Every potter knows how many variables there are in ceramics and how many things can go wrong between forming a ball of clay to taking a piece out of the glaze kiln. Most people though probably don’t realize what a potter is up against every studio day. We just experienced a ‘Crawling’. A first for us.
As you can see in the image, the glaze has separated into islands with bare patches of fired clay body. Research indicated that the most commonly cited cause of crawling is said to be grease or dirt on the bisque ware. As we use no grease of any kind and wash all the pots before waxing the foot, and glaze firing, those are not possible causes in our case.
Other possible causes we learned are a too thick application of the glaze – again not likely as we glaze the same way every time and have never had this problem before – or excessive drying of the glaze before firing – and here we think we have it.
When we came into the studio the day after firing the kiln was giving a ‘FAIL’ reading. On looking inside we saw all the glazes were in a powder form and discovered the reason for the fail was the thermocouple had ceased to function.
The thermocouple measures ‘heat work’, not temperature but temperature over time, which tells the controller when to supply current to the elements and for how long. If the thermocouple goes, there is no way for the controller to regulate its operation and it shuts down.
Having a spare I replaced the thermocouple and refired. Because the kiln had heated to around 400 degrees F., the likeliest scenario is that the glazes had become very dry, loosing their adhesion to the clay bodies, thus running off the pots when the glaze became fluid. So why not all the pots? The crawling only happened on three of the pots and a few of the small horizontal pieces.
The glaze that it happened to is the runniest glaze we have and the pots that crawled were in the hottest part of the kiln. We never mind terribly when we have a problem so long as we know the solution. Yes, we lost a few fountains but I have very little fear of it happening next firing.
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