in the ordinary metallurgical and ceramic arts. So also with alumina, which alone is a little more infusible than silica, and which nearly vitrifies at the heat of molten platinum.

But compounds of the two elements rarely, if ever, retain the characteristics of their component parts, either in fusing point or in other respects, and, furthermore, the fusing point of a compound is in nearly all cases lower than the average of its components and often lower than either alone. To this, kaolinite or silicate of alumina is no exception, though there is less difference here than any other case which might be cited. Its melting point is lower than that of silica or alumina, separately, but only slightly, for it requires a temperature nearly up to the heat of molten platinum, at which silica also fuses.

The importance of this fact can scarcely be overestimated. Consider for a moment what it means to the economy of mankind on this planet of ours. It means that the one material which has been gifted by nature with the supremely useful quality of plasticity, by which we make it take any shape our hearts desire, has also been given the power to become practically indestructible as well; that we can harden into permanence anything which our necessities lead us to model and design.

Pure clay substance, therefore, is refractory. It cannot be melted except in specially designed furnaces, at temperatures above those in practical use. But, as has been already said, pure clay substance is never found outside of museum specimens. Clays are composed of proportions of clay substance ranging from almost complete purity down to very low and unimportant amounts, and naturally their fusibility flunctuates nearly as widely as their composition.

The refractoriness of a clay, therefore, becomes a study of how these ever-varying mineral ingredients will affect each other and how they will affect the clay substance by which they are bonded together. And here the same laws come into play inside the anatomy of the clay, so to speak, as were shown to act on simple substances when heated together outside. We saw that a silica brick which could not be melted by heat alone would fail in a few moments if a basic clay was allowed to touch it when hot. Similarly, a clay which unites several ingredients in one body, each of which is in itself infusible, may fail wretchedly at a low heat when the test comes.

We must, then, observe in a clay two things, if we would form an estimate of its heat-resisting powers. First, we must know how much of its substance is really composed of clay or kaolin; and, secondly, we must know what its accessory minerals are, and how much of each.

Not only must we know these facts, but we must also know how to interpret them when they are obtained. And here is the key to the whole subject, after all, in this interpretation of the effect of these different minerals on each other.

Let us consider a very few of the principal minerals which we are certain to find in almost every clay.

First and foremost comes silica, quartz or sand, as it is variously called. This is an omnipresent ingredient of clays. None can be found which are absolutely free from it, and in most it constitutes a large, and often the principal, part. Now, the common and accepted idea among clay-workers, until very recently, has been that silica is an advantage to the refractory qualities of a clay. Indeed, I doubt not that if a vote were to be had among this assemblage many of you, possibly the majority, would be found in possession of that idea. Indeed, it is more than likely that some of you would arise as champions of the sand and say-not without truth, either-that you know of your own personal knowledge and from results of your own personal experience, that sand increases the refractoriness of your clay.

This fact cannot be gainsaid, but it will be my endeavor in a few moments to show how this apparent contradiction can be satisfactorily cleared up.

Some few years ago this subject was studied by the late Dr. Seger, in Berlin, who, with that wonderful clearness and simplicity which characterize all of his work, has presented the truth to us in a way which will never require improving.

Taking a pure white burning kaolin, which will endure a temperature nearly up to the melting point of platinum, he mixed. with it in aliquot proportions pure white silica. His mixtures had molecular formula and percentage composition shown in the following table:

From this but one deduction is possible, and that is that the silica has reduced the refractoriness of the clay. And as these are not theoretical considerations, but practical trials, there seems no escape from this deduction.

Seger's further experiments showed that silica reduces the refrac toriness of a pure clay even up to the proportion of seventeen molecules of silica to one of alumina, or in per cents up till the free silica amounts to about 77.7 per cent. and the koalinite to 23.3 per cent. of the clay mixture. After this point the continued ad

ditions of silica to the kaolin began to increase the melting point, gradually approximating to the melting point of pure silica, as the kaolinite becomes less and less.

It must be observed, however, that, after all, the total amount of this reduction in fusing point is not so very much in actual temperature. The whole of Seger's series, from the highest to the lowest, required temperatures greatly above that of a glowing whiteness. Indeed, the lowest point in this series is much above the fusing point of pure wrought iron or nickel, which requires 1500 degrees C. at the least, which is a heat tar in excess of that used in most of the metallurgical or ceramic industries.. So that these differences produced by sand on the fusing point. of a clay would be of no importance, if there were no other minerals present than clay and sand, or unless we needed for some special purpose the very maximum refractoriness which clay can be made to yield

Unfortunately, purity is easily obtained in a series of laboratory trials, but not in practical work. We must use such clays as can be had at reasonable prices and in large amounts, and as such clays invariably contain other ingredients than clay and sand, we must look further into the matter before we allow free entry of sand into our mixtures.

