626 



POTTERY 



Glase-Jciln. This is usually smaller than the biscuit-kiln, and contains no more than 

 40 or 45 bungs or columns, each composed of 16 or 17 saggers. Those of the first 

 bung rest upon round tiles, and are well luted together with a finely-ground fire-clay 

 of only moderate cohesion ; those of the second bung are supported by an additional 

 tile. The lower saggers contain the cream-coloured articles, in which the glaze is 

 softer than that which covers the blue-printed ware ; this being always placed in the 

 intervals between the furnaces, and in the uppermost saggers of the columns. The 

 bottom of the kiln, where the glazed ware is not baked, is occupied by printed 

 biscuit-ware. 



Pyrometric balls of red clay, coated with a very fusible lead-enamel, are employed 

 in the English potteries to ascertain the temperature of the glaze-kilns. This enamel 

 is so rich, and the clay upon which it is spread is so fine-grained and compact, that 

 even when exposed for three hours to the briskest flame, it does, not lose its lustre. 

 The colour of the clay alone changes, whereby the workman is enabled to judge ot 

 the degree of heat within the kiln. At first the balls have a pale red appearance ; 

 but they become browner with the increase of the temperature. The balls, when of a 

 slightly dark-red colour, indicate the degree of baking for the hard glaze of pipe-clay 

 ware ; but if they become dark brown, the glaze will be much too hard, being that 

 suited for ironstone-waie ; lastly, when they acquire an almost black hue, they show a 

 degree of heat suited to the formation of a glaze upon porcelain. 



The glazer provides himself at each round with a stock of these ball watches, reserved 

 from the preceding baking, to serve as objects of comparison ; and he never slackens 

 the firing till he has obtained the same depth of shade, or even somewhat more ; for 

 it may be remarked, that the more rounds a glaze-kiln has made, the browner the 

 balls are apt to become. A new kiln bakes a round of enamel-ware sooner than an 

 old one ; as also with less fuel, and at a lower temperature. The watch-balls of these 

 first rounds have generally not so deep a colour as if they were tried in a furnace 

 three or four months old. After this period, cracks begin to appear in the furnaces ; 

 the horizontal flues get partially obstructed, the joinings of the brickwork become 

 loose ; in consequence of which there is a loss of heat and waste of fuel ; the baking 

 of the gl;ize takes a longer time, and the pyrometric balls assume a different shade 

 from what they had on being taken out of the new kiln, so that the first watches are 

 of no comparable use after two months. The baking of enamel is commenced at a 

 low temperature, and the heat is progressively increased ; when it reaches the melt- 

 ing-point of the glaze, it must be maintained steadily, and the furnace-mouths be care- 

 fully looked after, lest the heat should be suffered to fall. The firing is continued 14 

 hours, and then gradually lowered by slight additions of fuel ; after which the kiln ia 

 allowed from 5 to 6 hours to cool. 



Muffles. The paintings and the printed figures applied to the glaze of earthen- 

 ware and porcelain are baked in muffles of a peculiar form. Fig. 1659 is a lateral 



elevation of one of these muffles ; fig. 





1659 



e 



1660 



1660 is a front view. The same letters 

 denote the same parts in the two 

 figures. 



a is the furnace ; b, the oblong muffle, 

 made of fire-clay, surmounted with 

 a dome pierced with three apertures, 

 k, k, k, for the escape of the vaporous 

 matters of the colours and volatile oils 

 with which they are ground up ; c is 

 the chimney ; d, d, feedholes, by which 

 the fuel is introduced ; e, the fire- 

 grate ; f, the ash-pit ; channels are 

 left in the bottom of the furnace to 

 facilitate the passage of the flame beneath the muffle ; g is a lateral hole, which 

 makes a communication across the furnace in the muffle, enabling the kiln-man to 

 ascertain what is passing within ; k, k, are the lateral chinks for observing the pro- 

 gress of the firing or flame ; J is an opening scooped out in the front of the chimney to 

 modify its draught. 



The articles which are printed or painted upon the glaze are placed in the muffle 

 without saggers, upon tripods, or moveable supports furnished with feet. The muffle 

 being charged, its mouth is closed with a fire-tile well luted round its edges. The fuel 

 is then kindled in the fireplaces d, d, and the door of the furnace is closed with bricks, 

 in which a small opening is left for taking out samples, and for examining the interior 

 of the muffle. These sample or trial-pieces, attached to a strong iron wire, show the 

 progress of the baking operation. The front of the fireplaces is covered with a sheet- 

 iron plate, which slides to one side, and may be shut whenever the kiln is charged. 



