FACTORS AFFECTING RATE OF FUSION.
Aiineralogical Composition—Synthetical studies of the fusion of mix tures of pure minerals, have shown that the same chemical elements, brought together as constituent parts of different minerals, produce mixtures having unlike fusion periods. The rate of fusion and the regularity with which it progresses, as well as the point of complete yielding, are affected very largely by the manner in which the various elements are previously combined. Because of the difficulty of mak ing a microscopic mineralogical analysis of a clay, we are not able to obtain information that would aid in an attempt to foretell or explain in full the fusing behavior of clays. Realization, therefore, of the fact that difference in the mineralogical make-up of clays of like ultimate chemical constitution, causes difference in their fusion behavior, is the only result of practical value that has so far come from the study of the fusion behavior of synthetical mixtures of minerals.
There is one very notable exception to the above, and that is in the case of calcium carbonate. The effect of calcium carbonate, depending upon size of grain and extent and homogeneity of diffusion throughout the clay mass, operates in a two-fold manner. If thoroughly blended with the clay in small particles a portion of it (on the average up to about 8 per cent of the total clay mass) operates as a very active flux. Its fluxing effect is most notable on account of the rapidity with which it combines with clay substance to form a molten mass. This reaction is in some instances so rapid as to make it very dangerous to ap proach the vitrification temperature. If the calcium carbonate is pres ent in nodules, the thermo-chemical reaction just described can take place only at the points of contact of the decarbonized lime and clay, the remainder of the carbonate being converted into quicklime. The different effects of lime in these two physical conditions one the rate and regularity of fusion of the clay mass is obvious.
In the very valuable researches recently published' by Dr. Reinhold Rieke, it is demonstrated that lime added in excess of a given amount does not act as a flux and cause sudden failure of ware with slight in crease of heat treatment. In fact, it happens that this excess lime
to counteract the effect produced by the smaller quantities. Experi ments in the compounding of pottery bodies have shown that notwith standing the fact that wares containing lime in excess of this amount do not fail by sudden fusion, i. e., with slight increase in intensity of heat treatment, they suffered a rapid decrease in porosity and specific gravity when the heat treatment had become sufficiently intense to cause the formation of the more fusible lime-silicate solution. It ac cords entirely with the facts to conceive of the fused portion as a mutual solution of minerals becoming saturated with lime. Up to the point of about one-third saturation the lime is very active as a flux and decreases in activity as the saturation approaches completion. It is easy to see, therefore, that any lime which may be present in quantities in excess of that which can go into solution will not have any fluxing action.
In most mineral mixtures (and this is true in clays) the first which fuses is not the most fusible individual mineral or substance which may be present. The first to fuse will be the most fusible mixture of the minerals present known technically as a eutectic mixture. This mix ture may consist of two or more of the clay ingredients. Whatever the mixture may be—and this depends largely upon the size and character of the grains—it will fuse some time before the fusing point of the most fusible mineral has been reached. This is shown in the curves given in the section of this report which deals with the chemical prop erties of clays.
Now (repeating for emphasis) any lime in excess of that which is required to form this most easily fusible (eutectic) mixture which is possible with the kind and condition of minerals present in a given clay, will not be active as a flux. That portion of the lime necessary to form this eutectic mixture goes into solution with a rapidity which is inversely as the degree of saturation. The lime which goes into solution is least active as a flux when sufficient is present to completely saturate the fused portion, most active at about one-third saturation.