Jet Opaque).
On the other hand, adding the extra covering of fabric helped, but as I said before, to me, that is mostly lowering the temp a bit.
My wife gave it her approval, so I'm probably good to go, but just saying that I do see a difference.
I haven't talked to my Neenah man yet, but I suspect that this stuff's lesser opacity is a trade-off for a thinner more supple end product.
Do you use a cloth cover sheet over the paper cover sheet?