|September 06 firing|
Loughborough anagama construction and firing.
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|Fifty cubic feet of pots, fire and ash.|
This text was originally
published in The
Fifty Cubic Feet of Pots,
Fire and Ash
It was decided to build a new anagama at the ceramics department of Loughborough University School of Art and Design (LUSAD), England, in June 2002. Our existing small anagama (15cu.ft./0.43m3) had been fired about 12 times over its 2-year existence, each firing lasting around 36 hours. While some lovely results - ash deposits and body colours - had been achieved, it was however just a little too small for all the effort of firing. The down-draught kiln (30cu.ft/0.85m3) in the department had also been fired regularly, but interest in the subtle fly-ash flashing which it produced on the wares, had waned over the past couple of years, with students now favouring the more dramatic results obtainable through a longer firing. It was proposed that we build a kiln with a packing capacity of around 50cu.ft. (1.42m3), which would be large enough for several students to have a reasonable amount of work in each firing. This seemed to make sense, as we could fire the kiln for the same period, putting in the same effort, and have many more pots to show for it.
So in October we started building the new anagama, having knocked down the two existing kilns and salvaged as many bricks as possible . We were lucky enough to obtain 1,000 brand new, straights 9 x 42 x 3 inches (23 x 11.4 x 7.6cm), 42% alumina bricks, which was a luxury as the two previous kilns had been built mainly with used and odd tapered bricks. Firstly a plan of the proposed kiln was drawn up, and an outline was roughly laid in the space, to help us visualise the dimensions. I had three students: Ben Saddler, Fian Andrews and Symeon Vickerstaff, who were keen to help and gain experience in kiln building. Having done many firings in several different anagama, I had a good idea about the particular features which we wanted to incorporate in the new kiln.
The main elements were a sunken firebox with a low grate, and a teardrop shaped chamber, tapering in from the sides and roof towards the back. Masanobu Izumihara, a Japanese potter with whom I have fired in the past, is adamant that the teardrop shape is crucial for the distribution of ash throughout the kiln. Having fired both the anagama at Wysing Arts in Cambridge-shire (which is teardrop shaped and built by Masanobu Izumihara to a traditional Bizen design - see The Log Book issue 5), and also the anagama at Rufford Craft Centre in Nottinghamshire (which is more of a tunnel), the better ash deposits beyond any doubt were from the Wysing kiln.
The kiln was to be built using a fireclay and sodium silicate mortar - not a binding mortar. The quantity of sodium silicate used is sufficient to make the mixture smooth and workable. Although it is not a bonding mix, it fluxes slightly with fly-ash in the firing, to form a hot face seal between the bricks. For a bulkier mix I added very course alumina as an aggregate.
As a result of conversations with American potter Dick Lehman, while he was on a visit to the LUSAD, I decided to make provision for the construction of a step-down grate in the new kiln. Dick had explained about his experiences of building and using a step-down grate, that allows him to carry out very long firings with minimum stoking for a large part of the firing (see 'A New Approach to Long-Fire Results' - Ceramics Technical No.9). Basically our kiln had to be as versitile as possible, given the educational environment within which it was to be used.
In order to achieve the
sunken firebox it was not an option to dig into the kiln yard, so we had to
raise the chamber. One hundred hollow concrete blocks were used, to raise
the packing space high enough to create a sizeable firebox area allowing room
for the grate. Once these were bedded on sand, the first layer of floor bricks
The concrete blocks were laid in such a way as to allow air to move freely beneath the structure, thus allowing heat to dissipate. The air inlets below the firebox grate would be 3 inches (7.6cm) in heigth, so the floor of the firebox would be 6 inches (15.2cm) above the base bricks, which I considered ample to minimise the heat transference to the yard surface beneath it.
