Cornish stone is used for almost all English wares, both in the body and the glaze ; 1.8 granite, in which the constituent felspax has been partially decomposed, but which retains sufficient alkaline silicate to render the mass fusible. It is quarried at St. Stephen's, in Cornwall, whence some of the best English kaolin is also derived. Pegmatite is a form of the same rock, but in a more advanced stage of disintegration ; whereas granite consists of inter mixed crystals of quartz, mica, and felspar, pegmatite retains no mica, and but a trifling proportion of quartz. Felspar, Cornish stone, and pegmatite are exceedingly hard, and are ground and diffused in water by mills of peculiar construction, illus trated in Figs. 1107-8. Fig. 1107 is a vertical, and Fig. 1108 a hori zontal section of the mill ; the ex ternal wrought-iron case or pan A is protected from injury from the grind ing stones by an internal ring ; B is the floor-level, and P is the shaft driven from below. The base of the pan is paved with blocks of chert, and the pavement slopes from the circumference towards the central shaft. To the shaft, are bolted, by the bolts R, the curved arms S. The shape of the arms F G is shown in Fig. 1107 ; to these, and to a con necting-bar M, the boards C are fastened, as shown at K. Blocks of chert H are chained to these boards, and are carried round by the arms. • When the substance to be ground and the water have been introduced into the pan, and motion has been communicated to the shaft, grinding takes place between the contiguous surfaces of the blocks and the pave ment.
To the glass-forming ingredients already mentioned, must be added the carbonate and oxide of lead, sand, borax, and the carbonates of sodium and potassium.
Class III. Indifferent Substances which do not contribute Plasticity or Translucency.—Flint are obtained from the upper strata of the chalk. Those nodules a,re preferred which are black compact, and free from iron and incrustation. When exposed to an intense heat, flint burns to a' pure white. The whiteness and stability of the calcined flint are availed of for neutralizing the colour and contraction inherent to clay. Calcination is effected in a kiln sinailar to that represented
in section in Fig. 1109 : A is the ground level, with the chimney or cone rising from it ; P, the charging-door ; B, the grate for supporting the layers of coal and flint introduced through the door P ; Ri the stoke-hole. The calcined flints are removed by withdrawing the bars of the grate B.
The masses of calcined flint are reduced to powder in a " stamping-mill " or a °rusher." The former consists of a row of vertical, heavily-weighted, iron clubs or stamps, with projections on their upper extremities. These projections are successively caught and released by cogs fixed at intervals upon the surface of a horizontally-revolving cylinder ; each stamp is raised and allowed to fall as its projection ie caught by or released from a cog, and the flint introduced beneath is gradually pul verized by its descent. The crusher shown in section in Fig. 1110 grinds successive charges of flint between its iron jaws H J.
The jaws are opened and shut by mechanism represented in the figure. The flint, reduced to a coarse powder, requires to be reground and mixed with water in a mill with atone runners, whence the mixture issnea as a pure white creamy liquid.
Bone-ash (caloio phosphate) in obtained by the calcination of bones, those being preferred which contain the smallest pro portion of oxide of iron. The bones are generally freed from grease by boiling (see p. 1449), and calcined in the same manner a8 flint. In some cases, however, the grease is allowe,d to re main in the bones, and to act as an auxiliary fuel. Calcination is considered complete when the bones are perfectly white, and adhere to the tongue. They are °rushed and ground with water. Bone-ash is the characteristic ingredient of English china.
Graphite is used in combination with flre-clay, for the manufacture of crucibles, and other apparatus employed in metallurgical operations (see Graphite, pp. 1087-93).
Class IV. Colouring Agents.—These are exclusively me tallic oxides and metals (see under Decorative Processes, p. 1597).