The formation of crazing in some early Chinese glazed wares

The crazing of ceramic glazes is generally given short shrift by those describing ancient pottery. Often it is ignored altogether; if not, it is dismissed as forming a 'random' pattern on the surface of the vessel. In fact the pattern is frequently very far from random, and in what follows an attempt is made to provide a theoretical basis to account for crazing and some of th~ various forms it may take. Crazing has long been the bane or delight of potters depending upon the aesthetic effects for which they are aiming. Nearly all that has been written by potters on this subject is, therefore, devoted to avoiding crazing, or to the propagation of crackle when desired; but very little attention has been given to the manner or nature of the crackle. Indeed, the literature that helps to illuminate this subject lies elsewhere, in that of mechanical and structural engineers who have perforce given a great deal of thought to the problems of cracks and crack propagation for very obvious reasons. And because glass is a brittle material and cracks readily, engineers have given the fracture of glass very considerable study [1]. Glass and glazes do not fracture simply because they are brittle. For cracking to occur, two conditions have to be met: first, the glass must be under strain. In the case of glazes this is almost universally the case, although some glazes are under greater strain than others. Strain alone, however, is not sufficient to cause cracking, and there must also be some factor to initiate the crack, often no more than minute lacunae in the glaze or the presence of surface scratches, no matter how slight. Once the crack has been initiated, provided that the strain remains, the crack will continue to propagate until the energy at the tip of the growing crack can be dissipated. In the case of glazes this most commonly happens when the tip of the crack encounters a second, already formed fracture. The cause of strain in a glaze is, more commonly than not, due to the higher coefficient of contraction of the glaze on cooling than that of the body to which it is applied. In simple terms the glaze does not 'fit', and this lack of fit is frequently due to incorrect formulation of the glaze. It is especially true of glazes.that contain too high a proportion of alkaline fluxes, which give a glaze a very high coefficient of contraction. However, a glaze may fit a body adequately, at least well enough to avoid crazing, when fired at a moderate temperature; yet the same materials when fired to a higher temperature may produce a ware that is crazed. This frequently occurs where the body contains a large proportion of quartz. When fired at moderate temperatures, due to the shrinkage of quartz on reaching the inversion temperature (573°C) during cooling it may shrink sufficiently to offset some of the strain developing in the glaze. Unlike quartz, the glasse.s resulting from higher temperatures will not shrink on cooling, and there is nothing to relieve the strain developing in the glaze. Strain in a glazemay also result from excessively rapid cooling of the kiln. Although little enough is known about the physical changes that occur when ceramics cool, some things are certain. Both glass and ceramics are poor conductors of heat, and the transfer of heat by conductance must be very slow. Heat loss, hence, has to be largely by convection and radiation into the surrounding kiln atmosphere. If the kiln atmosphere stands at a temperature too far below that of the wares, heat loss will be rapid, and since heat transfer takes place by way of the surfaces of the wares, these surfaces may cool, and therefore shrink, excessively compared with the underlying body. During very slow cooling, of course, adequate time is available to allow the