Once the floor of the chamber was in place, we laid the first layer of wall bricks. Next the firebox was built up to the level of the chamber floor, tied in, and the walls continued back as far as the chimney. The chimney base was a continuation of the kiln wall, so that the whole structure was keyed in. After five courses were laid, the flues were positioned and the bridging bricks tied into the main walls. This required the cutting of several precise wedges, and thanks to the sculpture department's 9-inch angle-grinder, the job was made significantly easier than cutting the bricks with lump hammer and bolster. A large stoke-hole and six inlets were built into the side of the firebox to be used as side-stoke holes, or if we chose to use a step-down grate, they could be used to control the air into the steps. At eight courses the main stoke-hole for the step-down grate was built in. At 12 courses the former for the door arch was put in place. Suddenly it started to feel like a kiln.
After two more layers of bricks it was time to begin the arch. A former was constructed using 1 inch (2.5cm) strips of thin plywood, which were woven and tied together to form an upside-down, boat/basket shape. This was then reinforced inside with four chipboard formers, spaced equally from front to back.
The back of the arch was tied into the base of the chimney. The chimney collection box was built of a double brick thickness up to the tenth layer, incorporating two passive dampers. The total height of the chimney, from the floor of the chamber is 12 feet (3.63m). The chimney was taken up through the yard roof with a flame viewing hole incorporated just below the roof line.
We had attempted to design the kiln so that it would be almost self-supporting, with an existing wall used for support on one side, two flying buttresses on the yard side, and an angle-iron bracing on either side of the door arch. (The kiln was given the name 'Stegosaurus' due to the two fins which it had acquired.)
As the arch began to take shape, once again the angle-grinder made the job of cutting the odd angled bricks required much easier. The arch was built using an initial one layer thickness, with tie-in bricks for the second layer about two thirds of the way up. Two spy holes and a blow-hole were incorporated into the top section of the arch.
A four-inch layer of insulation was applied to the arch. The second floor layer was put in place, and the firebox grate fitted. Finally, having built it throughtout the winter months, our new kiln was at last ready for the real test - firing!
Previously we had usually fired with pine slab off-cuts. However, due to our now larger firebox capacity, we decided to use some hardwoods as well. (I have used a two-thirds pine and one-third hardwood mix in firings of the Wysing Anagama). Our wood source could supply walnut and birch, comprising of slab off-cuts and waste wood from furniture production. This would enable us to build up the embers well, providing a hot bed for the pine. Due to the increased size of the kiln and the need to dry it out slowly, we decided to fire for three days. A rota was drawn up so that we had shifts covered through three days and two nights. The students had made up clays and porcelains developed for extended woodfirings (see 'The Perfect Body' - Ceramic Review No.196), and created a wide range of work to be stacked in the kiln.
The firing began at 08.30.am on Wednesday, with a small fire outside the firebox. As the temperature was slowly increased, stoking moved to the upper fire-mouth, and was held at a pyrometer reading of 700°C for twenty-four hours. Once stoking had moved to the upper fire-mouth the lower one was closed, allowing only a small amount of air in and the below grate air vents were opened. The ash was clearly visible churning around in the chamber, and by Thursday morning there was a good layer visable on the pots. The steam emitting from the kiln had subsided at this point, and the temperature was slowly increased.
Stoking was alternated between hardwood and softwood. By the early hours of Friday morning cone 12 was over at the front, but we only had cone 9 over at the back. This was not particularly surprising, since we had built the chimney flues much larger than required. We had done this with all the critical dimensions, working to the old adage that it is a lot easier to make an opening smaller, than it is to increase its size. We finished the firing at 09.00.am on Friday, clammed up the kiln, and began the agonising wait for it to cool. There were ten of us - two staff and eight students - who had worked for and with this firing, which had lasted a total of 49 hours.
For the first firing of a new kiln it was a great success, plenty of ash build-up - pooling and rivulets. The colours generated in the clays were good, with some of the pots showing a striking record of the movement of the flame.
The flues were subsequently reduced in size - although they are still slightly larger than specification. This increases our control of the atmosphere during firing and enables us to put the kiln into heavier reduction if desired, as well as bringing up the temperature at the back of the chamber.
The second firing, which lasted 56 hours, produced even better results than the first, and cone 12 was achieved at both the front and the back, with heavy ash deposits throughout the chamber. Despite the effort, the results achievable from a protracted woodfiring are unique, challenging, sumptuous, exhilarating, and unobtainable by any other means.