[Alumina reinforced porcelain jacket crown].
Aluminum oxides provide one of the most useful sources of high-strength and high-modulus crystals. New types of high alumina content glasses may replace feldspathic porcelains. Esthetics compare favorably.
Mechanisms Involved in Securing Dense, Vitrified Ceramics from Preshaped Partly Crystalline Bodies
The internal porosity of ceramics shaped before fusion has been improved by three firing techniques: (1) firing in vacuo, (2) firing in diffusible gas, and (3) finish firing and cooling under pressure. A method of measuring pore volume and texture has contributed to understanding the operating mechanics of these processes. This has led to recognition of the firing-process requirements for different porcelains depending on their crystalline content, matured viscosity, service conditions, and performance specifications.
Fracture of Glass by Spherical Indenters
The production of ring cracks in silicate glass, by the impact of steel balls and by the application of a static load to spherical indenters, has been studied for a large range of indenter curvature. It has been found that in impact tests a greater height of fall is required to produce ring cracks than predicted by a critical stress criterion of fracture. In static experiments, two different relations between the force required to produce fracture and indenter size have been found. Below a radius of curvature of approximately 3.5 cm the force is proportional to the radius and above this value to the square of the radius. Impact tests have also been carried out on a specimen of an organic glass-like material, with results similar to those of the impact experiments on silicate glass.