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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?
In the past, I have often written that I wedge all of my clay even though I take it right out of the bag. There are several reasons for this, and I wanted to take a few lines to explain them further and throw out some ideas for thought. My reasons for wedging clay for the last 30+ years are as follows: If the clay came out of the pug mill, it usually needed to be blended to an even consistency, remove air bubbles, dry it our some, and line up the particles so that for better strength. If out of the box at home, it had usually frozen over the Winter here in Central PA. Freezing drives water out of the clay to the outside, and leaves all sorts of striations in the clay block. I start wedging this by turning all sides into the center with cut and slam, then cut and slam the entire block about 5 times, finish by spiral cone wedging the clay into weighed out pot sized amounts. Wedging as exercise. I wedge clay for exercise, as crazy as that might seem, but when throwing for hours at a time, the break to wedge up the ball of clay to be used on a pot down the line is a welcome way of stretching the back and shoulders. This also brings me to the final reason I forced myself to not be lazy about wedging clay. Finally, wedging as therapy. Years ago I was in a bad auto wreck that left me with some damaged vertebrae. This caused lots of pain in the lower back for decades. Some days I could hardly get out of bed without levering my body out of bed with the weight of my legs. I found that hanging from an overhead bar would stretch the back and help some. However, my go to exercise became wedging because of the way the process worked. . . . at least for me. The process of wedging lifts the shoulders upward and back as you push against the clay and at the same time the shoulder lift and the body movement stretches the back muscles and the spine itself. With regular rhythm and movement in the wedging process where you are not trying to hurry the job, or wedge too much clay you can do wonders for you core and spine. So, I leave this open to comment, and I am sure many of you will go on about getting a pug mill, which for me would be a big investment, but as I get older I have been looking for a good used one. However, even with a deairing pug mill, I would still probably wedge for the last two reasons above.