The small kiln or furnace specific for the baking of pipes, forms on the outside, Fig. 18, Pl. X, a kind of turret mounted on a base 32 inches in diameter, Fig. 19, this tower is 5 to 6 ft high, (I am speaking at the moment of the smallest kiln.), the walls are about 7 inches thick and form on the inside an octagon traced on a circle about seventeen inches internal diameter. The inside of the furnace, or the diameter of the chamber, is 14+ inches.
To get a proper idea of this kiln, we must note that the pipes are expected to be completely white, in baking they must not be exposed to the slightest bit of smoke. This is why the general system of these kilns is to have an oven below B, Fig. 20, Pl. XI, where wood is burned, and above, the pipes are carefully enclosed either in perfectly closed cassettes (caskets) or boisseaux (bushels), or in well enclosed spaces. In both cases, the pipes are not heated directly by the fire, but the heat which warms up either the chamber walls or the boisseaux bakes the contents enclosed within, as in a crucible which has no contact with the smoke.
This understood, we can distinguish in the kiln, Fig. 18, Pl. X, of which we see the vertical section, Fig. 20, Pl. XI, and the horizontal section underneath the furnace, Fig. 21, same plate:- 1. The exterior walls of the kiln A, Fig. 20, Pl. XI, which we call the surtout. 2. The furnace B, or the oven in which is the fire. 3. The chamber C, or the container in which the pipes are enclosed. 4. The dome D, of the chamber. 5. The dome E of the surtout. 6. The chandelier or spindle F, which serves to support the pipes in a vertical position. 7. The boisseaux (bushel) G, which serves the same purpose.
The jacket A, which forms the outside of the kiln, is constructed on the measurements we have just given, conforming to these plans and profiles, from broken tiles and furnace earth mortar. The oven or hearth B, is in the form of a vault, 17 inches in diameter and 2 inches thick, constructed with the broken tiles and of the furnace earth. The underside of this vault is raised 14 to 15 inches above the ground. It forms a lintel rounded by about 2 inches, and supported by eight small pillars, each one projecting out by three inches and 2 inches thick, and all built like the rest from the mortar of earth and broken tiles. This can all be seen in B, Fig. 20, and the wood is put in through a door H, Fig. 18, Pl. X.
So that the heat of the furnace can reach all around the chamber C, or the container, in the vault of the furnace and between the pillars which support it, openings are made I, Fig. 21, Pl. XI, 18 lignes wide and 5 to 6 inches long, which serve to let the smoke escape and to bring the heat to the space between the container and the surtout, (outer shell), because the pipes must be fired as if in a sort of tourtier, (pie dish). The pillars which support the vault go up to the base of the dome, but they are indented to receive the broken tiles which form the chamber or container, which are well grouted with the earth, so that the smoke which passes between all the pillars, does not penetrate into the part of the container where the pipes are. So we must imagine that container C, is surrounded by 7 flues of the chimney I, which go all around it.
The chamber or the container C, like the one we see in Fig. 20, placed above the hearth B, is the place where the pipes are arranged to be fired, this we call emporter. Here the pipes are arranged in a circle around a small earthen pillar which we call a chandelier F, Fig. 20. This is placed in the middle of the chamber and is supported by an iron rod which passes vertically through the centre; by means of this rod, several chandeliers can be placed one on top of another to support a very high column of pipes, as is the practice in the large kilns. These chandeliers are 1 inch in diameter, 8 or 9 inches high, and their heads are fluted to receive the stems of the pipes.
When several rows of pipes are placed around the chandelier, a boisseau is used to support the weight of the pipes, which having been placed one on top of another tend to slip away from the chandelier, more pipes are placed around the outside of the boisseau to fill the chamber completely.
That which is here referred to as a boisseau, is an earthen pot which has no bottom. It is ten to twelve inches in diameter, and eight to nine inches high, the earthenware is six to seven lignes thick. In the following we shall see that in large kilns several are placed one on top of another.
The pipes are arranged in the chamber in a circle around the spindle, as has been stated, bowl facing down, as can be seen in Fig. 20; but when there are five or six rows placed one on top of another, three or four more rows are placed above bowls facing up, and you observe this alternative position is used in order to get more into the chamber. The chamber or the pot having thus been filled with pipes, a dome over is made twelve to fifteen inches high, with sheets of thick paper covered in a layer of earth four to six lignes thick, this we call dorure, (gilding). This dorure is powdered pipe earth, moistened with sufficient quantity of water, to make it workable so that the artisan can apply and spread it by hand on the sheets of paper which are placed on a ring of pipes already baked but waste, which are born by one end on the column of pipes to be baked, and by the other on the sides of the octagon which forms the chamber. Thus these baked pipes are like a sort of framework which supports the gilded paper.
A top is made to the surtout, eighteen lignes distant from that of the chamber, it is made with tiles overlapped and jointed with earth, this kind of dome is finished with a pot K, Fig. 18, Pl. X, and Fig. 20, Pl. XI, which is pierced in the middle to let the smoke escape.
The kiln is heated with white wood, which makes an intense heat, and very little smoke when it is dry. In these small kilns, six or seven hours are sufficient to bake the pipes. It requires fourteen to fifteen hours to bake them in the large kilns. When the pipes are baked, and we wish to empty the kiln, or like d.poter, (take out of the pot), we demolish the two domes which must be made again each time new pipes are to be baked, then the kiln looks like a round tower 4 ft high, with no cover; in place of the dome, we place on top of the kiln a plank or large tile to hold in the heat, so that the pipes cool little by little. We shall see in a moment that the domes of large kilns are never demolished. The small kilns of which we have been speaking can contain nineteen to twenty gross of pipes. These are the types used at Rouen for which Mr. Dubois has been kind enough to give me the plans.
Duhamel du Monceau 1771