To Dr. Richters, another German chemist, we owe the next important step in completing our information on the subject. In his now classic researches he showed, among other things, that while the various bases, such as iron, lime, magnesia and the alkalies, could attack and fuse even a pure kaolin at high temperatures, that their activity was very greatly stimulated by the presence of a little free silica or sand. Now that we know this fact, it seems only reasonable and rational that it should be so. We always feel so about great discoveries after some master mind has presented them to us. For pure kaolin is a silicate of alumina, already combined before the heat test begins. Moreover, it is refractory by and in itself. The presence of other bases, like lime and iron, is unfavorable, of course, but they did. not find anything of opposite chemical affinity with which to combine, for the alumina has already gotten all the silica preempted. Hence, great heat and large quantities of fluxes are necessary if we expect to break down a kaolin in this manner. But suppose there exists free silica; we now have the two chemical sexes present; when the heat rises combination will naturally ensue, as neither base nor acid are attached chemically to the clay substance. As a result a fluid slag is formed which in turn acts as a solvent to the hot clay substance. Its operation is not unlike that of a lump of sugar whose lower surface is dipped into water and allowed to absorb by capillary flow until the moisture appears on its upper surface. If this lump is tested with a knife, it is still hard and sound, though damp. If it is set aside for a ew moments, it will be seen to have melted into a syrupy pool.

It seems incredible at first that the small amount of water in this hard lump of sugar would exercise so important an effect. And in a clay it is equally wonderful how far-reaching is the effect of a small quantity of slag-forming ingredients under high heat. This, then, is the one first great law of refractory clays: "No clay can have high refractory qualities if it contains any appreciable quantity of free sand. For even if the clay contains naught else but sand, and if it will endure high heats when tested by itself, there still remains the fact, that contact with any kind of basic matter at any time when hot will result in the surface slagging and gradually wasting away, and as the dripping and flowing of the surface proceeds, new surfaces of the clay are constantly exposed to the fatal contact with bases. It matters not from what sources these basic substances may come. They may be slags or coal ashes or sparks of molten metal-their action is the same in any case.

This explains why so many clay workers have failed to reach success in entering the field of fire brick making. Scarce a month in the year passes in which some one does not write to ask my advice as follows: "I have a fine white burning kaolin clay and also a fine white sand. I wish to enter the fire-brick business, and would like to know how to mix these to the best advantage. They reason that clay is infusible and sand is infusible; that the shrinkage of the clay will be controlled by the unplastic sand, and that from the two an ideal brick will result. Not infrequently they refuse to accept the above principles as true, and persist in a trial before they are convinced. There are still other reasons which cannot now be taken up, connected with the shrinkage and physical strength of the product which prevent any mixture of kaolin and sand being well received in the trade.

BURNED CLAY AS A FIRE-PROOFING MATERIAL.
(From the Clay Worker. February, 1898)

A slow burning and fire-proof building are just as different as a brick house is from a frame one. A building made of steel beams, properly erected, filled in and protected with burned clay (hollow tile) can be made as fire-proof as the kilns in which we burn our brick and terra cotta, or the furnace in which the iron is heated to be rolled.

The only known material with which this can be done is burned clay, that is, hollow tile or brick. Of course this must be made properly, and if the tile is not made to cover the iron in the very best manner, and to stay there under all reasonable conditions, the desired result is not accomplished; or if it is made so light in weight and of such a brittle character that upon the least test it flies to pieces, then it is not of the proper kind.

Tile should be made with heavy webs, with rounded corners and of a porous nature, which then affords a material not only of

sufficient strength, but of a toughness which allows of contraction and expansion without injury to the material.

To make tile in this way means that the manufacturer must have a price which will justify him in using heavier material, especially where freight is one of the greatest items of cost, but this he seldom secures. It is usually the owner who is to blame, not the architect. The owner tells his architect he wants a building of such dimensions to cost so much. This cannot be built under a certain sum, but he thinks it can, and sometimes the architect agrees with him, knowing at the same time it is impossible, but, like all of us, he is anxious for business, and prepares plans and specifications, irrespective of the quality of the fireproofing required, which, to my mind, is of the most important materials necessary in the erection of a building.

The specifications call for hollow tile, but no weight is specified; one manufacturer, if responsible, has just as good a chance. as another; some one of them, having little to do and anxious for work, sharpens his pencil and figures how light in weight he can. make his goods for this particular job. He reduces the thickness of his webs as much as possible, cuts out one or two and possibly all of them, and he finds his weight decreased per square foot, say, for example, ten pounds. What is his saving on 50,000 feet, which is an ordinary job? It is 250 tons, which at $3.00 per ton freight means $750.00 without considering other savings in his manufacture.

There is no question but what heavy tile will withstand the action of fire much better than light ones. This has been proven in actual experience, of which I will speak later. The light tile fellow secures nearly the same price as the manufacturer of heavy tile, who believes in making his corners round, his tile tough, not brittle, and in every way giving a better and stronger job. This should not be any more than the iron manufacturer has a right to receive as much for a ten-inch beam, one foot long, weighing twenty-five pounds as he does for one of the same. depth and length but weighing forty-five pounds.

But such is competition among the tile manufacturers themselves. And the worst of all competition is what is known as concrete, plaster of paris, lime of teil and other so-called patent systems of fire-proofing, which have no more right to be called fire-proofing than the moon has a right to be called the sun.

However, will say here, there are very few architects who ask tile manufacturers to compete against such constructions as I have last named, as the great and learned majority will have nothing but burned clay for the purpose of fire-proofing buildings, which are to be living monuments of their skill and beauty of design.

These people of patents have nothing to lose and everything to gain; they need no capital, as the dealers supply them with. their requirements in the way of necessary cement, lime, or plaster of paris; the cinders are given to them for the carting,