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Scumming On Red Clay


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I pulled some bisque from the kiln and found this scumming on some of the beads made of Highwater Earthen Red clay. Fired to 04 on slow schedule. Terra Sig was brushed on the majority of these pieces, but not all (the one at top with black marks had no TS). I just installed a hood vent above the kiln, I was using crossdraft with fans and open windows before; this is my first firing with the hood/no fans. Could the scumming be from a lack of oxygen in the firing/bad atmosphere, the terra sig, or just bad clay? I also fired a bunch of porcelain extrusions with these beads, don't think that could be relevant to the scumming but if the atmosphere is the likely source should I avoid using the extrusions as well? Thanks for any help you can provide--(first time loading pics, hope this works) Edit: also forgot to add other things I'd changed this firing: cleaned the clay with new sponges that smelled heavily of salt, and new shelves, which I didn't wipe first and didn't apply kiln wash to.

 

Nancy A.

scumming on red clay

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Scumming is common with red earthenware clays, and is usually dealt with by adding barium to the mix. it's been a while since I've dealt with it, but I think it's in the range of 1%. I'm fairly certain Highwater is already doing that with their clay, so the culprit is probably the terra sig. What is your sig made from?

 

Also, but not likely related, are you leaving the top peep hole open during the firing?

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That Redart is the primary ingredient in many terra cotta bodies, so I would add a little barium carb to the sig and see if it helps. Based on some quick Google research I did, it may take as little as 1/10% or as much as 1 or 2%. From a safety standpoint, use as little as you can. Run a batch of tests from 1/10 to 1% in 1/10 increments and see what does the job.

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