An Experimental Observation of the Tempering of Flat Glass

Paying attention to both the thermal fractures taking place in glass tempering process and the fracture densities of tempered glass, the experiments of oil tempering of glass were performed. From these experimental data, the following results have been obtained.1) Critical thermal conditions on which the fracture takes place in tempering are depended on both the preheating temperature of glass and Biot's number. Furthermore, stress relaxation of glass surface in visco-elastic region can be determined on the basis of the conditions mentioned above.2) There exists the linear relationship between the fracture densities and the residual surface stresses determined by the procedure of extrapolation on the basis of the curve obtained by experiments.