RELATION OF CHEMICAL AND PHYSICAL CONSTITUTION TO BEHAVIOR IN FUSION.
Historical—Search in ceramic literature disclosed the fact that prac tically no data have vet been published that have a direct bearing on the relation of chemical and physical constitution, behavior of clays in fusion, and toughness of the burned ware. Ogden' did some prelimin ary work on the relation of composition to toughness in porcelains and found the remarkable fact that increase of clay content from 30 to 60 per cent caused a decrease in the toughness of porcelain. Inasmuch as he employed the "rattler test" in determining relative toughness of his bodies, his studies are directly applicable to the study of paving brick clays. While the development of toughness has not been shown to have a direct relation to the rate and manner of vitrification except in our own results, yet that such a relation exists can be assumed until other evidence proves the contrary. If this assumption is correct, Og den's results would show that the evidence developed by metallurgists to the effect that addition of either aluminum oxide or silicon oxide not only raises in degrees centigrade the period at which fusion is com pleted, but also increases the viscosity of the molten mass, and the rate at which verification takes place, is not applicable to certain mixtures It must be admitted that before Ogden published his results, ceramists entertained the belief that the greater the content of A1=08 and SiO in clays, the greater would be the toughness. The findings in the case of fire clays here reported confirm Ogden's ideas.
In the following paragraphs will be given such evidence as seems to bear on this point.
Effect o f Al Oa in. Ceramic has been known for some time that the addition of Os to clays and clay mixtures increases their refractoriness. Fire clays, high in A1,08, are, as a rule, the most re fractory. Al208 not only raises the actual period at which fusion is completed but also causes the wares made from aluminous clays to soften and deform very slowly. The slower softening and deformation of ware. made from aluminous clays has been attributed to increase of viscosity' of the mass caused by alumina.
The writer has shown' that the addition of Al2O3 as a constituent of clay to stoneware glazes until the proportion of alkali and alkaline earth to alumina was 2.5 to 1, not only rendered the glaze more fusible but
also less viscous. Additions of Al208 above this proportional amount in creased the refractoriness of the glaze, if not its viscosity. Addition of A103 as a constituent of feldspar did not have as great effect on the fusibility of the glaze as did the same equivalent of A103 from clay, notwithstanding the additional alkali that would be introduced by the feldspar.
From these stoneware glaze studies it was concluded that it was not so much a question of quantity of A103, but of the manner in which it was added. If added as a constitutent of clay it is already combined with silica and water. Whether it is this mutual solution of calcium carbonate and clay that caused greater ultimate fusibility in the stone ware glazes, when clay was increased to a definite amount, or whether it was a complex case of an eutectic mixture of several substances, is not yet determined. The fact remains that additon of clay did cause greater fusibility and less viscosity, notwithstanding the fact that with each addition of clay the A103 was being increased.
Bleininger' has shown experimentally that calcium carbonate reacts with finely pulverized feldspar as readily as with washed kaolin. From his results it would seem as though fusion is initiated between calcium carbonate and feldspar as early as between calcium carbonate and kaolin (pure clay). This being the case it would seem as though the addition of clay to stoneware glaze mixtures was merely the formation of a eutectic mixture of Evidence thus far developed in the case of simple mixtures is sum marized in the following table : Any increase or decrease of Al208 outside of the limits given in the above table results in increase of refractoriness of the mixtures as shown in the several curves to which reference has been made. Similar points of greatest fusibility have been noted in the case of glazes, but data have not been obtained that permit showing the facts in tabular or curve form. A1=08 then increases the fusibility of mineral mixtures when added in amounts not exceeding a given proportional limit, the limit being different for different mixtures.