Firing

I have made it this far and now have a piece ready to go through the kiln of doom, the kiln of gods or as a say, the raku gods decision not mine. I carefully hand carry each and every one to the kiln which has already been setup and very carefully lower it down onto the kiln floor. Time must be taken to make sure the heat will circulate around the piece in an even manner or it may explode later on..

Room temperature - 130C (possible steam or air bubble expansion)
I make this preheating stage a slow and careful one, this is just delicate clay after all. As the work reaches 90C - 110C any moisture that might still be present in the middle of the walls will expand and destroy the piece. It’s always best to leave a piece for as long as possible before firing.

130C - 400C 
Still being extremely careful the piece is now slowly pushed upwards to 400C. There is still a risk of any joints or micro bubbles expanding and destroying the work. At 400C the clay will go through a huge change. From the room-temperature form of quartz, quartz undergoes a reversible change in crystal structure. This phenomenon is called an inversion, A to β quartz inversion is accompanied by a linear expansion of 0.45%. This can cause stress and further failure.

400C - 700C
We have now made it through the most dangerous stages and want to travel onwards to a form of under-fired raku ceramic at 700C. I still have pots crack or explode during this travel. Once we hit 700C it is likely the work will survive the rest of the temperature increase. 

700C - 1000C
I usually start to relax a little more during the final push. The kiln starts to rumble and the pot starts to glow. We are now making the piece into genuine raku and starting to melt the glaze and chemicals. The glazes reach a maturity point around 1000C - 1010C and left to soak for a few minuets before the kiln is shut down completely. 

1000C downwards
The next huge risk now facing the fully fired work is to remove it from the kiln while still glowing red hot. We may hear a crack immediately and this whole exercise has been a waste of time and angst. With some luck the ceramic will survive plummeting in temperature whilst setting fire to combustibles in the reduction chamber such as sawdust or newspaper. The fire will starve itself of oxygen and this is when the chemical glazes can cool all the way back down to room temperature in a carbon dioxide environment, along with a vast array of other chemical compounds within the chamber. Experimenting with different combustibles can produce different finishes. The smoke also gives a huge benefit of adding carbon blacks to any unglazed parts of the work. 

Room Temperature 
The journey is now over and hopefully we have a true piece of raku that started as an idea many weeks if not months ago. Opening the reduction chamber is one of my favourite parts of the process, sometimes as the smoke clears they take my breath away.  A quick tap to ensure there are no fractures and from here the work will have a deep clean and left to dry out before being sealed. The chemicals need to be sealed otherwise they will slowly react with oxygen and change colour or become dull over time. 

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