An Encyclopaedia of Indian Archaeology v1.c21.s12: 16.12 Pottery Kilns
An Encyclopaedia of Indian Archaeology v1.c21.s12
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16.12 Pottery Kilns
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The three essential components in a pottery kiln are: (1) a fire-chamber with one or more fire-places, where fuel is burnt in a continuous supply of air; (2) vents to transfer the hot gases generated in the fire-chamber into the baking chamber; (3) a baking chamber which is an enclosed space, like an oven, where the sun-dried clay objects are carefully stacked and heated to the required temperature. Depending upon how the draught of hot gases moves within the baking chamber, the pottery kilns are called: (1) vertical up-draught kiln, (2) vertical down-draught kiln, and (3) horizontal kiln.
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Evidence of the earliest pottery-kilns in the subcontinent comes from Mohenjo-daro. One of them in DK area in Block 9 in late levels enables us to visualize its shape and technical aspects. It is a vertical up-draught kiln of a cylindrical shape. At the bottom is the fire-chamber, 2·25 m in diameter and 1·04 m in height, nearly three-fourths of it being below the ground level. Above it is the baking chamber, with its floor also 2·25 m in diameter, supported by the circular wall of the fire-chamber and a column of bricks at the centre. It is perforated with 14 holes, to act as vents, each measuring about 11 cm in diameter, 13 of them arranged in a ring near the edge of the floor and one near the middle. The enclosure wall of the baking chamber is built of brick laid in mud mortar, plastered inside with clay. As extant it measures 40 cm in height and is without any vertical curvature. The height of the kiln cannot therefore be determined. It was probably domed with a chimney at the top like the 6th century B.C. Corinthian pottery kilns. LOTHAL (Harappa [4.9]) and INAMGAON (Chalcolithic [4.14.6] also have pottery kilns. [See also DAIMABAD.]
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While pottery in a vertical up-draught kiln is baked hot gases rise up in the baking chamber and pass out through the chimney, inducing a natural draught of air. This draught is controlled to raise the temperature of the wood fire to 800° to 900° C in the baking chamber. But the heat within the chamber is not equally distributed. It is higher near the bottom and becomes progressively less towards the roof. Vertical down-draught kilns and horizontal kilns are better for a uniform distribution of heat within the baking chamber. In a vertical up-draught kiln the potter stacks the heavier thick-walled storage-jars at the bottom of the chamber and smaller thin-walled vessels higher up in the pile. Thus he sees to it that the heating is adequate for all the ware stacked in the kiln.
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When pottery is baked in a kiln there is not much wastage. It is also uniformly and adequately baked, free from smoke stains. But the process is not inexpensive. Kilns can be fired only by high-priced dry billets of hardwood or charcoal that gives intense, smoke-free flame. Also, the construction of a kiln involves a certain amount of capital expenditure. It would become profitable only if it is continuously used to meet the demands of a large pottery market. It is uneconomical for a peasant-potter working to meet the needs of his village. But it is doubtful if all the Harappa pottery was baked in kilns; it is likely that much of it was baked in open fires, which, as Mackay, 1938, points out, is common in Sind today. Mackay further observes that successful baking of pottery does not depend upon the type of baking device. In Kutch also there are potters who use open fires to produce as good pottery as the Harappa. The rarity of ancient Indian pottery kilns suggests that more than 99% of ancient Indian pottery was baked in open fires or in *gajaput* fires. But these baking practices do not leave behind any tangible relics.
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No clearly intelligible evidence of ancient Indian open fires or *gajaput* fires used for baking pottery has so far been discovered in excavations. However the simple and inexpensive but adequate pottery-baking techniques used by the modern Indian village potter enable us to visualize the techniques of the ancient potter. An open fire for baking pottery consists of a shallow circular pit, 4 to 5 m in diameter and 50 to 75 cm in depth. It is loosely filled to the brim with different kinds of easily available inexpensive fuels like dry brushwood, rice husks, dry grass or hay and cow-dung cakes. Pots are carefully piled up within the circle. The heavier thick-walled storage jars are kept at the bottom near the middle of the circle and progressively smaller pots are kept one above the other in the pile. Thus the potter maintains the equilibrium of the pile by retaining its centre of gravity near the bottom. Cow-dung cakes, hay and rice husks are put among the pots and also above them as they are placed in the pile. Finally the whole stack is completely covered with brushwood and hay and plastered with a thin layer of clay. At the top of the pile a 50 to 75 cm long cylindrical clay pipe is fixed like a chimney. Around the bottom of the pile, at the ground level, are arranged 10 to 12 air inlets made up of rims of broken pottery. While these holes are also used to set fire to the pile, they primarily act as vents for natural draught of air into the burning pile.
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The firing may take 8 to 16 hours or more, depending upon the size of the pile and its contents. The potter continuously keeps a watch on the pile to cover any hole through which heat may escape. In laboratory experiment the temperature of an open fire using cow-dung cakes as the main fuel has been found to reach 950° to 1000° C (a temperature of 800° C is adequate for baking pottery). The potter allows a sufficiently long time, two days or more, for the pile to cool so that the risk of breakage is minimized.
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Some important ancient Indian wares like the PGW [11.14], Black-slipped Ware [11.13] and NBPW [11.16] were baked in a reducing atmosphere. Even now a group of potters produces a shining black pottery similar in appearance to the NBPW. They use the following simple technique: they stack their sun-dried ware inside a large-mouthed, thick-walled, well-baked jar; the space inside the vessels and between them is filled with goat droppings; and then the jar is sealed with several layers of clay and cloth to make it completely air-tight. The pottery in the jar is then baked in *gajaput* fire. [See also 11.16.]
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The *gajaput* fire consists of a pit of about 90 cubic cm. At the bottom it is closely packed with cow-dung cakes up to a height of 20 to 30 cm. The sealed jar is placed at the centre of the pit above the cow-dung cakes. It is then fully surrounded with cow-dung cakes and excess cakes are piled up above the pit. It is then set fire to and allowed to burn first slowly for a few hours, and then with strong intense fire for 8 to 12 hours. Within the jar the pottery is baked in a reducing atmosphere of excess carbon monoxide. Under the reducing condition the pottery develops a black surface finish.
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Hegde, K.T.M., 1975, in *Antiquity*, 49, 195, pp. 187-90 (for PGW). Leach, B.A., 1945, *Potters' Book*, London. Mackay, E.J.H., 1938, *Further Excavations at Mohenjo-daro*, 1, pp. 177-8. Mitra, D., 1972, *Excavations at Tilaura-kot & Explorations in Nepalese Tarai, Kathmandu*, pp. 250-2 (for pottery-baking technique in the *tarai*). Norton. F.H., 1974, *Elements of Ceramics*, Reading For sites see respective entries in vol. II.—K.T.M. HEGDE