Ceramic Review, Nov.-Dec. 1975
Two Chambered Kilns - A New Approach by Robert Compton
In this article Robert Compton, an American Potter working at the Mad River Pottery, Vermont, USA describes how he has used exhaust hear from his glost kiln to fire his bisque ware. Robert Compton makes a range of functional stoneware and architectural pieces. Comments are included from potters in Britain who have built multi-chambered kilns.
The exhaust of a stoneware firing approaches 1260 C (2300F) a fact that disheartens every potter with today's high cost of fuel. Since bisque need only reach 900C (1600F) the logic of utilizing exhaust from a glost to fire bisque is obvious. I have successfully developed a means of controlling a slave chamber fired entirely off the exhaust from a glost firing. The value of this to the potter is threefold. Since the bisque requires no additional expenditure of fuel, there is a savings of money. The simultaneous firing of glost and bisque requires the potter to kiln sit only once and achieve the result of two firings, so there is a saving of time. Less tangible, but not less important, is the increase of efficiency; since the glost chamber demands greenware before the next glost is begun, thus a steady flow of work is encouraged.
While most downdraft kilns could be adapted to use with a slave chamber, a few basic principles must be followed. The glost chamber must be significantly larger than the bisque chamber, since there is a good deal of heat loss in transfer, but this presents no problem because a small bisque fires enough pots to fill a larger glost. The glost kiln is my shop is 64 cubic feet and fires a slave kiln of 27 cubic feet with its exhaust. The most important aspect of the design involves the placement of the chimney and dampers. Two separate chambers sharing one chimney permit a shirt and direct flue configuration. The simplicity of this becomes clear when examining the damper arrangement. Three dampers control exhaust from the glost before it is permitted to escape up the chimney. When closed, a horizontal damper #1 in the chimney prevents the exhaust from taking a direct course up the chimney, but when opened, allows a by-pass of the bisque chamber. Two vertical dampers (one below #2 and one above #3 the horizontal damper) control the passage of heat into and our of the bisque chamber. When the two vertical dampers are open the horizontal damper is shut and the exhaust from the glost travels under the chimney and into the bisque chamber before escaping up the chimney. When the two vertical dampers are shut the horizontal damper is open and the exhaust from the glost is vented directly up the chimney (see diagram).
Thus, two kilns may be fired for the cost of one, and each has the exacting control necessary to insure good firings. Doing a glost firing in the 64 cubic foot camber consumes about 52 US gallons of Liquid Petro. Gas (present cost 37 cents/gal) costing about US$20. This figure could be reduced by firing at a faster pace, but I find it necessary to safely dry very large green pots.
The glost kiln has four atmospheric burners, one in each corner of the kiln. Their flames are controlled by bag walls and target bricks, this helps to even the firing (there is less than a half cone difference top to bottom, front to back).
FROM John Malby, Stonehill Pottery
Bisque fired with the excess heat from previous glost chambers was, I'm almost certain, a common feature of traditional Chinese and Japanese climbing kilns: certainly such an arrangement was included in the plans for the first St.Ives climbing kiln designed upon traditional lines by Mr. Matsubayashi for Bernard Leach (see diagram page 187A Potters Book, B. Leach). Note that the "first damper stops the draft thorough the last chamber (which was usually I'm sure used for bisque-though I see that saggers and shelves are drawn in the diagram!) And with the second damper open, the draught would by-pass this chamber under the floor (far simpler than the American concept).
My own experiences is that, probably as a result of relatively quicker firings with fuels other than wood (which as such a long flame and therefore hearts the bisque chamber rather more), there is no danger of overfiring - experience with my 3 kilns built over a period of years using this principle indicates that the second chamber usually manages to get to about 800C when the first finishes at 1300C. It is then a simple matter to boost it up to approx. 980C - with the burner from the finished glost firing, though I have done it at the same time as the glost was firing with an extra burner.
I do believe that the biscuit done in this way is beneficial - not only from an economic angle but the long duration of the second chamber firing (about 20hours) means a gentle evenness; very important with large square dishes for instance (which invariably crack in the electric bisque firing: hardly ever inn the second chamber). In order to save the cost of a burner one of the burners in the first chamber pivots on its air pipe into the bisque chamber.
