Pure water is first run into the pans formed by the beds and sides of the mills, and the mate rials to be ground are then added. As the " runners" are forced round and round, the flint or Cornish stone is respectively ground to an impalpable powder, and worked with the water so as to form a compound of a thick creamy consistency. The mixture in either case is allowed to settle for a short time, and is then drawn off through plugs at different heights in the sides of the pans. From the top plug, water is drawn off ; from the second, the slip or mixture which is to be used ; and from the bottom plug, a sediment which requires to be reground. The flint and Cornish stone slips are thence conducted into circular tanks, where water is gradually added, and where the material is mixed with the water hy the revolution of an agitator or spindle with arms and paddles, until such time as the mixtures respectively attain a standard density. The density is determined, as in the case of the clays, by weighing a fixed quantity. The standard weight of a pint of flint slip is 32 oz., and that of Cornish stone is generally the same. The creamy liquids are run through pipes into store-tanks in the clay hlunging-shed, the inlet to the pipes being slightly above the level of the bottom of the agitating-tanks, in order that any coarse sediment still present may be retained for regrinding. The flint and Cornish stone slips are now under the same roof with the clay blungers and mixing-tank, and are introduced into the mixing-tank, the quantity of each being regulated by the measure attached to the side of the tank. At this point, also, any colouring mixture that may he required is introduced. In the mixing-tank, the clay slips, together with the flint, Cornish stone, and any colour that may be present, are thoroughly mixed by agitation, either by hand or by power. The mixture is then passed through three sifting-machines or three sets of sieves, each sifting-machine or set of sieves being covered with lawn of increasing fineness. The coarsest lawn contains 50 threads in an in. ; the finest, 120 threads. After mechanical sifting, the mixture is subjected to magnetic or electrical sifting, in order that it may be purified from minute particles of iron. For this purpose, the trough through which the fluid is conducted is furnished with a series of horse-shoe or elsetro-magnets, and the fluid passes through their field of action. After this double process of purification, the liquid mixture or slip reaches an underground store tank or " ark," whence it is raised by pumps to be partially solidified by pressure and filtration.
The following table shows the number of inches of each liquid material of standard density required to make up 100 in. of the different earthen-ware mixtures in a liquid condition.
The table serves rather es an illustration of the attributes of the different materials than as a bssis for manufacture. Ball-clay (eee p. 1558) supplies plasticity, but acquires a slightly yellow tint when flred, which ie especially noticeable when the commonest qualities of tbe clay are used. Kaolin contributes plasticity, whiteness, and infusibility ; flint contributes whiteness, etability, and an earthy fracture; and Cornish stone acte as a flux, and produces vitreosity. Whenever any one of these ingredients is in excess, the quality or qualities which it possesses show themselves in the resultant ware. Thus the mixture B will produce as white but a lees vitreous ware than A, for although the proportion of ball-clay in the former is greater than in tho latter, the proportion of the whitening ingredients, kaolin and flint, in B is greater than that possessed by A. In E, the proportions of ball-clay, kaolin, and flint are reduced, as deficiency in whiteness and plasticity is supplied by the addition of 25 per cent. of the shavings and waste of unburnt ware. The colour of the cream-colour ware is due to large proportion of ball-clay, which, in the samples given, amounts to Eta much as 18 and 65 per cent. " Granite " is vitreous, owing to a large proportion of Cornish stone; but part of
this effect is balanced by the presenoe of an excess of kaolin, which renders the ware white, and somewhat refractory. "Iron-stone " is vitreous, owing to a comparative deficiency of clay, and a great excese of Cornish stone.
Fig. 1139 represente an ordinary pump used for raising the liquid clay mixture from lower to higher levels, whilst it is undergoing the different processes of refinement. The pump employed for forcing the elip from the store-tank into the press is of special construction, and of considerably greater power. The press is made up of a series of large, oblong, wooden, shallow trays or frames, usually 24 in number, which are grooved or ribbed on both faces, and, when placed vertically side by side, leave intermediate spaces of about in. In °Bch of these spaces, is laid a piece of strong calico, of rather more than double the size of one of tho frames; and the calico is so folded as to form, in the space between the frames, a bag, into the upper side of which, a small brass pipe is fixed, which can be adjusted to a main eupply-pipe running above the frames and connected with the slip pump. When the frames, with the bags between them, have been set together, and tightened up by means of screw-bolts, and when the email distributing-pipes have been connected with the supply-pipe, and the pump has been set in motion, the slip is drawn from the store tank, and forced successively into the 24 bags. As more and more of the slip is driven in, the pressure in the bags is increased, which is resisted by the strength of the calico, the wooden framework, and the tie-bolts. The pressure causes the water to separate from the clay, and filter through the calico, and, passing through openings in the wooden framework, to run in a channel beneath the press into a tank placed for its reception, whence it is pumped for use in the " blungers" or elsewhere. When the water ceases to flow from the bags, the pump is checked, the main supply-pipe is dieconnected from the distributing-pipes, the tie-bolts are loosened, and the frames are separated. When the calico between the frarnes is unfolded, a thin sheet of plastic clay, corrugated by the ridges in the wooden frames, is discovered. In order to prevent loss during the action of the pump, through the buretiog of any one of the bags, each of the small distributing pipes is furnished with a etop-valve, by which its connection with the supply-pipe can be closed, without interfering with the process of filtration in the remainder of the bags.
The clay or clay mixture is now practically pure, and requires only be freed from the air which is still retained in its pores, and to be rendered close-grained and tenacious. This result is obtained partly by a mechanical and partly by a manual process. The mechanical process is performed by the " pug-mill," Fig. D40. It consists externally of a conical, cast-iron, horizontal case, with a hopper on the upper part of the large end of the cone, and with a comparatively small square or octagonal orifice on one side of tbe small end. A shaft passes through the length of the cone, having iron splayed blades fixed spirally round it. The shaft is driven by power, and the clay, which is thrown into the hopper, is mangled and driven forwards by the revolving blades. The shape of the mill causes the clay to be more and more compressed, until it issues from the orifice in a compact stream. The stream of clay is cut into blocks by a tightened wire, and the blocks are conveyed to the different workshops. The manual process of compression is only necessary for clay which is intended for the finest work. It consists in placing the block of clay, as it comes from the pug-mill, on a bed of plaster, and repeatedly cutting it hot i zontally with a wire, and hurling one half upon the other with all the power the operator can muster.