POTTERY 605 



the factory the clay is cut to pieces, and then kneaded into a pulp with water, by 

 engines; instead of being broken down with pickaxes, and worked with water 

 by hand-paddles, in a square pit or water-tank, an old process, called blunging. The 

 clay is now thrown into a cast-iron cylinder, 20 inches wide and 4 feet high, or into 

 a cone 2 feet wide at top and 6 feet deep, in whoso axis an upright shaft revolves, 

 bearing knives as radii to the shaft. The knives are so arranged, that their flat sides 

 lie in the plane of a spiral line ; so that by the revolution of the shaft, they not only 

 cut through everything in their way, but constantly press the soft contents of the 

 cylinder or cone obliquely downwards, on the principle of a screw. Another set of 

 knives stands out motionless, at right angles from the inner surface of the cylinder, 

 and projects nearly to the central shaft, having their edges looking opposite to the 

 line of motion of the revolving blades. Thus the two sets of slicing implements, the 

 one active, and the other passive, operate like shears in cutting the clay into small 

 pieces, while the active blades, by their spiral form, force the clay in its comminuted 

 state out at an aperture at the bottom of the cylinder or cone, whence it is conveyed 

 into a cylindrical vat, to be worked into a pap with water. This cylinder is tub- 

 shaped, being about four times wider than it is deep. A perpendicular shaft turns also 

 in the axis of this vat, bearing cross spokes, one below another, of which the vertical 

 set on each side is connected by upright staves, giving the moveable arms the appear- 

 ance of two or four opposite square paddle-boards revolving with the shaft. This 

 wooden framework, or large Hunger, as it is called, turns round amidst the water and 

 clay lumps, so as to beat them into a fine pap, from which the stony and coarse sandy 

 particles separate, and subside to the bottom. Whenever the pap has acquired a 

 cream-consistenced uniformity, it is run off through a series of wire, lawn, and silk 

 sieves, of different degrees of fineness, which are kept in continual agitation backwards 

 and forwards by a crank mechanism ; and thus all the grosser parts are completely 

 separated, and hindered from entering into the composition of the ware. This clay- 

 liquor is set aside in proper cisterns, and diluted with water to a standard density. 



Hints. These are obtained in great quantities from the chalk formations. See 

 FLINTS. 



Chert, which is a flinty substance, found in the Mountain Limestone, is also em- 

 ployed. This is calcined and ground. See CHEHT. 



Felspar. Used both in the body and in glazing. See FELSPAR. 



Bone. Bone-ashes, (phosphate of lime), also enters into the composition of pottery. 



Steatite, or Soap-stone, is occasionally employed. See STEATITE. 



China-stone. A decomposed granite. See CHINA-STOKE. 



These may be regarded as the substances which enter into the body of the ware. 



Clay alone cannot form a proper material for pottery, on account of its great con- 

 tractility by heat, and the consequent cracking and splitting in the kiln of the vessels 

 made of it; for which reason a siliceous substance incapable of contraction must 

 enter into the body of pottery. For this purpose, ground flint, called flint-powder 

 by the potters, is universally preferred. The nodules of flint extracted from the chalk 

 formation are washed, heated red-hot in a kiln, like that for burning lime, and thrown 

 in this state into water, by which treatment they loso their translucency, and become 

 exceedingly brittle. They are then reduced to a coarse powder in a stamping-mill or 

 a crushing-mill. The pieces of flint are laid on a strong grating, and pass through 

 its meshes whenever they are reduced by the stamps to a certain state of comminution. 

 This granular matter is now transferred to the proper flint-mill, which consists of a. 

 strong cylindrical wooden tub, bottomed with flat pieces of massive chert, or horn- 

 stone, over which are laid large flat blocks of similar chert, that are moved round 

 over the others by strong iron or wooden arms projecting from an upright shaft made 

 to revolve in the axis of the mill-tub. Sometimes the active blocks are fixed to these 

 cross arms, and thus carried round over the passive blocks at the bottom. Into this 

 cylindrical vessel a small stream of water constantly trickles, which facilitates tha 

 grinding motion and action of the stones, and works the flint-powder and water into 

 a species of pap. Near the surface of the water there is a plug-hole in the side of the 

 tub, by which the creamy-looking flint-liquor is run off from time to time, to bo passed 

 through lawn or silk sieves, similar to those used for the clay-liquor ; while the par- 

 ticles that remain on the sieves are returned into the mill. This pap is also reduced to 

 a standard density by dilution with water ; whence the weight of dry siliceous earth 

 present may be deduced from the measure of the liquor. 



The standard clay- and flint-liquors are now mixed together, in such proportion by 

 measure, that the flint-powder may bear to the dry clay the ratio of one to five, or 

 occasionally one to six, according to the richness or plasticity of the clay ; and the 

 liquors are intimately incorporated in a revolving churn, similar to that employed for 

 making the clay-pap. This mixture is next freed from its excess of water by evapo- 

 ration in oblong stone troughs, called slip-kilns, bottomed with fire-tiles, under which 



