Origins in a raku kiln.
When I designed and built the prototype for the wood-fired stoneware kiln, Fred Olsen's fastfire kilns were all the rage and my design had much in common with Olsen's although its origins were in the little wood fired raku kilns which we built in the 1970s.  These little kilns had two fireboxes and the chimney sat on top of the fireboxes just behind the tiny firing chamber.  I don't know where the design had originally come from but we built more than a dozen of these little kilns out of housebricks salvaged from demolition sites.  They were a joy to fire and some of the pots that emerged from them were unbelievable gems!  We could scrape 1050 degrees centigrade and we used lead and borax glazes flavoured with copper, iron and (sometimes!) uranium!  Heavy reduction was achieved by blocking the chimney with a pice of broken kiln batt and adding a bit of used sump oil to the fireboxes.  Magic!
         

The kiln at Blackhills was built to the same design which we had used for an earthenware kiln at Pantasaph in North Wales five years earlier.  Three large fireboxes run underneath the kiln, each about six foot long.  The 64 cubic foot chamber and the chimney are built on top of the fireboxes.
Although I started off firing the kiln to 1300 degrees centigrade in about ten hours, I now take thirty hours to reach 1250 - and would fire longer if exhaustion didn't intervene.  I also cool as slowly as possible, clamming up every crack with clay.  The cooling takes a couple of days and it is this long cooling cycle  that give the glazes their soft maturity and bring out the rich reds in the iron bearing clay.  The importance of the cooling cycle has not been given enough attention.  The vogue for fastfire and speed cooling produced bright, colourful, shiny glazes but with none of the depth and mellowness of the same glazes cooled slowly.

We fire to a thousand (centigrade) in oxidation over about twenty hours, care being taken to minimise smoke and flame entering the ware chamber.  At a thousand, we reduce hard for an hour or more without any temperature rise.  Then the temperature in the kiln is allowed to rise slowly (a mixture of reduction and oxidation) until the temperature reaches 1200.  The kiln is then soaked for a couple of hours and reaches about 1250, by which time cone 11 is down at the top and bottom.  I try (not always successfully) to leave cone 12 standing.