Microscopic, and X‐ray examinations of a series of steatite compositions reveal that the body having the best dielectric properties Also has the simplest and most homogeneous physical structure. It is believed that the attainment of similar physical structures in other ceramic bodies will lead to improvements in dielectric properties. This empirical finding is in accordance with present theories of conduction in crystals. It depends on the contribution to conductivity and dielectric loss made by imperfections of crystal structure. However, even in homogeneous materials, alkaline earth ions tend to cause less conduction under like conditions than alkali ions, and there are high‐dielectric constant materials like the titanates which, though homogeneous and pure, have a high loss.

During the past 30 years, the principal ceramic insulating materials have been the electrical porcelains, the steatites, and the high alumina bodies. The compositions, the structures, and the properties of these materials are discussed. Other porcelains and insulating compositions, containing as their principal crystalline phases mullite, zircon, forsterite, cordierite, wollastonite, or β‐spodumene, have unusual properties and are useful for specialty items. Predictions of future trends in ceramic insulating materials are made.

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Journal of The American Ceramic Society-Rigterink am? Grisdale also a t all test temperatures for specimens of both brands of siliceous fire-clay, one silica, and one highalumina brick; large deformation was obtained a t ' 850' to 95OoC. for the fire-clay brick and a high-alumina brick containing 80% alumina.The creep values for specimens tested in compression were not proportional to those for specimens tested in per day tensile creep at 95OoC. ranged from 3 lb. per sq. in. for the fire-clay brick containing 48y0 S O 2 to 120 lb. per sq. in. for one containing 65% SiOz.Unstressed specimens of silica brick showed growths as large as 0.27% resulting from repeated heatings at several temperatures without load.tension. REFRACTORIES SECTIONThe stress that would produce a 1 X in. per in. ~~~~=~~~~. o~. s T A N D ABSTRACTAlkaline earth porcelains have been prepared from mixtures of clay, flint, and synthetic fluxes consisting of clay calcined with at least three alkaline earth oxides. These porcelains possess excellent dielectric properties, have low coefficients of thermal expansion, are white, and are especially valuable as bases for deposited carbon resistors for which they were developed. Their characteristics make it probable that other uses will be found for materials of this type.An illustrative composition is 50.0% Florida kaolin, 15.0Q/, flint (325 mesh), 35.0% calcine (200 mesh). The composition of the calcine is 40.0% Florida kaolin, 15.0% MgC03, 15.0% CaC03, 15.0% SrCQ, 15.0% BaC03, calcined at 1200°C. The electrical properties of this body at 1 mc. are Q at 25"C., 2160; Q at 25OoC., 280; Q at 350°C.. 90; speci6c resistance at 15OoC., lOl3.6 ohm-cm. and at 300°C.. 1010-7 ohmcm.