I went to the ceramics studio last night with high hopes of collecting my latest two holey bowls that had been fired. The kilns had been broken for a while now so I was really eager to see how they turned out now the kilns were running again.
It was not as I expected!
I was faced with a kiln shelf that held the shattered remains of the two bowls. Strangely the glaze had vitrified and there was no slumping, so the bowls must have cracked and shattered during the cool down period of the cycle. Amazingly, another artist’s piece that had shared the shelf was unaffected, so although it looks as though the pieces exploded they probably just cracked, shattered and fell apart. At least that was good, as I would have felt dreadful if someone else’s hard work had been destroyed too.
We discussed this for a while and the studio director said she had never seen anything quite like it.
The glaze on one bowl had sloughed off, a situation she described as “shivering”, but it was odd that it had happened in quite large areas.
There was nothing unusual about the way I glazed the bowls, they were both dipped in the glazes and then left to dry as usual. With the kilns out of order for a couple of weeks I can definitely say the bowls were dry, so moisture should not have been a problem.
Afterwards, a search on the web resulted in the introduction of a new term into my vocabulary – “dunting,” and also provided me with some fascinating scientific background to explain the probable cause.
As the ceramic cools down in the kiln below 1063°F (573°C) the silica molecules in the clay abruptly rearrange themselves and cause the pot to contract rapidly. The same occurs as the temperature drops to 439°F (226°C). These two temperatures are known as silica “inversion points,” and they set up lines of stress on the pot due to differential cooling. This effect is exacerbated by the amount of silica in the clay used, the pot shape and even the thickness of the glaze applied.
It would appear to me that the holes in my holey bowls probably created additional areas of stress by causing uneven cooling of the sides of the bowl, resulting in catastrophic cracking.
At least that’s what I think happened!
It is annoying but it does also serve to illustrate just how precarious is the work of a ceramacist. Problems with creating the initial piece by hand or wheel merge into issues of potential cracking when the piece initially dries to leather hard, possible damage when turning the piece, and chances for chipping when dried ready for the bisque firing. The possibility of cracking and breaking during the initial fire is the next risk, followed by the risk of damage when removing and storing prior to glazing. And lastly, the risk of failing in the final glaze fire – both from the kiln cycle not being correct leading to glaze problems through under-firing or over-firing (I’ve had both) or, in this (the worst) case, catastrophic structural failure.
When I look at some delicate ceramic ware that artists produce I am truly amazed it actually survived all this – only then to be placed precariously on a shelf for someone to knock onto the floor!
I have another two holey bowls ready to be fired but, to be honest, I am a little worried about what will happen with these pieces too. I also need to create a couple more to replace those that were lost, although I may vary the design a little to reduce stress on the pieces, as I really liked the glazes of these two!