Thursday, September 11, 2014

What Disaster Can Tell You

It makes no difference how much experience I garner, every now and then I will do something monumentally stupid. Several weeks ago, I had a ware board full of ramekins. I placed the board on a shelf with about half of the board extended out over the edge of the shelf. Then I set to work unloading the board.
Starting at the wrong end.
Almost immediately, the ware board with all those beautiful ramekins began tipping like an insane seesaw causing several ramekins to "introduce" them selves to the floor. Sancho Panza put it best when he said, "...whether the stone hits the pitcher or the pitcher hits the stone, it's going to be bad for the pitcher."
I lost four ramekins to that little act of idiocy, but it's interesting what such a disaster can tell you. Not so much about my working methods, but about the product I make.
Many years ago (actually, it was decades ago), I was working in the pottery studio of a private school. I actually worked in the kitchen of that school, but they allowed me to putter about in the pottery studio when I was not at work. It kept body and soul together for the few months I was there. The students were aghast at my habit of cutting pots I had just thrown in half so that I could see how the wall of the pot looked. I can remember the teacher telling them, "You guys should be doing that more often."
I rarely cut pots in half these days. I don't really need to as the lesson of an even wall has been well learned. Still, it's always interesting to look at the profile of a pot when it meets an untimely end. And truth be told, I have actually broken pots on purpose to get the sort of information the little beauty in the picture above revealed.
So what can I tell from what I see? Three things.
First, I can see that the wall is nice and even. No surprise there. You may also notice the slightly thicker rim. That makes the pot less prone to warping during manufacture and chipping during use.
Second, I see that the pot only broke into about four pieces - most of them quite large. That means that the pot is extremely strong. A weaker vessel would shatter into a lot of small pieces.
Third, I see that the glaze perfectly follows the same break pattern as the clay. That means the glaze fit is exceptionally good. I knew that, given the tests I put the glazes through. But it's good to see it up close.
Most people don't realize it, but a glaze has to fit the clay it's applied to perfectly. The critical measurement is what happens when the pots cool in the kiln. Everything expands while heating and contracts while cooling. If the glaze contracts more than the clay does, it will be under a lot of tension. In a case like that, something has to give, and the glaze will form a fine network of stress cracks. Potters call this crazing. Crazing weakens a pot and will inevitably shorten its life. Some glazes are specially formulated to produce the same crackle pattern seen in crazing without causing problems for the pot, but crazing as a gaze flaw is something to be avoided.
If the clay shrinks more than the glaze, then the glaze can pop off, resulting in tiny, razor sharp pieces of glass that can end up in the food or beverage the pot was holding. This problem is called shivering and it is one issue that can keep a potter up at night.
Ideally, a glaze should shrink slightly less than the clay, but not enough to cause shivering. That assures that the pot will be the strong and last a good long time in normal use.
And the best way to prove the strength of a pot is to break it on purpose - or, in this case, by accident - and see how many or few pieces it breaks into. The fewer the pieces, the stronger the pot. If the glaze margins align perfectly with the edges of the broken clay, the glaze fit is perfect.
I make high quality pottery. The picture proves it.

No comments:

Post a Comment