1. Crazing. Fine cracks appear in the glaze but do not penetrate the body. There are many causes. The body may be underfired or overfired. In the former case the crazing does not always appear at once and it grows worse upon standing. In the latter case the glaze is found to be crazed when taken from the kiln and it does not extend even after long standing. The glaze may be underfired. In this case the lines of the crack are broken and irregular, one often changing its direction without meeting another crack. In all these cases the remedy is obvious.
Crazing also occurs when both body and glaze are correctly fired but there is an inherent disagreement in expansion. In such a case a little flint added either to the body or to the glaze will tend to cure the trouble but it must be remembered that the addition of flint to the glaze is apt to render it less fusible and therefore while one craze may be cured another may be caused. The addition of flint to the body is the simplest remedy.
2. Shivering or peeling. This is the reverse of crazing and is caused by the glaze being too large for the body. It almost always appears immediately the ware is cooled. The symptoms are that edges or convex surfaces are pushed off and even the ware itself is shattered. The remedy is to decrease the flint in either body or glaze.
3. Blistering. Glazes, both bright and matt, are apt to develop blisters at times. These may be yet unbroken when the kiln is opened or they may have melted down to a small crater, a ring with a depression in the center. The cause of this fault is usually to be found in the body. All clays contain sulphur and when a clay is aged this develops an acid which rises to the surface of the ware when dried and causes a scum. The glaze attacks this sulphate scum and a gas is generated which boils out and causes the blisters. If old clay blisters and new clay does not it may be regarded as certain that this is the cause. A little barium carbonate added to the clay will help to effect a cure. About one per cent. is usually enough. Clay so treated, however, must not be used in plaster molds as the barium attacks the plaster. If the cause be not found in the clay it may exist in the glaze itself. Some glaze ingredients contain impurities in the form of sulphates and these will cause blisters.
4. The glaze flows, leaving bare places. It is too fluid, add a little clay and flint.
5. A matt glaze burns to a bright surface. Matt glazes must be used in a very thick coat. If too thin they will inevitably brighten. The fire may be too high. The fire may be "reducing," that is, with insufficient air.
6. The glaze crawls or rolls up in lumps. Notice whether the glaze is cracked before burning. If so it will surely crawl. Too fine grinding is usually the cause of this trouble. Too much clay in the glaze may cause it, or a too porous body. A body which is underfired will almost certainly cause the glaze to crawl.
7. Pinholes appear in the glaze when cool. Too rapid cooling is the cause.