Thursday, 6 November 2014

Where the magic happens......


The kiln was set to start firing at 5.30 this morning, and gradually begin to work though the firing schedule which I'd programmed into the computer.

Here it is in its spot in the dining room.  I always clear away all furniture etc and put it up on a heatproof board when firing.  It is extremely well insulated, but even so, it does get hot.

It's currently 11.30 am, and the controller is showing an internal temperature of 1162 degrees Celsius.

Which is very, VERY hot.

When you consider that the maximum temperature of a normal domestic oven is around 300 degrees, which will burn just about anything to a crisp in next to no time, try to imagine it almost 4 times hotter and you will have some idea of what it's like inside my kiln right now.

I had to get very close to the kiln to take this photo and I can attest to the fact that it was definitely HOT.  There is a tiny, hairline gap between the lid and the base.  The white stuff you can see is the edge of the insulating fire-proof lining, and just below you can see a glowing yellow line.  That's the heat inside...... it goes from red hot, to yellow hot, to white hot.  Every tiny piece of greenware inside, carefully placed on the shelves, will also be glowing and incandescent, as the heat works to turn the soft, porous, fragile, powdery porcelain into hard, vitrified, impervious porcelain. Each already tiny piece will shrink by up to 1/3 and attain its final colour.

The controller also tells me that it's in the end phase of the second temperature ramp.  In order to properly mature and vitrify the porcelain there has to be a carefully controlled combination of time and temperature.  Both work closely together.  Too much time and not enough heat won't work.  Neither will too much heat and not enough time.  It's a delicate balancing act.

It still has over 50 degrees to climb before it reaches optimum temperature.  And it must stay at that top temperature for a specific period of time.  This is called the soak.

It takes a lot of power to maintain that very high temperature for a sustained period, which is why I time my bisque firings to end early in the day.  In the late afternoon or early evening, when a large number of households are drawing power from the local grid, there is a small but noticeable drop in everyone's electricity supply.  You probably wouldn't notice it at home, but it does affect my kiln, and even a drop of half a degree will disrupt and extend the soak time, with potentially catastrophic effects.

The firing should end at about 2pm or thereabouts, and I always make sure that I'm around to monitor the final stages.

So although it's all very technical and scientific... a combination of chemistry and physics, I can't help feeling that it's also rather magical.

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