FROM Peter Dick, Coxwold Pottery, York
Basically this seems a sound scheme and should be of interest a number of potters. Most of us must be aware of the potential saving in firing a bisque chamber with the waste heat from the glaze but it would do no harm to remind readers of the fact now that it could be added to existing kiln/chimney where provision for a second chamber had not been made. It might be a good thing if the contributor make this clear. It would also be valuable if he went into some detail of the damper construction, working life, etc.,- also some suggested flue sizes of different capacity kilns.
FROM Wally Keeler, High Wycombe, Bucks
I like this idea very much. It seems - on paper at least - to be far better than the usual two-chamber layout with the chimney after the second chamber. It offers a high degree of control in the simplest possible way (one can end up with a labyrinth of by-pass flues etc., with the old layout). What is more one could add a chamber to most existing kilns in this way (providing there is room behind the chimney) with relative ease. The comments about the relative sizes of the chambers seem to be vital - too large a bisque chamber would mean underfiring or providing it with a burner.
FROM David Winkley, Vellow Pottery
The premise, that there is enormous economies to be made in terms of both time and money is unassailable. But I think the ideas are a bit clumsy and the saving in fuel achieved with this plan are not as great as they could be given different solutions. Building a separate chamber for biscuit takes up more room, uses more bricks, is more complicated to build and lacks the flexibility of many other plans I know or use myself. There is, I think some chance that if it is published one or two aspiring kiln builders may follow the idea and could come unstuck, particularly if they scaled the plan u or down inn size. The fact that the kiln works for the US potter as well as he says it does is, I think, more fortuitous that planned, just because of the relative sizes of the two chambers.
FROM Ray Finch, Winchcombe Pottery
This seems quite an ingenious idea but it seems to me that the main problem with 2 chamber kilns is achieving an even distribution of heat in chamber 2. I can see that this plan would avid overfiring but wonder if you might not get considerable variation in the porosity of the bisque (a great nuisance!)
We fired 2 chambers here at Winchcombe for many years and out solution was to have both chambers of the same capacity. This meant that chamber 2 never reached more than 700C in the exhaust from chamber 1. This was supplemented by 2 burners in the second chamber, which took the bisque up to temperature with the necessary control to ensure an even heat throughout - with the added bonus that chamber 2 could be fired to glost if required.
Robert Compton replies: -
With regard to damper construction; my dampers are a fireclay composition (purchased from a potters supply company) and measure 9" x 18". The flue openings measure 9"x 9". These dampers have gone though 40 firings to date and show no sign of wear.
When firing only the glost chamber damper #1 is opened to a maximum of 3", a total of 27 square inches of opening (3" deep by 9" wide). When firing both chambers damper #1 is closed, damper #2 is opened about 7" and damper #3 is opened a maximum of 1.5", a total of 14 square inches of opening. I could have had smaller flues, but would rather over design these areas when building to be on the safe side.
The height and composition of the chimney is also a factor in draft and thus affects the size of the flues. My chimney is 20 ft high and composed of blocks with a flue tile liner.
As several of the British potters indicated in their comments, there is little chance of over firing the bisque with the exhaust from the glost. I fire with L.P. gas, which has a short flame, and my bisque reaches about 780C when my glost is cone 9. A shutoff between the two chambers allows firing the chambers separately, or the heat from the glost can be used to pre-heat a gloss in the second chamber.
The ability of my bisque chamber to be used without the gloss chamber is the primary difference between my design and that referred to by John Maltby (in a Potters Book p.187). In 1971 I built a different two-chambered kiln with a by-pass under the floor, but I have found the chimney between the chambers the least complex system. With this system a chimney and second chamber can be added to an existing kiln without great difficulty.
With the cost of materials being quite high, I design and build with as much thought as possible to the future. My kilns are laid dry (without cement) so it may be dissembled and rebuilt if necessary. In fact some of the bricks in my present kiln have been in four previous kilns.
I do not claim to have developed any new breakthrough in kiln design. I know of very few US potters who use multi-chambered kilns and those who do, have climbing kilns. My design is best suited to the features I wanted in a kiln, without the limitation to firing both chambers at once. It has flexibility, an attribute I have not found many kilns here in the US.