Have you ever been run over by a train?
You know, the huffing, puffing, hooting, fire breathing monster sort!
Have you been tied to the rails like a starlet in a movie,
as the express thunders down?
Well, not really, but.... he loved the hot tin roof and enjoyed keeping me company as I fired the kiln yesterday.
The kiln "clammed up" after the firing. All airways to the fire box well covered with bricks, ash and sand.I fired single handed as we had the studio open to the public yesterday. Laura kept me supplied with water and things to eat. I find that I don't feel all that hungry when firing the kiln, and eating is difficult as I am stoking almost continuously most of the time, but water is absolutely essential as it is hot work, even on a cool day, and it is easy to become dehydrated.
After four hours, the kiln was at about 800 degrees Centigrade (1472 F) and I went from firing in oxidation* to firing in heavy reduction* for the next hour and three quarters until we were at about 950 degrees Centigrade (1742 F). From then onwards I changed to a more efficient stoking pattern of adding enough wood to cause some, but not a huge cloud of, smoke. Wait for the smoke to clear. Then repeat! I have some copper red glazed pieces in this firing that need a good reduction atmosphere to develop the red. Hopefully I have got it right... if not, they will be green! The firing took 10 hours to cone 11* in the medium temperature part of the kiln (some parts hotter than this, some cooler). As my pyrometer is probably 100 degrees Centigrade out towards the higher end, I won't bother with temperatures, but... Cone 11 is probably around 1270 degrees Centigrade (2320 F) for the speed I was firing in the last few hours.
Useful things to watch for when firing a kiln.
Whilst stoking I do take notice of all sorts of clues as to what is going on in the kiln. Colour and quantity of smoke, colour of flame in the fire box, sound of the fire in the fire box, and, very importantly "pressure". When a kiln is starved of oxygen, as it is when firing in reduction, the flame and smoke will try and find their way out of every opening. Gaps in the brickwork and spy holes will puff smoke. Potters sometimes call this "pressure". When the kiln has lots of oxygen, it will inhale rather than exhale through gaps and spy holes. Pressure is a useful indicator of what is happening inside the chamber of the kiln, there may on occasions be pressure and a reducing atmosphere in the chamber of the kiln, but little smoke from the chimney.
Firing in oxidation means that there is more than enough oxygen available for combustion of the fuel. In oxidation, the flame in the firebox will be bright, active, and yellow. Inside the kiln visibility will be good and the colour bright. There will be no smoke from the chimney. Spy holes will suck in air.
Firing in reduction means that there is not enough oxygen available to burn all of the fuel. Carbon Monoxide will be present in the kiln, and it will greedily seek out oxygen from where ever it can find it, including stripping oxygen atoms from glaze materials and clay. There will be some smoke from the chimney, there will be "pressure" in the chamber and smoke and flame out of open spy holes. The flame in the firebox will be lazy and orange. Inside the chamber visibility will be poor.
A little cone shaped object of ceramic material which is formulated to flop over when at a certain temperature. Cones are one of the most useful ways of telling how things are in a kiln as, being made of similar material to the glaze on the pot, it is subject to the same variables as the glaze, namely: time, temperature, and atmosphere. Cones are numbered and can be formulated for temperatures as low as 586 Centigrade. Potters usually use a sequence of cones to give them warning when the target temperature is approaching, and a guard cone... to show if the target temperature was exceeded!