Bodies and glazes are compound silicates, or are, at least, largely composed of compound silicates, and the conditions under which glazed goods are made are such that the glaze must possess a lower fusing point than the body upon which it is used. Under this law, then, there is always a difference between the two in the amount of expansion or contraction produced by changes of temperature. This difference is greater or less as the point of fusion of the body differs more or less from the point of fusion of the glaze. When this difference is great enough and the body possesses sufficient strength to rupture the glaze the result is crazing. When the body is so weak that the strain ruptures the body, the result is shivering. We have, then, these two opposite manifestations of strain having existed between body and glaze, the only difference between the two being as to whether the body or the glaze fails under the strain.
The law above refers only to perfectly free or unimpeded expansion and contraction. In glazed work the expansion or contraction of glaze is restrained or impeded by the body and the expansion or contraction of body is impeded by the glaze. The measure of this restraint is the amount of strain produced. One limiting element of the possible amount of strain is the strength of the weaker, and another limiting element is the elasticity of the more elastic.
For instance, were a bar of iron coated with an absolutely non-elastic coating weaker than the iron, and then the whole subjected to a change of temperature that would enlarge the iron 1 per cent the coating would be ruptured immediately, while a volume of air might be inclosed in the same coating and subjected to a change of temperature that would double its volume, if unimpeded, without producing a strain near great enough to rupture the coating. Again, the iron might be coated with a very weak coating possessing some degree of elasticity, and an expansion of 1 per cent would not produce a material strain upon the coating, while the air might rupture the coating long before its volume had doubled. These are extreme cases, but thoroughly illustrate my meaning.
So much for theory. Now, if the theory is correct, it should perfectly suggest the proper relation between bodies and glazes, and its suggestions should harmonize perfectly with practice. If the theory does this perfectly, it can be relied upon and can be carried out to its utmost limits, and by means of such theory advances in practice can be made in a short time that would require a long time without it.
Right here I will restate the law. All silicates produced by fire expand an equal amount between zero and their melting points. For "silicates produced by fire" I will substitute "bodies and glazes," when it would read as follows: "All bodies and glazes expand an equal amount between zero and their melting points."
Now, what is the obvious suggestion of this theory in order that strain between body and glaze may be reduced to a minimum? Evidently that their melting points should be identical. This is obviously impracticable, and the practical suggestion would then be to bring their melting points as nearly together as the conditions will admit This would suggest that when the glaze is melted the body should at least be vitrified, and the more thoroughly vitrified the better. A thoroughly vitrified body is a strong body and has large strain producing capacity. We would therefore expect to see a vitrified body craze its glaze at once or else not at all. If it crazes its glaze at once, it must be from one of two causes - either the melting points of body and glaze are too widely separated or the glaze possesses too little elasticity to stand the slight difference in amount of expansion and contraction. I think these state-ments are all directly deducible from the law and correctly stated. Now, how do they agree with practice? Our most perfect types of glazed vitrified bodies are found in the porcelains of the Chinese, Japanese, French, Germans and others. These porcelains do not craze in use, and can be relied on through time and almost through eternity. Chemists' porcelain stands sudden alterations of temperature, even from almost zero to a white heat, and even stands heat applied to one portion, while the balance remains comparatively cool. Household porcelain also stands large changes in temperature perfectly; in fact, when failure occurs, it is not from crazing, but from cracking of the whole mass. I have never seen a piece of genuine hard porcelain craze in use. Yet this type of goods does craze, but the crazing, when it occurs, occurs in the kiln. The manufacturers of hard porcelain understand the simple laws which govern crazing in their goods thoroughly, and they state that when goods are found crazed in kiln it is either because glaze is not thoroughly fused and possesses very little, if any, elasticity, or because the body is not thoroughly vitrified. The first is an accidental occurrence due to bad burning, the second a constant occurrence so long as improper body is used.
The remedy for the second cause of crazing is to add flux to body and continue burning at the same heat or take some flux out of the glaze and use a higher heat. We then see that practice in the best type of vitrified glazed bodies agrees perfectly with the law. I am sorry I am unable to give the experience of the stoneware makers in this country, but have not the slightest doubt but that it also agrees perfectly with the law.
For many reasons it is impossible that we should all be makers of glazed vitrified bodies. Now, what suggestions does the law offer to makers of glazed porous bodies. We can easily see that in such bodies there must be a large difference between the melting points of body and glaze. From the law we would then deduce that there is a great difference in the tendency of the two to expand and contract under changes of temperature, consequently a great tendency for body and glaze to mutually strain each other, consequently a great tendency, unless overcome by other conditions, to craze. The tendency to craze is there, and it is unfortunately too often in evidence.