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OK, this never happened before! I cleaned a shelf that I hadn't used in years and applied my shelf primer as I always have. I used the same primer 5 days earlier with no problems. I put the shelf in my kiln and fired it as always relatively slowly to 500 f. I also vented the kiln and observed steam/smoke at around 150f. At 465f I gloved up and checked the shelf as it was still smoking. The shelf primer had hardened and peeled off the shelf in small sheets, sort of like a sun burn. The other variable in the mix was the fact that I was firing in an unheated garage with a small monsoon coming down, 42f outside! This hardly ever happened in NJ, but now we are in WA so I'm learning to roll with it. This is one of our larger kilns and I really don't want to bring it inside unless I have to. Any suggestions on what went wrong other than I moved to Seattle, just kidding
As always, thanks!!

Chuck,
I haven't had that issue, but the outside of my skutt has peeled. It is in an unheated, enclosed space. (When I got it I read that that might happen.)

I have been using thin fire for years, but may begin using kiln wash as I run out of the previous. But I have several friends in the area and they have never raised this issue. I will be interested in what more experienced fusers say.

wet shelf. scrape it, put it back in the kiln and dry it out.

Thanks for the replies! I was thinking last night what could have caused this and I think you're right about the wet shelf. As we hadn't fired the larger kiln since our move, I decided to put shelf wash on the base of the kiln as well as the shelf, which was on legs, and fire them both together. I think that the moisture from the kiln floor evaporated up and condensed on the bottom of the kiln shelf, again which was also just washed, but not it's bottom. That's, possibly, where my moisture came from! Bottom of kiln fired fine but moisture trapped in the kiln shelf prevented the wash from adhering.