Mechanical Strength of Porcelain

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Abstract
The mechanical strength of porcelain, in which a glassy phase is the major part, is shown to be influenced mainly by stresses set up in the glassy phase rather than by the amount or size of mullite crystals formed. A crystalbe material (quartz) having a coefficient of expansion higher than that of the glassy phase will establish favorable prestresses in the glassy phase. As the tensile stresses induced in the quartz are relatively high, uniformly sized grains of about 15 to 30 μ will best withstand these stresses. If finer‐sized quartz grains are used, they will dissolve on firing and will not contribute to favorable prestressing of the matrix. The maximum prestresses are obtained when the radius of the quartz grains is three times that of the surface layer dissolved on firing. Illite, as a flux, tends to dissolve only a thin layer up to the maturing of the porcelain, but increases its action at the maturing temperature, thereby increasing the favorable prestressing. Another factor is the temperature gradient inside the body during firing, which causes gradual differences in the coefficients of expansion because of quartz dissolution. As the coefficient of expansion is lower near the surface than near the center, additional favorable prestresses are established.

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