POTTERY AND PORCELAIN. 453 



dried and undergone a red heat for a time, of losing this plasticity, 

 becoming irretentive of water to any considerable degree, hard, 

 close in texture, and able more or less perfectly to confine all 

 liquids contained within its hollow. 



Clay, however, is in all instances a very compound substance : 

 it owes its plasticity to alumine, which forms a constituent part of 

 it ; but the proportion of alumine varies considerably in different 

 species, and almost as much as the other substances with which it 

 is combined. 



It may hence be supposed that many of the impure- coloured 

 natural clays are of themselves sufficiently mixed with other earths 

 for the potter's use without any addition; but the white and finer 

 clays mostly require dilution with silex in some form or other, 

 which may be done to a considerable extent without taking away 

 the plasticity requisite for working. 



The most important circumstances requisite to be considered in 

 selecting the materials for pottery are plasticity, contractility, soli, 

 dity, and compactness after drying, colour, and fusibility. 



The plasticity seems to be simply owing to the proportion of clay 

 used, or relatively to the original plasticity of the clay itself; for 

 all clays are not equally plastic, and the superadded substances in 

 no instance increase this property, and in many cases considerably 

 diminish it. 



The texture, including the qualities of hardness and compact, 

 ness, depends partly on the mixture of siliceous (flinty or sandy) 

 ingredients with the clay, and partly on the heat employed in the 

 burning of the pottery. The purer natural clays are almost in. 

 fusible in any furnace heat; their hardness is nearly progressive 

 with the intensity of the fire, but they have the essential defects of 

 drying very slowly, of shrinking very considerably, and of becom- 

 ing rifty or full of minute cracks when dried, so as on this account 

 to be porous. It is therefore necessary to mix them intimately 

 with any other earth of qualities opposite to those of clay ; that is 

 which absorbs but little water, and quickly parts with it, (qualities 

 directly opposite to plasticity) and which dries compact and close. 

 The colour of the earths use*d is also of essential importance in 

 the finer pottery, in which the great desideratum is to find a clay 

 which after burning remains perfectly white. The appearance be- 

 fore burning cannot always be depended upon, for though in gene, 

 ral the whitest clays before burning are those that remain white af- 



