"First—In regard to the decomposition of calcium carbonate, it is clearly shown that it begins to break up between 610° and 650°C., and before 700° is reached the evolution of carbon deoxide is going on quite rapidly. At 1.000° the evolution is practically at an end." "Acco;td—On examining the amounts of insolublei residue and comparing the percentage with the known amount of quartz in the mixture, 18.66 per cent, and making allowance for the small amount of quartz in the kaolin itself, it is seen that the kaolin is decomposed completely at 850°C., and al most completely at 800°C." Third—Free quartz seems to be attacked by the calcium oxide' soon after the completion of the decomposition of kaolin, probably at about 950°C., which reaction continues at an increasing rate up to the highest tempera ture employed in these experiments. It is quite evident, also, that the length of time of burning influences the amount of quartz attacked somewhat, so that by longer burning, at least with temperature over 1100°, more quartz may be rendered soluble than in a short period of ignition." Prof. Bleininger, continuing, says : "A very interesting fact was brought out by the tendency to dust observed with the mixture at temperatures above 1200°C. While at 1200° the bri quettes were hard, at 1250° they dusted very rapidly, and at almost instantaneously." "On calculating the formula of this mixture from the composition we find it to be 1.77 CaO, 0.108 SiO2, that is not quite a singulo calcium sili cate, and hence must properly be classed within the group of natural ce ments. It is not difficult to understand that the dusting must be coincident with a significant molecular change from the condition of the loose, friable mixture to a hard body breaking down at once to a powder. Might not this fact indicate that up to 1200° 'these calcareous mixtures are but pozzuolane like, simple silicates, consisting of silica and base which on further applica tion of heat become chemically more complex and non-or but slightly hy draulic? This view is strengthened by the results of another investigation which have shown that on increasing the free silica, with but sufficient base to convert the quartz into the active state, the hydraulicity is practically as great as with a greater amount of base." 1 Rieke's data is evidence that Bleininger's query can be answered in the a;liimative, for it was at this same temperature, 1200°C., that his body ceased to increase in porosity and began to vitrify. From 1200° C. on, Rieke's body vitrified quite rapidly showing that "a significant molecular change" is taking place. From Nauss' results it must be conceded that the clay has suffered a. very significant change. Na doubt it has passed completely into solution with lime and silica. In fact Bleininger's results given in Table XXXVI page 248 proves this to be so.
Rieke's porosity data show also that prior to this critical tempera ture, 1200°, (rough approximate) the grains must be changing form and size, for the mass is getting more porous with each increase in heat treatment, yet, according to his shrinkage data (1.2 per cent at cone 05 and 3.7 per cent at cone 5) the mass as a whole is decreasing in vol ume. Similar simultaneous increase in porosity and decrease in volume was noted in several instances in our own researches, so this phenom enon is not alone peculiar to simple mixtures high in lime.
Important as are these observations, and especially that of complete solution, and possibly the formation of entirely new compounds before the mass begins to decrease in porosity, i. e., vitrify, the more important
item to note at this time is, in the writer's opinion, the difference in the ultimate fusion behavior of the two bodies, the one containing fred ilica and the other supposedly none. It was shown by Bleininger's result' that quartz is not nearly as readily attacked by CaO as is kaolin or feldspar, and hence it could be inferred that the higher the content of quartz in a mixture, the later and slower would the mass fuse. In decided contradiction to such an inference we find that in Nauss' body, containing 18.7 per cent quartz, the original minerals have been com pletely broken down and the whole began to "melt" at the same tem perature at which Rieke's body containing no quartz exhibits a porosity of 10 per cent, but complete fusion does not take place until a tem perature of about 1600° C. has been reached. We are learning not to wonder at such apparent discrepancies in experimental work where simple mixtures two minerals are compared in their fusing behavior with more complicated mixtures of minerals.
Summarizing these observations the following facts appear: First, Watts has shown that a small quantity of lime toughens a porcelain mixture. Second, Rieke has shown that in a simple mixture of kaolin. and 1 to 10 per cent calcium carbonate there is quite a large vitrification range and slow fusion, while in mixtures with kaolin containing more than 10 per cent of calcium carbonate the body does not vitrify until late and then rather suddenly fuses. These findings by Rieke and Watts agree with ours in support of the assumption that long vitrifica tion range and slow fusion generally result in the production of tough ware. Third, the results of Bleininger, Nauss and Rieke studied to gether show very forcibly that chemical alterations and reactions may take place long before vitrification and fusion begin.. Also, that each mixture has its own peculiar pyro-chemical and physical behavior, and, as the mixtures become complicated in composition, the deductions drawn from simple mixtures are found to hold true only in very small part.
Beyond these studies in simple mixtures by Bleininger, Nauss, and and the observation in complicated porcelain mixtures, we have no data that have a bearing on the effect of smaller or larger quantities of lime on toughness of burned wares made from shales. Contrasting the work of Rieke and Nauss, the difficulties that are encountered when attempt is made to trace the effect of,lime in such severely complicated mixtures as shales are clearly shown.
Effect of Other Oxides in Ceramic Mixtures—Practically nothing is known concerning the influence of oxides other than those considered above, except that in slags titanium causes increased viscosity; that potash silicates are more fluid than soda silicates, and yet, as a rule, less fusible; that phosphoric acid is expelled from ceramic mixtures only at high temperatures, and that, before expulsion it is combined with the bases forming phosphates that are analogous to the silicates. A detailed study of the influence of the several oxides, alone and to gether, on the fusion of silicate mixtures and the toughness of the burned mixtures, offers a very fruitful and interesting field for re search.