The rate of formation and the amount of fused material formed in a brick very obviously determine the rate at which the open pores will be eliminated. Since lime readily forms solutions with silicates, and particularly with clay substance, those clays which contain from 2 to 8 per cent of free lime will vitrify rapidly. Other clays having the same ultimate chemical composition as the rapidly vitrifying ones, but in which the lime is already combined as in a lime-bearing silicate, will not vitrify rapidly, other factors which influence fusion being equal. We must recognize, therefore, that when the other factors which effect fusion are the same, the amount of lime which will combine to form this most easily fusible mixture depends upon whether the lime is free or. com bined, as well as upon the kind and relative quantities of the other oxides present.
The per cent of calcium oxide which Rieke found would form the most fusible mixture of the formula XCaO 1 Al203 vSi02 were as fol lows : XCaO 1 1 ; — 25.6 per cent CaO XCaO 1 2 SiO, ; — 33.4 per cent Ca01 XCaO 1 S SiO, ; — 33.1 per cent CaO XCaO 1 4 SiO, ; — 24.6 per cent CaO In each of these mixtures the per cent of calcium oxide taken into solution up to the point where the rate of solution began to decrease as shown by his curves, were as follows: XCaO 1 1 ; — 7.9 per cent CaO XCaO 1 A1,0, 2 ; —10.0 per cent CaO' XCaO 1 3 ; — 9.8 per cent CaO XCaO 1 4 ; — 7.5 per cent CaO Size of full significance of this factor can be appreciated only by considering extreme cases, as in the case of calcium carbonate, above cited, or as in a mixture of two minerals such as feldspar and flint. When feldspar and flint are mixed as fine powders in the pro portion of 75 per cent feldspar and 25 per cent flint, the mass will be fused to a fluid at approximately 1100°C in a comparatively short time. If, however, these two minerals were placed side by side in the shape of rectangular pieces having the same proportional weight as in the first case, the only fluxing action that would take place at 1100°C would be at the points of contact. Even if the heat was held at 1100°C, com plete fusion of the two pieces of mineral could only take the glass, formed at the point of contact, enveloped and slowly ate into the unfused portions, and thus produced an intimate mixture of the two minerals by diffusion or surface tension. It is common experience that if complete fusion of the two minerals at 1100°C is desired when brought together in the form of coarse particles, considerable time must be allowed, and that to effect complete fusion in a shorter time, the heat must be raised from 1100°C to 1230°C (approximately), or the fusing' point of feldspar. At this temperature the feldspar melting would completely' envelop or perhaps float the flint particles, and slowly attack and dissolve them, just as water will attack and dissolve a piece of loaf sugar.
The above illustration, while an exaggerated case, nevertheless is descriptive of the effect of fineness of grain on the fusion of any two minerals which the mutually soluble, and also descriptive of the fusion of a mixture containing particles of several minerals, as a clay.
In the burning of clay wares, where time is an important and un avoidable factor, the effect of fineness of grain influencing the fusing of clays is particularly noteworthy. By the manufacturers of pyrome tric cones it has been recognized as such a powerful factor that the ut most care is taken to maintain uniformity in size of grain in their materials, both before and after manufacture into powdered cone stock.
The statement has been made in preceding paragraphs that differ ences in mineralogical constitution cause differences in behavior of clays during fusion. That statement is correct for the heat treatment or time and temperature required to affect either the partial or complete fusion of the mass. It would not be correct, as will be shown, if the tempera ture alone was considered.
The mixture of minerals in a clay which has been ground in a dry pan is far from being homogeneous. Our discussion earlier in this chapter of the constitution of the grains should make it plain that even if they were as finely ground and as thoroughly disintegrated as is prac ticed in the potteries, the mixture would lack very much of being homo geneous. Now the molten silicates are so viscous that diffusion in them is exceedingly slow compared with diffusion of salts in water' and hence a very. long time would be required to obtain the homogeneous mixture that is necessary before the mass will fuse at its true melting point.
Walker has been quoted to the effect that crystalline substances have a definite melting point while amorphous substances do not. The reason for this is based very largely upon this matter of absolutely perfect homogeneity of constitution. When a substance crystallizes, its com ponents are as intimately and homogeneously blended as it is possible to conceive of, hence, when the mass fuses the components are in a position to dissolve in one another as soon as a temperature is attained at which the solution is affected. In amorphous compounds' we have not this intimate molecular mixture' and hence not a sharp melting point. In the case of clays and clay mixtures, where we are not able to cause a mixture of the components that is any other than a compara tively very poor approximation to intimacy and homogeneity, it must be expected that either an inordinarily long time will have to be taken, or a temperature higher than the true melting point of the mixture be maintained in order to effect the fusion. This is why in research laboratories they either remelt the mixture at least once before determ ining its true melting point, or, take it to complete liquid fusion and note the temperature at which the mass solidifies. This is also the reason why potters find that a mixture will melt more easily the second and third time. This is also one of the reasons why so much stress was laid by the writer upon the mineral constitution of the grains as Grout found them in the West Virginia clays.