Reviewing the known facts about the effect of silica on either the fusion of clays or development of toughness in clay wares, it must be admitted that we have not at present much positive evidence.
Effect of Magnesium Oxide in Ceramic Mixtures—In figure 20 on page 209 is shown graphically in fluxing effect of magnesium oxide with kaolin. Metallurgists report that magnesium oxide is a much "harder" flux than calcium- oxide and produces a much more viscous slag. Ceramic investigators have reported conflicting results in their attempts to use MgO as a flux, some claiming that -it is more active than CaO and some that it is less active. Claims have been made by some that in glazes it gives greater fusibility and slower fusion, while others claim opposite results. From this accumulation of apparently conflict ing data it has been shown that in short quick burns, as in experimental kilns, MgO is an active flux causing more rapid fusion, but in longer burns its fluxing action begins as early as in the shorter burns but pro gresses less rapidly and requires more intense heat treatment to effect complete fusion.
The lag in the fusion of mixtures containing magnesium oxide is at tributed to either the viscosity of the resulting magnesium silicate, if it enters into combination with the glassy matrix that fills and seals the pores of vitrifying wares, or to the formation of non-fluid magnesium compounds. Cement investigators claim that the alkaline earth sili cates formed by heating mixtures of clay and calcium or magnesium car bonate at temperatures below that required to cause sintering of the mass into a hard cake or brick are simple silicates of calcium or mag nesium oxide which are not necessarily fluid. At any rate, the effect of magnesium in ceramic mixtures differs from that of calcium in that the magnesium mixtures fuse very slowly over a long heat range, while the calcium mixtures, especially when present in amounts equal to or more than 10 per cent, remain porous up to the time that fusion begins, and then fluxing ensues very rapidly causing the ware to pass from por ous into the overhurned condition within a very short range of heat treatment.
The only known facts concerning the influence of magnesium oxide in ceramic mixtures are: (1) magnesium oxide increases viscosity; (2) mag nesium oxide causes slower rate of fusion, at least when it is the pre dominating flux; (3) as has been stated earlier in this report, clays which make good paving bricks contain a larger amount of magnesium than calcium oxide; (4) the Italians are now making low-fired porce lain of which toughness is a special feature, and in which magnesium is the only Ro or fluxing base present.
Effect of Calcium Oxide in Ceramic has shown that the presence of a small amount of calcium oxide in porcelain mixtures results in increased toughness of the ware. His investigations are not, however, sufficiently exhaustive to warrant more definite statement.
It is known that lime causes a breaking down of the silicates with comparatively little heat treatment, and - also that the new silicates formed are probably very simple in composition until higher tempera tures are attained, in which event these simple silicates suddenly fuse, causing the whole to pass rapidly into a fluid mass.
Dr. has shown that in mixtures of from 1 to 10 per cent of calcium carbonate with kaolin very close tight bodies are obtained which have quite a large range of vitrification and in the end fuse quite gradually. Mixtures containing more than 10 per cent of calcium car bonate remain quite open until final fusion begins, at which time the whole mass fuses very rapidly.
In comparison with Rieke's work it is of interest to study results obtained by who worked with a mixture similar to Rieke's high calcium body.
The two bodies were as follows: In the following table are Nauss' results and in a separate column are placed the data obtained by Rieke. Rieke measured his heat by cones and hence the temperatures obtained in these two studies cannot be com pared closely. Since, however, Rieke used a Seger trial kiln and very short firing periods, his cone readings can be approximated in of degrees centigrade within the accuracy of and discrepancy between the method by which each research was executed.