Normally, I clean the kiln shelf after every firing and reapply kiln wash but I recently had a lot of pieces to make so in hopes of reducing some prep work and speeding up the process I’ve been using thin fire instead. The strangest thing is happening. I fire a project, take it out, vacuum the kiln, put down fresh thin fire, and fire the next project up. When I removed the latest project fired on thin fire out of the kiln it looks like there is a reaction of the previous project on the bottom of the piece (see photo above).
You’re right, the problem is caused by a reaction with residue left over from a previous firing. Most likely, it’s a reaction between the sulphur in the glass and copper residue left on the shelf. That can happen when using thinfire shelf paper. It’s even possible if you scrape kiln wash off the shelf and reapply. Sometimes that works, but more often you need to fire the shelf higher (to at least 1650F/900C to get rid of the fired-on residue.
If you have access to a sandblaster, you can remove the reaction from the piece by blasting off the top layer of glass. You can also use loose grit and silicon carbide to do the same thing, but blsting is easier and quicker. Once you’ve removed the residue, you’ll probably need to re-fire the piece to get the specific look you want.
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