Technical Investigation of Chinese Pottery 91 



preparation of the glaze. It must have been made by the addition of 

 approximately one part of limestone, or the lime burned from it, to 

 two parts of the clay from which the body was prepared. It is also 

 possible, but not certain, that small quantities of soda and oxide of 

 iron were added to rectify minor defects. 



The calculation follows: It is assumed that the limestone is a pure, 

 more or less magnesian, limestone, such as would naturally be employed. 

 The limestone is taken to be somewhat magnesian, partly from inspec- 

 tion of the analyses, and partly because a non-magnesian limestone is 

 rather an unusual rock. As such a limestone is practically free from 

 silica, the silica of the glaze must come from the clay, and the ratio 

 of the silicas in body and glaze will give a measure of the quantity of 

 clay used in the mixture. As the body contains 71.61% silica, and the 

 glaze 54.17%, it is evident that, ignoring for the present losses in 

 burning, 75.66 parts of clay were used per 100 parts of glaze. The 

 following table may then be readily calculated: 



Table showing Relations between the Composition of the Glaze and of 

 a Mixture of 75.66% of the Pottery Body with 24.34% of Lime 



75-66% 



BODY OF BODY 



Silica, SiOi . . . .71.61 54. 17 



Alumina, AljOj 

 Iron oxide, FeO 

 Lime, CaO 

 Magnesia, MgO 

 Soda, NaiO . 

 Potash, KiO . 

 Carbonic Acid, COj 



18.67 1412 



3-57 2.70 



059 0.45 



o.33 °- 2 5 



4-43 3-35 



1.37 1.04 



100.57 76.08 99.27 — 36.93 



In the column marked "excess" are recorded the differences between 

 the actual and computed compositions of the glaze. These differences 

 are trifling. The absence of potash from the glaze is in line with the 

 known volatilization of potash from the surface of wares subject to 

 the kiln fires. 



The slight excess of iron oxide and soda in the mixture is not sur- 

 prising, as crude, untreated earths of the kind used are by no means 

 uniform in composition, and greater discrepancies than this are to be 

 expected in analyses of consecutive batches of such material. Especially 

 common is such an interchange of potash and soda as appears in this 

 instance. The correspondences between figures and theory are, in 

 fact, so close, that it is probable that the material employed was care- 

 fully selected by such physical characters as color, texture, etc. 



