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I have been having a strange issue with my Cone 6 Black Stoneware Clay from Standard (266). It gets these big cracking bubbles upon glaze firing. I bought this clay to marble with Porcelain (Standard 365) because it had a similar shrink rate. The marbled pieces came out SUPER bubbly, really horrible. I thought it was just poor clay preparation on my part because of the mixing of the two. However it has been happening with pots I make solely out of the Black Stoneware. I prepare this clay the same way I do all my other clay bodies and never really have this issue otherwise. Has anyone else had this problem with this clay? Any advice?

 

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Gas bubbles in the 266. The glaze is maturing and sealing the surface before the gas bubbles from the clay body are released. Most common remedy is to hold top temperature during bisque to allow more of the Sulphur and other impurities to burn out. Also, stack the 266 loose in the bisque to maximize surface area to promote burnout of impurities.

 

Not an uncommon problem with 266 and similar clay bodies.

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This is a very common issue with dark clay bodies, as Bruce said. I haven't used it myself but everybody I know who does has had a similar issue for the reasons described above. I hope a hold works to solve the issue for you! :)

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I had the same problem.  I like the colour of the dark body PSH- black clay, so I made a slip out of it and I use it with my regular Plainsman cone 6 clay. So far- so good. I bisque to 04 and I have an old bisque kiln with just a kiln sitter so I couldn't soak it at 04. The slip seems to be a good solution but I have also just made some things out of the dark clay without glaze and that works too. My profile picture is of unglazed black PSH and a cone 6 porcelain mixed together.

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Actually you are dealing with bloating. Follow Bruce,s recommendation first and see if that solves the problem. Bloating is caused by a couple of things; even though both are related. High iron/magnesium/ titanium clay ( better known as dark brown clay) has high levels of impurities. If the above recommendation does not fix the problem, then you will need to change your glaze firing schedule. From 2050f, up to 2230f becomes more critical in your glaze fire.

Nerd

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Thanks everybody. I will try a bisque soak. The ones that became more bloated were usually stacked deeply in the bisque firing, which confused me because some things came out fine and some were horribly bloated! Thanks again for the info.

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If none of the above works, try a one hour soak at around 800C (1470F) during the bisque firing. If organic matter in the clay is the problem, that is when it burns out (and is also before almost anything else in the clay starts melting or ceramic change starts happening). Make sure the kiln lid is cracked or all bungs are out, since the organic burnout process needs plenty of air to get completed.

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