Cracking/flaking unfinished crock pot

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Cracking/flaking unfinished crock pot

Postby kioke on Tue Mar 31, 2020 6:35 pm

Hello all!

Any folks who know anything about making or maintaining ceramics? I recently purchased really awesome crock from a local kid who's just getting into ceramics. The final product is beautiful! However. As I understand it, he fired the lid and the pot at the same time to get the fit right. In order to keep the two from fusing, he left the rim of the pot and the bottom of the lid unglazed. I don't think this would be a problem since I don't intend to fill this thing all the way to the brim, however I have been noticing some flakes or powder developing as I continue to take the lid on and off. He assures me this was made with food-friendly, lead-free materials and glaze (I'm getting a lead tester just in case the kiln he used has some history he's not aware of), but still don't want powder in my ferments!

Wondering if anyone can give advice on some food-safe ways resolve this issue. I was thinking of sanding the rim and the lid and then coating in some sort of varnish? I don't readily have access to a kiln, so I'd like to avoid re-glazing if possible.

Anyways, thanks in advance for any advice! Any recommendations welcome and I can give more info on the materials/process if it would help.

Thanks!
kioke
 
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Re: Cracking/flaking unfinished crock pot

Postby Christopher Weeks on Wed Apr 01, 2020 8:50 am

I'd want to know more about the "flakes or powder" you're seeing before speculating. Is it possibly and efflorescence of salt from your brine?

Is the crock stoneware and do you know how high it was fired?
Christopher Weeks
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Re: Cracking/flaking unfinished crock pot

Postby kioke on Wed Apr 01, 2020 12:28 pm

Thanks for the response Christopher. I know that these flakes are from the crock itself as I haven't started anything in it yet! As for the makeup of the crock, here is what the maker sent me:

The jar is wheel thrown out of a stoneware clay. The dark coloring on the lid comes from manganese within the clay, while the lighter clay has no coloration. It has sand particles so that it would be strong enough to support the weight of the sides while it was wet. The sand acts a disruption to the natural order of clay molecules protecting from both collapse and cracking by disrupting any weak faults that might form. When it was leather hard black engobe was painted onto the top 2 inches of the outside and then carved through. This is called scrafito. It was then dried and bisque-fired to about 1800 degrees F over the course of 2 days. This was to slowly evaporate all the remaining water in the clay, leaving it partially vitrified (some of the silica in the clay had begun to turn to class). This made it substantially lighter and stronger. Then it was glazed. The outside of the main pot used a white orange shino glaze, that tended towards white. Shino glazes are Japanese in origin, specifically from the Gifo prefecture, and have been used for over 500 years. The colors come from crushed feldspar. The lid also had a shino glaze on it, this one tended towards orange instead of white. The inside of the pot uses a yellow salt glaze, a recent invention. All the glazes are composed of elements or minerals and crushed clay. The glaze firing was in a gas kiln with reduction (although reduction did not affect either of these glazes) and the kiln was fired to cone 10, about 2000+ degrees. At this point the clay fully vitrifies, along with the glazes, meaning they are 100% food safe and water proof. During the firing process the glazes fully melt and drip somewhat so the inside of the lid and the lip and top 2 inches-ish of the lid had to be left unglazed so that the lid would not be glazed onto the pot. Unfortunately the kiln was slightly over fired so there was some warping on the lid and pot. Upkeep wise, it can be put in the dishwasher or washed just like any other ceramic dish.
kioke
 
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Re: Cracking/flaking unfinished crock pot

Postby Christopher Weeks on Wed Apr 01, 2020 6:46 pm

All that stuff the potter wrote sounds entirely legit and if it accurately describes the situation, the crock was made correctly. So, what about the powder? I don't know. Maybe scrub it off once and see if any more forms. I wouldn't expect much to come out of it without a lot of serious abrasive work. Also, if the flakes are big enough to have sharp edges, that sounds like something you want to be really careful about but if it's just a powder, even if it's not exactly desirable, it should be harmless to ingest.
Christopher Weeks
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Re: Cracking/flaking unfinished crock pot

Postby kioke on Sat Apr 04, 2020 2:36 pm

Awesome thanks for the peace of mind. I think I might try giving it a bit of a sand to reduce the amount of interference between the lid and crock so I don't run the risk of the big chips and then I'll just monitor it as I go!
kioke
 
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