POTTERY 



617 



into one of the tuos A' ; but as this semi-liquid matter may still contain some granular 

 substances, it must be passed through a sieve before it is admitted into the tub. 

 There is, therefore, at the spot upon the tub 

 where the zinc pipe terminates, a wire-cloth 

 sieve, of an extremely close texture, to re- 

 ceive the liquid paste. This sieve is shaken 

 upon its support, in order to make it dis- 

 charge the washed kaolin. After the clay 

 has subsided, the water is drawn off from 

 its surface by a zinc syphon. The vats A' 

 have covers, to protect their contents from 

 dust. In the pottery factories of England 

 the agitation is produced by machinery in- 

 stead of the hand. A vertical shaft, with 

 horizontal or oblique paddles, is made to 

 revolve in the vats for this purpose. 



The small triturating mill is represented 

 in fig. 1651. There are three similar grinding-tubs on the same line. The details 

 of the construction are shown in fig. 1652, where it is seen to consist principally 

 of a revolving millstone, B (fig. 1653), of a fast or sleeper millstone, B', and of a 

 vat, c, hooped with iron, with its top raised above the upper millstone. The lower 

 block of hornstone rests upon a very firm basis, 6'; it is surrounded immediately 

 by the strong wooden circle c, which slopes out funnel-wise above, in order to 

 throw back the earthy matters as they are pushed up by the attrition of the stones. 

 That piece is hollowed out, partially to admit the key c, opposite to which is the 

 faucet and spigot c', for emptying the tub. When one operation is completed, the 

 key c is lifted out by means of a peg put into the holes at its top ; the spigot is 

 then drawn, and the thin paste is run out into vats. The upper grindstone, B d, 

 like the lower one, is about two feet in diameter, and must be cut in a peculiar 

 manner. At first there is scooped out a hollowing in the form of a sector, denoted by 

 d e f, fig. 1653;. the arc d /is about one-sixth of the circumference, so that the 

 vacuity of the turning grindstone is one-sixth of its surface ; moreover, the stone 

 must be channelled, in order to grind or crush 

 the hard gritty substances. For this purpose, 

 a wedge-shaped groove d e g, about an inch and a 

 quarter deep, is made on its under face, whereby 

 the stone, as it turns in the direction indicated 

 by the arrow acts with this inclined plane upon 

 all the particles in its course, crushing them and 

 forcing them in between the stones, till they be 

 triturated to an impalpable powder. When the 

 grindstone wears unequally on its lower surface, 

 it is useful to trace upon it little furrows, pro- 

 ceeding from the centre to the circumference, 

 like those shown by the dotted lines e' e". It 

 must, moreover, be indented with rough points by 

 the hammer. 



The turning hornstone-block is set in motion 

 by the vertical shaft H, which is fixed by the 

 clamp-iron cross, i, to the top of the stone. When 

 the stone is new, its thickness is about 14 inches, 

 and it is made to answer for grinding till it be 

 reduced to about 8 inches, by lowering the clamp 

 i upon the shaft, so that it may continue to keep 

 its hold of the stone. The mannner in which 

 the grindstones are turned is obvious from in- 

 spection of fig. 1651, where the horizontal axis i, 

 which receives its impulsion from the great water- 

 wheel, turns the prolonged shaft i/, or leaves it 

 at rest, according as the clutch I, I', is locked 

 or open. The second shaft bears the three bevel 

 wheels M, M, M. These work in three corre- 

 sponding bevel wheels M' M'M', made fast respec- 

 tively to the three vertical shafts of the millstones, 

 vishich pass through the cast-iron guide tubes M"M". 

 These are fixed in a truly vertical position by the 

 collar-bar m", m',fig. 1652. In this figure we see at m how the strong cross-bar of 



1652 



1653 



