The same effect may be easily produced upon all terra-cottas of which the paste is more sensible to the changes of temperature than the exterior coating or glaze. In fayence this accident is of frequent occurrence; the red porous clay, being more expansive, draws away the enamel, which, being less elastic, is separated into fragments, and the greater the resistance the more they are multiplied. Now one of the qualities of porcelain is precisely to avoid this double action. Its paste is composed of a felspathic rock, decomposed and infusible, called _kaolin_; the cover or glaze comes also from a felspathic rock, slightly crystallised; these melt and assimilate together harmoniously in vitrification, and a complete affinity is evident between the two elements of porcelain. Nevertheless the Chinese, in modifying the glaze, are able to render it more or less expansive and to break the harmony between its own shrinkage and that of the paste or body which it covers. [Illustration: FIG. 332.--PLATE. _Egg shell porcelain._] Hence the crackle, at the option of the potter, is made of large, middling, or small size. Various kinds of crackle are thus produced, sometimes upon one and the same piece, as by exposing the porcelain or portions of it when at its greatest heat to a sudden cold or contact of water, large fissures may be obtained. These cracks are sometimes filled in with black, red, chocolate, or purple colours. Others may be classed among the curiosities of porcelain--for example, cups or bowls which have an outer reticulated coating, pierced or cut out into arabesques, completely insulated from the inner vessel, except at the rim at top and bottom where it is joined; these have been used for tea or hot liquids, and may be held in the hand with impunity, notwithstanding the heat enclosed within it. [Illustration: FIG. 333.--PLATE. _Egg shell porcelain._] Another variety consists in cutting or punching out pieces of the paste or body of the ware in patterns before it is baked; the pieces so cut out are small ovals like grains of rice placed in more or less numerous stars, rosettes, &c. The vase thus ornamented is dipped into the glaze which fills up all these small holes, and then placed in the kiln. The pattern, being much more transparent than the body of the ware, is distinctly seen, but especially so when held to the light. Another beautiful effect is produced by means of the glaze itself, which is of a light or dark shade according to its intensity or thickness; for example: a fish, animal, or other object is stamped incuse on the upper surface of a plate, it is then filled in with a coloured glaze and vitrified, and is consequently shaded according to the thickness of the glaze on each portion of the design, the surface being perfectly smooth.