Good linen is a bit hard to find, most bed "linen" is actually cotton or cotton synthetic blend. I bought an assortment of linen fabric online and of the four I bought only one was a good tight weave and that measured .020" thickness. However, I soon found that linen does not compress as much as cotton so that the loading effort, ie: tightness, of the .020" linen canvas was very similar to cotton canvas of .028" thickness. Linen was the preferred material for patching back when muzzleloaders were state of the art firearms but you'll need to do some searching and experimenting to find a good linen patch cloth today.
