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We are relatively new to pottery (18 months) and have recently experienced bloating during our glaze firing. We have two kilns and the problem happens in both of them. When we opened the kiln this morning we found that the top shelf is the worse, the next shelf down has moderate bloating and the lowest shelves rarely have bloat. We've slowed down the bisque firing process to allow more organic matter to burn out of the clay, then cured the glaze for 3 days before firing. We are using Seattle Pottery Supply Vashon Red clay (cone 4-6) and are using Seattle Pottery Supply glazes (cone 4-6). Does anyone have any helpful ideas on how we can eliminate the bloating? After the holidays we are switching over to buff clay to see if that's where the problem is.

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Bloating is most often the result of over firing the clay body. Occasionally the problem can stem from firing too quickly or having a high degree of volatile material in the clay body that is not given sufficient time to escape, but not usually. Some clay bodies, especially those with a high iron content are susceptible to over firing if fired even slightly beyond the stated cone (in this case cone 6). I would consult with Seattle Pottery supply and ask how critical the cone 6 limit is.

 

Are you firing using a computerized kiln? Are you also placing cone packs (cone 4-5-6-7) visible through the peep holes and in locations throughout the kiln to assess the firing? 

 

Steven

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Thank-you for your help! Seattle Pottery told us that this clay should be fired to cone 5 even though the label says cone 4-6. We contacted our previous instructor and he said it didn't make sense that what they sell as cone 4-6 is actually cone 5.  We will take your advice and fire with the witness cones. Our kilns are manual with infinite settings.

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Connie!!

 

SPS clay SUCKS.

 

Especially that Vashon stuff, ick... It has the most finnickey temperature range preferences and omg, I won't EVEN get into their effing redart. I have never suffered more losses from cooling cracks in my life than with SPS clay. Yuck!!

 

You want excellent clay at great prices? Go to Clay Art Center in Tacoma! ♥ It's literally two minutes off the I-5 exit--super easy to find. Their clay is nothing short of FREAKING AWESOME. I have NEVER had a cooling crack/bloating problem with it, and it throws and handbuilds like a dream! Their mid-range bodies are gorgeous and they have a wide selection of colors. SPS is the place to go for overglaze and posts, but get shelves, clay, and their amazing homemade underglazes (if you use them) at Clay Art Center. They have, by far, the best selection. Oh! And DO NOT DO NOTTTTT get EPK from SPS! They completely botched a ton of their bags of it and wouldn't refund me, though it totally ruined my glaze.

 

Nice to meet another Washingtonian! ^_^

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I'm excited to have so much help! I have a little Vashon Red left so I will first test my kiln with the witness cones then fire at cone 5. Yes, I will try Clay Art Center for new clay! We didn't know about them! Our instructor always got clay from SPS so we did too. However, he used cone 10 (reduction kiln) while we fired cone 6.

   Guinea Potter: which part of the state is the crappy part? We're in the central basin :wacko:

 

Thanks again for everyone's help!

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  • 2 weeks later...

Okay, we test fired the kiln with #4,5,6 and 7 witness cones and a #6 in the sitter. I put the witness cones on 3 different shelf levels in the kiln. Cones 4 and 5 bent on all three shelves as one would expect. Cone 6 bent best on the middle shelf and cones 7 barely bent at all. As we were looking to see if the kiln was over firing our cone 6 clay, I think the test was inconclusive. Maybe it is the clay since we didn't have the problem when we used Georgia's clay. I will use the last bag of Vashon Red and fire to cone 5.......but I'm always open for more ideas and suggested to you with more skill and knowledge!

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  • 4 weeks later...

Argh. There is nothing quite so heartbreaking than to open the kiln after a glaze fire to find sooo many destroyed pieces! ALL of the pieces made with SPS Vashon red clay are trash. Not only did they bloat but the surfaces look like 60 grit sandpaper. The pieces made of SPS Vashon white clay fared a little better. I had considerable bloat in one medium size bowl. And the newest, latest problem is that many pieces warped out of shape. I had a half dozen soup bowls with handles, the handles melted right down until they were resting on the shelf.  We called Clay Art Center in Tacoma and their employees were wonderful about helping us. They guided us through testing the kiln to assure they are firing correctly. I am now convinced that we have multiple problems. First is the Vashon Red clay, and I believe it can be isolated to one bad 50# box.Although the sandpaper finish has me baffled. We used SPS glazes. Tomorrow we will pick up a bag of Georgia's red clay and see what happens. I love pottery (working on the wheel), but firing problems are getting the best of me! 

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Oh my sweet lord have mercy.

THAT is INEXCUSABLE.

Seriously, I would never buy clay from SPS again. It SUCKS. You are gonna love CAC's clay! It throws like a dream and has great temperature resistance. Best stuff in the area!! :) I am SO SORRY about your ruined load. I know that exact feeling... Fred ruined over $2k worth of commission work once...I literally ralphed. UGH...

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About 50% were commissioned pieces. I am sooo ready to be done with SPS clay!! Any ideas about the rough texture ? I'm just marking it up to the red clay but I just don't know. One other thing I forgot to mention in my previous rant: it seemed like the (bar) cone adhered slightly to the sitter pins. It broke as we pried it off. Makes me wonder if it caused the kiln to fire too long. Also, we replaced the plug. Once the pass is clear we'll be making a trip to Tacoma.We'll do a test run on their clay before buying a huge amount. We'll probably order in quantity and have it drop shipped, it's cheaper than driving our 1 ton Ford pick-up! I'm trying real hard to not be discouraged, I doubt my abilities to figure this out.

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I used their Vashon Brown, and I got that crap sandpapery surface, too. I don't know if the clay just eats the glaze or if it just refracts it off, but man, lesson definitely learned. I am so freaking sorry this happened to you...why SPS continues to manufacture such awful clay is beyond me!!

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