We have seen, in glazed vitrified bodies, that when the tendency to craze existed, crazing occurred at once, and that when crazing did not occur at once it never shows, and we therefore assume that the tendency does not exist or is very slight. In glazed porous bodies crazing may occur at any time - in the kiln, in a day or a week, in months, or not for years. When it does occur it proves that the tendency was there during the whole period, and has either been increased through changes in condition, or glaze has failed, due to crystallization or loss of elasticity. Crazing in porous bodies is so general that I think we have a right to assume that the tendency is universal. If so, the law is upheld by practical experience.

The law offers no direct remedy, but simply tells us that in glazed porous bodies there must be a constant struggle between the body and the glaze, and leaves it for us to determine the best means of securing to the glaze the winning of the struggle. These means are either to make the glaze so elastic that it does not feel the straining of the body or to make the body so weak that it is incapable of producing sufficient strain to rupture the glaze.

I am perfectly willing to confess that I do not feel much at home on the first of these two plans. I do not know what makes a glaze elastic and what does not, except to a very slight extent These facts can only be ascertained by long continued researches - either by testing different glazes on a fixed body and noting which glazes stand the longest under certain fixed severe tests, or by actually testing glazes, in the form of bars of glass, in a testing machine. Either method would be extremely slow and probably unreliable, the first especially so, from the possible double variations in the firing of body and glaze, and the second from variations in the firing of glaze and impossibility of preparing perfectly uniform test pieces. A certain quantity of lime in a glaze, I believe, increases the toughness and elasticity of the glaze very much. Care must be taken with low heat glazes not to use too much, as the lime, being a resistant at a low heat, will cause the glaze to come from kiln imperfectly fused. I made some exhaustive experiments in this line, some years ago, with a glaze containing 8 per cent of lime. This glaze crazed on a semi-vitreous body. I tried additions of lime in the form of paris white, 5 per cent at a time. The first addition of lime materially affected the crazing for the better. The second and third additions stopped it altogether. At the fourth addition crazing reappeared, although the glaze did not show any signs of imperfect fusion until I reached the sixth addition, or 30 per cent in all. I repeated this experiment a number of times, with the same results. I have been told since that my results were not due to increased elasticity of glaze, but were obtained because the lime prevented the glaze from expanding and contracting so much. This later view I will admit to a certain extent, for it is just in agreement with my whole argument. I was evidently raising the fusing point of glaze by the addition of lime, consequently was making its expansion and contraction per degree less, but I think there must have been an effect beyond this which I can only attribute to increased elasticity.

As directly bearing upon this point, I will cite the great claims made as to the toughness and durability of what are known as lime glass lamp chimneys. When quite low heat glazes have to be used, I feel positive that a good lead glaze possesses greater strength and elasticity than a glaze depending upon alkaline fluxes. Underfiring produces a weak, non-elastic glaze, and is a frequent cause of crazing. This I think reasonable, as an underfired glaze is not thoroughly fused; the full and complete chemical combinations have not taken place, and there undoubtedly is more or less material merely held in suspension in an unfused state, and adding nothing to the strength or elasticity of the glaze; in fact, by separating the molecules of glass, detracting from these qualities. This brings me to the subject of enamels. The opacity of enamels is caused by unfused material held in suspension, and, as stated above, this tends to a lower ultimate strength and elastic limit My whole experience with enamels has been in accord with this belief. I find it more difficult to produce enamels that will not craze than to produce glazes that will not craze. As there has been much discussion as to what constitutes an enamel, I will state when I use the term I mean an opaque glass, opaque due to holding in suspension unfused lime, barium, tin, wolfram, arsenic or any other material that does not combine and flux with the glass containing it. The natural glaze, known as Albany slip clay, I know from experience, is an extremely tough, elastic glaze, standing almost any amount of hardship, even to being taken from the kiln at full heat and rapidly cooled. I understand there are several other natural glazes possessing the same qualities. I have often tried to imitate or copy the qualities of Albany slip clay in an artificial white mixture, but without perfect success. The analysis of Albany slip clay, as given by Professor Orton, is as follows:

Albany slip clay is a moderately high heat glaze, possessing the property of standing almost any amount of beat without blistering. An examination of the analysis shows us nothing especially peculiar, but if we try synthesis we may find something that will look different The same chemical mixture can be produced by the following:

We now notice one great peculiarity, or, at least, it is so to me; that is, the large amount of clay - and national china clay is a tough, plastic clay, that stands a high heat. Such a glaze would, in my opinion, be an impossibility without the iron, for it is so aluminous that it would surely blister before it would melt The question is, Does the Albany slip clay derive its extreme elastic qualities from the iron or from the alumina? I think from the alumina, for artificial glazes stained with iron do not show any marked superiority in elastic qualities over the unstained glaze. We notice that all the natural glazes possess this aluminous or clayey property in a marked degree. This is also one of the marked peculiarities of hard porcelain glazes, yet they will not stand one-tenth part of the hardship that Albany slip clay will; in fact, the adjustment of body to them has to be made with great exactness, whereas Albany slip clay will stand on nearly any clay that does not shiver.