The kiln was fired with lower gas pressures than last time, as I had been advised that the optimum for this burner is likely to be around 4 Psi ( less than a third of a Bar). At higher pressures you are also more likely to suffer from tank freeze (frost appearing on the outside of the tank up to the level of propane remaining) and this will slow down the outflow of gas to the burner. However, even at 3 to 4 Psi frost still appeared on the tank as it got down to around half empty .. perhaps this was what caused the temperature to stall but I was unable to get close to 1280 degrees C (cone 10) according to the cones and pyrometer.
As a result, I was expecting the results to be rather dismal but I was very pleasantly surprised! I suspect the cones and thermocouple were in a cold spot on the same wall as the burner port and had given a false impression. Further into the kiln and closer to the "fire boxes", the majority of the pots were well fired, with the ash and celadon glazes completely melted. None of the pots was over-fired and it was pleasing that there was no evidence of bloating, despite the fact that some of them must have been fully blasted by the gas flame. Levels of reduction were also very promising, no doubt partly due to the wood which I threw in several times during the firing.
After ten hours, some heavy reduction cooling was induced by putting wood and charcoal into the side stoke hole and as much charcoal as I could stuff into the fire box through the burner port. The kiln was then clammed up around 8pm (this does generate a fair bit of smoke!) and left to cool overnight. At lunchtime the next day it was still too hot to open, but by 4pm I was prepared to risk a peek as it was a warm and sunny day.
Here are some of the pots which came out well:
|Ash-glazed tea bowl|
|Vase with wood ash glaze|
|Iga style vase with crusty ash glaze|
|Round bottomed bowl|
|Vase with slip glazes|
|Pot, approx. 3 ins tall (n.f.s.)|
I am now considering moving to a downdraft design, mainly as I would like to fire more quickly and efficiently. The updraft experiments have been interesting, but gas is rather expensive these days at £63 for a 47kg tank!
Thanks for reading.