History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t4.85

v1.c13.s2.t4.85

After about a week’s time when the clay wall was still moist the front opening of about 300 mm width and 200 mm height was cut open as shown in the figure. This opening was made for preparing the bottom of the furnace, fixing of the clay tuyere pipe and removing the molten slag. The top diameter was measured by old craftsman using a piece of stick and then the excess clay was scrapped away. The taper and bottom diameter of the furnace was checked by dropping a wet clay ball from the top inner edge and then measuring the distance between the centre of the ball mark and the wall. The process was repeated several times all round the circumference of the top inner edge. After correcting the furnace shape and size the inner wall was coated with a locally available non shrinking refractory clay. The bottom of the furnace was made from a mixture of charcoal powder and the mud clay to have a taper towards the opening. Inside the bottom opening a clay pipe having shape like a trumpet with about 50-60 mm diameter at the larger end and 20-25 mm at the smaller end was fixed as shown in the figure and then the mouth was sealed with the clay-charcoal powder mixture. The bamboo pipe of the two air bellows or foot operated *Bhāthi* were fixed at the mouth of the clay trumpet as shown in *Fig.29*. Finally the furnace was dried by kindling a fire inside it and slowly raising the temperature to bake the furnace. Now the furnace was ready for operation.