Clifton College Scientific Society. 39 



times in the furnace before they are rendered useless. The largest 

 and coarsest saggars are usually placed on the floor of the oven, 

 which must be previously covered with a layer of sand. 



If the heat be not equally distributed through the whole area, 

 some pieces would be injured by excessive firing, while others 

 would be inadequately baked. The bottom of the saggars being 

 flat, each, as it is placed upon another, forms a cover to the one 

 beneath, and the entrance of smoke is further prevented by placing 

 a ring of soft clay on the upper rim of each case. In this way 

 the saggars are built up to the top of the oven, the uppermost 

 being always left empty. Each of these piles, as it stands, is 

 called a hung. The process of baking usually lasts from forty- 

 eight to fifty hours, during which time the heat is gradually in- 

 Creased. In order to ascertain when the baking has been carried 

 far enough, the ovenman places trial pieces in different parts of 

 the oven, but so disposed that they can be readily taken out for 

 examination. These pieces are rings made of common Stafford- 

 shire fire-clay, which is found to have the property of changing its 

 colour with each accession of heat. By comparing these rings 

 therefore with pieces of the same clay which have previously been 

 sufficiently baked, and which serve as a standard, the actual pro- 

 gress of the wares in the oven may at any time be ascertained 

 precisely. When the appearance of these trial pieces is judged 

 satisfactory the firing is then discontinued, the furnace and ash- 

 pit doors are closed, and the oven, with its contents, left to cool 

 gradually twenty-four or thirty hours. From the similarity of 

 the ware in this state to the appearance of well-baked ship-bread, 

 the ware is now called biscuit-ware. 



If it were attempted to apply the glaze to articles of porcelain 

 and earthenware without their previous conversion into biscuit- 

 ware, their texture and shape would be injured by the absorption 

 of water from the glaze. It must be borne in mind that the 

 shrinking of clay upon the application of heat is permanent, and 

 that no alteration of its bulk will occur, unless it be subjected 

 to a still higher degree of temperature. By limiting, therefore, 

 the heat of^the gloss-oven, in which the baking is finished, below 

 that applied to the biscuit, the risk of cracking the glaze through 

 the contraction of the ware is obviated. The glaze usually em- 

 ployed for common kinds of earthenware is compounded of 

 litharge of lead and ground flints in the proportion of ten parts 

 by weight of the former to four parts of the latter. Cornish 

 granite is sometimes substituted for flint, and used in the pro- 

 portion of eight parts to ten of litharge. This method of glazing 

 is objectionable on account of the injury which, notwithstanding 

 every precaution that can be taken, it occasions in application to 



