Hi, all! Just wondering if there's been any studio-wide discussion about the issues with bloating on the charcoal clay. I love the clay, and have been using it a lot -- but it definitely has severe and unpredictable bloating problems. (The photo here shows one of 7 large bloats on one of my recent bowls, none of which were present after bisque firing. Have had bloating on several other charcoal pieces as well, and know other people have too!) Very grateful for the new clay, and also wondering about the possibility of discussing/trouble-shooting the bloating issues. Thanks so much!!
Hi Sarah - sorry to see your bowl falling victim to bloating, but thanks for voicing the issue. It's something we've been seeing since we starting using the 710 clay. There's been a consistent rate of bloating that frankly would have already deterred us from keeping the clay. There has been so much excitement at both studios though over the colors and textures this clay produces that we've stuck with it, and even invested the time and energy to create a glaze board in Belmont.
Bloating in clay bodies can come and go if it's due to manufacturing impurities. In this case, the bloating is because of the high metal content in the clay which gives it the dark color.
I talked to Julie at Standard Clay. She is their tech rep. She offered the idea of bisquing at a higher temp or glazing at a lower temp (like some of your pottery forum responses suggest).
As a matter of principle, I almost never make system-wide changes in our firing cycles to accommodate a specific clay or glaze. The effects are cascading and it's not worth it. In this case, Jared and I will experiment with some adjustments. If we can make a change that's practical and doesn't throw off everything else, we will.
In the meantime, our message to students is that you should use the charcoal brown clay knowing the risk of bloating. If you're spending a lot of time on a single piece, it might not be worth it to use the clay. For context, we've used approximately 5,000lbs of 710 between both studios since February. I'd guesstimate that each piece has a 2-4% chance of bloating.
TLDR: We know about the bloating and are working on a solution. If we can't find one, we may get rid of it.
If it’s at all helpful, I posted in a large pottery forum, and these were a few of the responses from different people there:
“Bisque at 04 (not 06); fire to cone 5 (no higher) for either 710 or 266. I had body bloating with 06 bisque! No problems since above schedule.”
“Yes, Laguna's black clay does bloating at cone 6 also. Had to Lower the firing to cone 5 with a 10 min hold. Stopped all bloating. If they continue to carry that clay they will have to adjust the temperature a lil bit.”
“I regularly fire my home kiln to cone 5 and with a 10 minute hold get a solid cone 6. I know there is probably some way to re-calibrate the kiln but this works for me. Every kiln has its own particular issues...”
“Are they using witness cones to verify the cone reached? Controllers are often not accurate.”
“Call manufacturer and get the detailed firing schedule for bisque firing to prevent bloating.”
“We have issues with bloating in Standard 266 if it gets overfired at all or any cone 6 re-fire. We haven't had that issue with 710 that I've heard of at all. I haven't used that much 710 but have had no bloating issues. We have occasionally had a glaze kiln that wanted to overfire at our cone 6 standards and it resulted in lots of bloating problems with Laguna B-mix 5 Speckled. Because of that we don't have access to that clay anymore. Also because of that, I do cone 5 glaze firing at our home studio. I still have issues with the B-mix 5 Speckled and can't re-fire it at all. I have had no issues firing either 266 or 710 at cone 5. My kilns (Olympic & Scutt ) both tend to overfire based on witness cones. I crank in a setback for both glaze and bisque firing.”