I do not mean to give the impression, from what I have said, that glazers generally do not know how to mix bodies and glazes that are adapted to each other, and in which the glaze will not craze, but their knowledge is usually limited by their actual experience. With the bodies and glazes with which they are familiar they know that if crazing occurs a little of this or a little of that added to the body will stop it, or some different mixture of glaze will overcome the difficulty. I do not refer to this knowledge from experience with particular mixes, but to that broader knowledge which is able to take clays as they are found, possibly entirely different from anything ever presented to it before, and, after a few preliminary experiments, adapt a glaze to it that will not craze or adapt the clay to a glaze. This adaptation I am not foolish enough to claim as being so complete as to be perfect under improper conditions of firing; in fact, as I will point out later, the proper heat is a most important factor.
Clay, in the unburned condition, varies very much, indeed. There are not only the chemical variations, which are very many, but also the physical differences. These chemical variations and physical differences are largely independent of each other. Toughness is usually considered a sign of a large percentage of alumina in the clay and shortness a sign of a large percentage of silica, but this is not necessarily true, as the physical condition of the material in the clay affects its plasticity wonderfully, and clays are met with containing large amounts of silica that are very plastic and clays containing large amounts of alumina that possess very little plasticity. Burning eliminates many of the physical differences, and only the chemical variations remain. The nearer we approach the fusing point of the clay the more completely are the differences in the original physical characteristics eradicated, until, at the point of complete fusion, they are wholly eradicated. As an illustration of my meaning I will imagine two materials, one a pure ground felspar, containing, say, 25 per cent alumina, 55 per cent silica and 20 per cent potash, the other an artificial mixture of day, silica and potash. This artificial mixture can be made so that when burned it will contain exactly the same chemical elements as the felspar, and in exactly the same proportions. The first of these two materials will be nearly white in color, and will not possess any plasticity; the second may be white, yellow or gray, and will possess considerable plasticity. For the benefit of the hypercrit-ical, who may claim that a pure enough clay does not exist for this imitation of the chemical elements of felspar to be possible, I will say that they may imagine impurities added to the spar so as to obviate this difficulty that will answer for purposes of illustration just as well. If these two materials are made into cakes or brick while wet and placed in a kiln, and specimens drawn out at different heats, it will be found that, up to the point where the combined water is driven out of the clay, there will be about as great a difference in the physical characteristic of strength or bond as there was in the raw material. After this point is passed the felspar samples will harden more rapidly than the other samples, and at the point of vitrifaction the two materials will be almost identical, physically and chemically, while at the point of thorough fusion they will be exactly identical, every iota of physical difference having been eliminated.
The difference in fineness of material probably shows through a greater range of heat than any other physical characteristic, and is only finally eliminated by complete fusion. Difference in fineness also affects chemical combination to a considerable extent, and at points below that of complete fusion leads to different chemical results, even though the chemical elements may exist in exactly the same proportions. For instance, coarse silica combined with a certain percentage of flux, subjected to a certain heat, may result in a glass, holding in suspension much uncombined silica, whereas fine silica, combined with the same percentage of the same flux, subjected to the same heat, may result in a vitreous, semi-vitreous or even porous mass. The first will be a silicate mixed with silica, the second will probably be either a different silicate or a silicate mixed with a smaller quantity of silica. The flux will take up more of the fine silica than it will of the coarse. In the clays used for glazing extreme difference in coarseness of material is not apt to be encountered, and the thorough grinding and working which is given to the clay tends further to reduce this difference.
The problem of crazing has to do only with burned clays, consequently it can have nothing to do with the original physical characteristics that have been eliminated by the burning. In vitrified clay products it is then almost entirely a chemical problem; in semi-vitrified bodies it may be a trifle less of an absolute chemical problem, and in porous bodies original physical characteristics may affect the problem to a marked degree. I am free to confess that on this latter point I do not feel thoroughly posted, but my opinion is that within the limits of the probable variation to be met with in clays used for glazing the effect of physical structure is slight
The chemical elements found in clay are necessarily aluminium and silicon, with more or less iron, and there are frequently any or all of the following elements: Sodium, potassium, calcium, magnesium, sulphur and carbon, also, of course, hydrogen and oxygen; besides these, there may be others, such as barium, fluorine, chromium, manganese, gold, titanium, etc., etc., but these latter are usually in such small quantities as to have very little effect upon the burned product from the point of view of this article. The sulphur, carbon and hydrogen and oxygen in the form of water are mostly or entirely eliminated by the burning. The binding material in all conditions, either raw or burned, is the silicate of alumina, or pure clay, which contains silica, 46.3 per cent; alumina, 39.8 per cent, and water, 13.9 per cent, when unburned, and, when thoroughly burned, contains silica, 53.82 per cent, and alumina, 46.18 per cent. All silica in excess of 46.3 per cent in the original clay is a weakening element, except as acted upon by the fluxes which the clay may contain. Sodium and potassium are the fluxes at all heats, having an affinity for silica and readily forming silicates therewith. These alkaline silicates are binding elements. Iron enters into the fluxing department at different heats, depending upon the form in which the iron is present, but it is safe to say that it does not enter so early as the sodium and potassium. Magnesia is a flux at a higher heat. Lime is peculiar, and evidently possesses a double property. I have not investigated it in all its combinations, especially when combined with large quantities of iron, but so far as I have gone the indications are clear and positive that in reasonably pure clays lime is actually a resistant and consequently a weakening element, below a yellow or bright yellow heat or, say, below the fusing point of Albany slip clay, and becomes a strong flux beyond this heat. Lime also differs from sodium and potassium in the fact that it has a limit of fluxing power. A proportion of one of lime to about five of silica is the most fusible mixture of these two substances. Further additions of lime add to the refractoriness of the mixture, whereas the greater the amount of soda or potash the more fusible the mixture.