That's probably the reason why when folks find a good batch of cloth, they buy a bunch of it because the next batch may not be the same. :D
Printable View
When I measure fabric I use a caliper rather than a micrometer. I squeeze the jaws with both hands to get my measurements. There is "pillow ticking" and there is also "mattress ticking". Mattress ticking is a bit thicker and, IMO, makes better patches. The "pillow" is about .015" or a little less; "mattress" is about .016" or sometimes + or -. While I still use them from time to time, I mostly use either cottton duck (.019") or my favorite heavy, unbleached canvas (.024"). Snugged in the caliper jaws the canvas measures about .030"; but compressed I get around the .024" mentioned above. For me, the canvas actually seats more easily than the duck. Denim is popular and I have used it at times. But I've consistently had it seat rather grudgingly.
Really, that thick? I've been wearing Carhartt made in usa double layer knee pants to work for years. They certainly aren't short on toughness, as I have some that were worn weekly for 5 years or more, and the outside knees are just starting to wear thin. I've got a couple well used pairs that I've been using for canvas patch material. I never thought to use it in the muzzleloader, as it is at least twice as thick as pillow ticking.
What size ball are you using with those thick patches, and what gun?