As is well-known, fixed routines are standard in the firing of oil, gas, and electric kilns but are unknown, as far as we know, in the firing of wood kilns. We set up a trial to establish a fixed firing routine for a wood-fired kiln. The kiln we used is based on the Stubbs-Schloessingk design published in Ceramic Review, No. 170, 1997, with some modifications: it has seven-and-a-half feet of high-fire insulating fire brick, and the chamber volume was reduced from sixteen cubic feet to twelve and a half. The large firebox of this kiln, equal in volume to that of the chamber, allows for fast firings.
After some trial and error, we established a system for glaze firings to Cone 10. We kept a low fire in the lower firebox for two or three hours using about one wheelbarrow full of scrap firewood. Then we began stoking in the upper firebox according to a strict and unvarying schedule, which requires 1) stoking at five-minute intervals, and 2) weighing out all firewood in thirty-pound bundles.
To ensure a steady temperature increase and consistent firings, each bundle is separated, approximately by eye, into a number of equal mini-bundles and stoked as shown on the chart at right.
The current schedule required eight bundles of willow wood followed by six bundles of ash wood, but early records make it obvious, against all expectations, that these two fuels are equal in calorific value. This came as a surprise to all of us. Householders know very well that a trailer load of ash gives more heat than a trailer load of willow, but this comparison is made on the basis of volume, not weight. Firewood dealers in Norfolk, United Kingdom, do not consider willow to be salable and burn it off in the woods as useless rubbish.Following this schedule, our last three glaze firings to Cone 10 have required 444 pounds of fuel burned in 605 minutes, 426 pounds of fuel burned in 590 minutes, and 438 pounds of fuel burned in 600 minutes. (The wheelbarrow of starting firewood is here disregarded.) Bisque firings to 860 degrees Celsius (Cone 013) required 260 to 270 pounds of firewood and follow a similar, naturally shorter timetable.
The Forestry Commission has published figures showing that different species of wood have almost the same calorific value. Our practice is to dry the willow under cover for one summer and the ash for two. Beyond any doubt willow will easily take this kiln to 1,300 degrees Celsius. The large firebox, equal in volume to the chamber, is almost certainly the reason for this....