Tuesday, 29 November 2011

Getting the best out of Bertha.

Having a kiln not far from Buxton, the highest town in England, built on an exposed hillside open to the elements, little in the way of insulation and a firing planned for the end of November is not the ideal. But with a need for pots for the impending Christmas events there was little option.
Bertha is an Olsen fast fire, built along the lines of the Joe Finch wood fire version found in his book. Bertha is beginning to show her age, largely due to exposure to the elements and lack of tender loving care. Being single skin and of high temperature bricks, the effects of several over enthusiastic stokers have taken their toll and the back of the fire boxes bulge and many of the bricks are cracked and have large gaps around them. The kiln also features the tallest chimney I have ever seen on a kiln of this size and must qualify for red lights to warn low flying aircraft. Why is it that Potters whack another six rows of bricks on the chimney when they experience firing difficulties?

The usual routine is to get up to Blackwell, about two hours drive, clean out the fire boxes, and start the pack. Slowly the other participants arrive with offerings for the kiln and we usually finish packing around six, close the wicket, put on the gas burner and head for the pub for a pint and dinner.
It takes some courage to leave the log fire in the pub and head out to the kiln site. It was bitterly cold packing the kiln but now the temperature is plummeting and it is below freezing. The pyro says 130 and I light one of the fires with off cuts of oak strip wood. We fire with fire box lengths of oak, quite small in section, 25 X 25 up to 25 X 50 millimetres. Sue managed to get a huge stack of the stuff and this must be the 5th firing using this wood.

Everyone has gone, there is just the two of us left to fire the kiln through the night. I light the second fire for the fire box next to the chimney and the temperature slowly rises. Bertha responds to a light touch, two or three lengths each stoke otherwise there is masses of black smoke and the fire comes back out of the fire box. Gently does it and by 4 am the passive damper in the base of the stack is out and we are aiming for some body reduction. The sliding damper broke off some time ago and still isn't fixed. We leave the passive damper out to get some heat to the bottom of the kiln and hit 1000 degrees around dawn. It is a beautiful dawn, the Owls have gone to sleep and the Blackbirds are kicking off their territorial squabbles.

There was a loud thump and the sound of breaking ceramic around 300 degrees. Can't see anything, so we carried on, pretending nothing had happened. It will be revealed in two days time.
Norman arrives around nine so I grab an hours sleep in the back of the van and around twelve o'clock cone nine is well on it's way. Still easy on the wood, three pieces each stoke, black smoke and the temperature drops. The smoke clears and the temperature rises slowly, disregarding the pyrometer which has a mind of it's own and claims it is only 1150 degrees we have cone ten over. At one o'clock Sue decides we have finished and we call it a day, clam up and I head off home.
I couldn't make it for the opening,  I thought about the firing as I got ready to head off for Welbeck and the Harley Christmas Fair. Would it be alright. Would the Shino be ok. It was risky to try it on so many pots without testing first. All that work and effort invested could be ruined at a stroke.
Was it a good firing?
What was the sickening thump during the night?
 Would there be Christmas pots for all?

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