I've not heard of clay content in paper but also suspect it might be abrasive.

I have used commercial high concentrate "cotton rag" paper for patching since the 70's - seems to me at the time I bought a huge supply, it was from Trackofthewolf.

Symplicity patterns use an onion skin paper which is about the thinnest available - .0015" I think.

In the States, Buffalo Arms sells or used to sell several different thicknesses of paper specifically for patching cast bullets.