AbstractViscosities and surface tensions of glasses and slags are properties of great practical importance but they are frequently difficult to measure. The methods which yield results in absolute units are time‐consuming and are not applicable to a study of the behavior of partly devitrified silicate melts.This paper describes an empirical method for estimating not only the relative fluidity of glasses and slags but also for determining the lowest temperature at which crystallization from a previously chilled vitreous slag will occur on reheating, the approximate rate at which devitrification proceeds at various temperatures, and the range of temperature within which devitrification is complete enough to prevent flow of the slag under its own weight.By an extension of the method to mixtures of crystalline substances, the behavior of such mixtures upon heating may be studied advantageously. By a further extension of the technique, the temperature at which the solid‐liquid interfacial tension becomes low enough to permit wetting of a solid by a glass or slag may be determined. The applicability of these techniques to vitreous enameling problems is indica