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emphasize the importance of the surface behavior in its relation to strength. Below firing temperatures of G O o , t h e chemically treated clays were generally weaker than the untreated clay, whereas at higher temperatures they were generally stronger. The NaC1-treated clay fired to temperatures below 50O0 was stronger than the CaC12-treated clay, which was in turn stronger than the HC1-treated clay. These results agree with those cited in t h e literaturezv7 (i.e., the dry strength decreases with the adsorbed ions: N a + > Ca++ > H+) and also indicate t h a t the number, behavior (e.g. hydration tendency), and charge of the exchangeable ions are all important factors.There was no evidence of reaction between the clay particles and adsorbed ions a t temperatures above 500° where the sudden increase in strength occurred. The highest strength values were obtained at 80Uo firings where increases as high as 131yo were observed. The hydrogen clays (HCI and electrodialyzed) and t h e H202-treated clay had t h e greatest strengths after being fired to this temperature. AcknowledgmentsThe authors wish to express their appreciation to R. W. Grimshaw for his comments and criticisms and to the Edward Orton, Jr., Ceramic Foundation, Columbus, Ohio, which granted the Fellowship that made this study possible.A method is described whereby safe cooling schedules for structural clay products are developed, based on a criterion for thermal failure. Resistance to thermal failure may be indicated by the following equation: Resistance = M R ~/ E c Y ,where M R is the modulus of rupture, E is the modulus of elasticity, a is the thermal diffusivity, and CY is the coefficient of linear thermal contraction. The modulus of rupture, modulus of elasticity, linear thermal contraction, and relative thermal diffusivity of three commercial bodies were measured in the temperature range 1600'F. to room temperature. Using the experimental values, relative cooling schedules are developed in terms of ware temperature versus dimensionless time, the total time for cooling being a function of size and shape. The order of importance of the physical properties is discussed with respect to cooling schedules and it is concluded that the coefficient of linear thermal contraction has the greatest effect. The mechanisms acting in clay bodies during cooling are discussed and the idea of quartz separation is presented. The hysteresis of the modulus of elasticity and coefficient of thermal expansion while heating and cooling is correlated with quartz separation and the literature is cited to support the idea. Some tests are described to show that rebonding of the quartz occurs on reheating to sufficiently high temperatures.