The average proportion of fusible to infusible ingredients in different wares is approximately illustrated in the following table of the results of analyses RAW MATERIALS.—The materials used in the manufacture of pottery may be divided into four classes :—(I) Plastic clays ; (II) glass-forming materials, used either in the body or the glaze ; (III) indifferent substances ; (IV) colottring agents.
Class I—Kaolin, Cornish or China clay (see p. 635) ; aztificial Cornish clay of Belleek (see p. 639); Fire-clay (see p. 638).
The " blue," "ball," or "pottery" clay of Dorsetshire and Devonshire is highly plastic. The upper beds of this clay frequently contain a large proportion of sand, and furnish a body which, without further admixture, is suited for the manufacture of ordinary stone-ware. The finest quality of the clay is found at a considerable depth ; it is of a uniform blue-grey colour due to organic matter, is unctuous, and free from grit ; it mixes with water with some difficulty ; when treated with acids, no effervescence takes place ; when subjected to a moderate heat, it becomes white, hard, and but slightly absorbent ; with an intense heat, it is rendered so hard as to resist scratching vvith a steel point, assumes a yellow tint, and becomes non-absorbent. There is but a trifling proportion of iron intimately mixed with the clay, although nodules of pyrites are of common occurrence ; the free silica present in the clay is in a state of exceedingly fine division.
Preparation of Claye.—All clays, after extraction, are heaped up in the open, and exposed to the weather for as long a period as possible. Lengthened exposure tends to disintegrate the mass of the clay, and it is certain that ware containing clay which has been long exposed, is less liable to shrinkage than if the clay has been mixed with the other ingredients of the ware without previous exposure. In China and in France, it is customary to preserve the prepared mixtures for a long period ; whereas in England, directly the clay has been incorporated with the other necessary substances, the mixture is considered ready for use. Fire-clay and the dry sandy day derived from the upper strata of the blue-clay de posits, are prepared for use by grinding ; kaolin and superior blue clay, by dif fusion in water. The mills
employed for grinding dry clays resemblo ordinary mortar.mills. Diffusion is effected by stirring the masses of clay in tanks of water, by means of paddles workcd by hand or ma chinery. The process of mixing the clay and water is known as " blunging," and the machines in which the process takes place, as " bluugers." The simplest form of machine-blunger is a hori zontal wheel, with paddles attached to the circumference, revolving in a round or octagonal pan. Power is communicated to the shaft of the wheel from beneath by suitable gearing. Fig. 1105 represents three blungers A B C, intended respectively for ball-olay, kaolin, and "shavings " of unburnt ware (vthich represent the plastic) constituents of earthenware), driven as described. Agitation may be increased by fixing projecting per forated arms to the inside wall of the pan, against and tlirough which the clay is driven by the revolution of the paddles. The most recent form of blunger consists of an octagonal cast iron pan,—the octagonal form aiding ( the process of disiutegration,—with circular casting to ward off the ma terial from the bearings of a central spindle. To the spindle, are fixed six oblique blades, arranged in accordance with the principle of an arehimedeau screw. When water and clay bave been introduced, and the spindle has been set in motion, the clay is gradually raised to the level of the topmost blade, where it is dashed against splash-beards, and thrown to the bottom of the pan, to he once more raised and rejected, until such time as the mixture is complete. Fig. 1106 is a view of three communicating blungers driven from the centre. The pans are so arranged that the mixture is forced to pass successively from one to the other, by which means, friction and agitation of the mixture are increased.
Class II. Glass-forming Materials.—Felspax, which may be regarded as a natural type of e glaes (the potassium felspax being represented approximately by the formula K20., Al203 6 Si02), is used both as a glaze and as a glass-forming ingredient in the body of porcelain. It. is generally obtained from Sweden, in masses of a salmon-red colour. It becomes white when calcined. .