606 POTTERY 



a furnace-flue runs. The breadth of this evaporating trough varies from 2 to 6 feet; 

 its length from 20 to 50 ; and its depth from 8 to 12 inches, or more. 



By the dissipation of the water, and careful agitation of the pap, an uniform doughy 

 mass is obtained ; which, being taken out of the trough, is cut into cubical lumps. 

 These are piled in heaps, and left in a damp cellar for a considerable time ; that is, 

 several months, in large manufactories. Here the dough suffers disintegration, pro- 

 moted by a kind of fermentative action, due probably to some vegetable-matter in the 

 water and the clay ; for it becomes black, and exhales a foetid odour. The argillaceous 

 and siliceous particles get disintegrated also by the action of the water, in such a way that 

 the ware made with old paste is found to be more homogeneous, finer grained, and not 

 BO apt to crack or to get disfigured in the baking, as the ware made with newer paste. 



But this chemical comminution must be aided by mechanical operations ; the first of 

 which is called the potter's slapping or wedging. It consists in seizing a mass of clay in 

 the hands, and, with a twist of both at once, tearing it into two pieces, or cutting it with 

 a wire. These are again slapped together with force, but in a different direction from 

 that in which they adhered before, and then dashed down on a board. The mass is 

 once more torn or cut asunder at right angles, again slapped together, and so worked 

 repeatedly for 20 or 30 times, which ensures so complete an incorporation of the 

 different parts, that if the mass had been at first half black and half white clay, it 

 would now be of a uniform grey colour. A similar effect is produced in some large 

 establishments by a slicing machine, like that used for cutting down the clay lumps 

 as they come from the pit. 



In the axis of a cast-iron cylinder or cone, an xipright shaft is made to revolve, from 

 which the spiral-shaped blades extend, with their edges placed in the direction of 

 rotation. The pieces of clay subjected to the action of these knives (with the reaction 

 of fixed ones) are minced to small morsels, which are forced pell-mell by the screw- 

 like pressure into an opening of the bottom of the cylinder or cone from which a 

 horizontal pipe about 6 inches square proceeds. The dough is made to issue through 

 this outlet, and is then cut into lengths of about 12 inches. These clay pillars 

 or prisms are thrown back into the cylinder, and subjected to the same operation 

 again and again, till the lumps have their particles perfectly blended together. This 

 process may advantageously precede their being set aside to ripen in a damp cellar. 

 In France the earthenware dough is not worked in such a machine ; but after being 

 beat with wooden mallets, a practice common also in England, it is laid down on a 

 clean floor, and a workman is set to tread upon it with naked feet for a considerable 

 time, walking in a spiral direction from the centre to the circumference, and from 

 the circumference to the centre. In Sweden, and also in China (to judge from the 

 Chinese paintings which represent their manner of making porcelain), the clay is 

 trodden to a uniform mass by oxen. It is afterwards, in all cases, kneaded like 

 baker's dough, by folding back the cake upon itself, and kneading it out alternately. 



Although we have abundant evidence proving to us the importance of the so-called 

 fermenting process, of the treading operation, and of the slapping, we are not in 

 possession of any explanation, which is in the slightest degree reliable, as to any one 

 of the changes which may be effected in the mass by these manipulations. 



The basis of the English earthenware is a clay, brought from Dorsetshire and 

 Devonshire, which lies at the depth of from 25 to 30 feet beneath the surface. It is 

 composed of about 24 parts of alumina and 76 of silica, with other ingredients in very 

 small proportions. This clay is very refractory in high heats, a property which 

 joined to its whiteness when burned, renders it peculiarly valuable for pottery. It is 

 also the basis of all the yellow biscuit-ware called cream colour, and in general of 

 what is called the printing body ; as also for the semi-vitrified ware of Wedgwood's 

 invention, and of the tender (soft) porcelain. 



The constituents of earthenware are, Dorsetshire clay, the powder of calcined 

 flints, and of the decomposed granite called Cornish stone. The proportions are 

 varied by the different manufacturers. The following are those generally adopted 

 in one of the principal establishments of Staffordshire : 



For cream colour ware, Silex or ground flints .... 20 parts. 



Clay 100 



Cornish stone 2 



Composition of the Paste for receiving the Printing Body under the Glaze. 



For this purpose the proportions of the flint and the felspar must bo increased. The 

 substances are mixed separately with water into the consistence of a thick cretim, 

 which weighs per pint, for the flints 32 ounces, and for the Cornish stone 28. The 

 china-clay is added to the same mixture of flint and felspar, when a finer pottery 

 or porcelain is required. That clay-cream weighs 21 ounces per pint. These 24 



