Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/625

Rh POTTERY 601 by the addition of broken pots, crushed or ground, an ex pedient practised during the earliest stages of the develop ment of the art of pottery. Classification. Many attempts have been made to classify pottery and porcelain according to their mode of manufacture. The classification of M. Brongniart (Traite des Arts Ceramiques, Paris, 1854) has been followed by most later writers. With some modifications it is as follows : ((a) Biscuit. Simple baked clay, porous and without gloss. Example, a common modern flower-pot. (b) Glossy. Fine clay covered with an almost imperceptible vitreous glaze. Example, most Greek vases. (c) Glazed. Clay covered with a perceptible coating of glass. Example, common white earthenware plates. (d) Enamelled. Clay covered with a vitreous coating made opaque by white oxide of tin. Example, Italian majolica. lead 1. Soft pottery, easily fusible. 2. Stoneware, very hard and infus ible. &amp;lt; 3. Porcelain, white, semi - transpar - ent, and only fused at a high temperature. ( (a) Very silicious clay covered with a vitreous glaze. Example, old grey I Flemish ware. 1 (b) Silicious clay covered with a salt glaze. Example, a modern brown ginger -beer I bottle. ((a) Hard Porcelain. Natural kaolinic clay covered with a felspar glaze. Example, porcelain of China and Japan. (b) Soft Porcelain. Artificial paste covered with a lead vitreous glaze. Example, I early Sevres porcelain. This classification is necessarily imperfect, some pottery coming under two heads, as, for instance, much of the Italian majolica, which is both enamelled and glazed. For this reason in the following article pottery will be treated according to its age and country, not according to its method of manufacture. Porcelain differs from pottery in being whiter, harder, less fusible, and (most essential difference) in being slightly translucent. The paste of which it is formed is a purer silicate of alumina than the clay of which pottery is made. It will therefore be de scribed under separate heads (p. 633 sq., infra). For the sake of clearness it will be well to define the sense in which technical words relating to pottery are used in this article. Body or paste is the clay of which the main bulk of a pot is made. Slip is clay finely ground and mixed with water to the consistency of cream. It is usually applied over the whole surface of a vessel in order to give it a finer face or a different colour from that of the body of the pot. It is also sometimes applied partially, forming ornaments in relief, as in the case of some Roman ware and the coarse 17th-century pottery of Staffordshire described below. Glaze is a thin coating of glass, evenly fused over the surface of a clay vessel to make it harder, and also to render it impervious to water. Clay simply baked without a vitreous coating is called bisciiit ; its surface is dull, and it is more or less porous. The sim plest and oldest form of glaze is a pure silicate of soda ; the addition of oxide of lead makes the glaze more fusible, but less hard and durable. For decorative purposes glazes may be coloured by various metallic oxides without losing their transparency. Enamel is a glaze with the addition of some substance to render it opaque. Binoxide of tin has the peculiar property that when even a small quantity is added to a transparent glass it renders it opaque and white without otherwise altering its character. Great con fusion has been caused in various works on pottery by a careless use of the terms &quot;glaze &quot;and &quot;enamel&quot;; they are both of the nature of glass, but the best distinction to make is to apply the word &quot;enamel&quot; to a vitreous coating that is opaque, and the word &quot;glaze&quot; to one that is trans parent ; both may be coloured. The method of applying vitreous coatings to clay, whether enamel or glaze, is this. The materials are ground fine and mixed with water to the consistency of cream. The pot is dipped in the mixture, or the fluid is applied with a brush ; it is then set to dry, and finally fired in the kiln, which must be heated sufficiently to fuse the component parts of the glaze or enamel into one smooth vitreous coating, while on the other hand it must not be hot enough to soften or melt the clay body of the vessel. The use of oxide of lead enables a glaze to be applied to a clay body which would not stand the high temperature necessary to combine and fuse a pure silico-alkaline glaze. In order to prevent the glaze or enamel from blistering or cracking off there must be a certain similarity of substance between the clay body and the vitreous coating. A fine silicious glaze or enamel will not adhere to a soft fat clay unless the proportion of silica in the latter is increased either by admixture of a harder, more silicious clay, or by the addition of pure silica either in the form of sand or of ground flint. The Potter s Wheel. All pottery, except the rudest and Potter s most primitive sorts, is moulded or &quot; thrown &quot; by the aid wheel. of a very simple contrivance, a small round table fixed on a revolving pivot. Fig. 1, from a tomb-painting at Thebes, shows its simplest form. The potter at intervals gives a spin to the table, which continues to re volve for some time without a fresh impulse. This form of wheel, used by the Egyptians (as is shown by existing fragments of pottery) about 4000 B.C., is still employed without any alteration by the pot ters of many parts of India. A later improvement introduced in Egypt under the Ptolemies was to FIG L _ Potter moul(lin have another larger circular table, vessel on the wheel) from fixed lower down on the same axis, which the potter set in movement with his feet, and thus was able to keep up a regular speed and leave his hands free for the manipulation of the clay (see fig. 2). No process in any handicraft is more beautiful than that of a potter moulding a vessel on the wheel. The ease with which the plastic clay answers to the touch of the hand, and rises or falls, taking a whole succession of symme trical shapes, and seeming, as it were, instinct with the life and thought of the potter, makes this art beautiful and striking beyond all others, in which tho desired form can only be at tained by comparatively slow F IG. 2. Potter s wheel of the and laborious methods. Ancient time of the Ptolemies, moved poetry is full of allusions to this. b ? the foot. from, a ^all-re- TT / n AA lief at Philas. Compare fig. Homer (II,, xvm. 600) com- 55j the wheel on Bright, pares the rhythm of a dance to the measured spin of a potter s wheel; and the rapid ease with which a clay vessel is made and remade in a new form is described by Jeremiah (xviii. 3-4) in one of his most forcible similes (compare Horace, A. P., 21-22). Among the Egyptians of the Ptolemaic period the potter was used as a type of the Creator. Nouf or Knourn, the divine spirit, and Pthah, the creator of the mundane egg, are symbolized by human figures moulding clay on the potter s wheel. 1 The wheel and egg are shown above in %. 2. .. 1 See Rosselliiio, Monumenti dclV Egitto, pi. xxi. and xxii., 1844. XIX. 76 a painting in a tomb at Thebes about 1800 B.C. Compare the wheel on the left in fig. 55.