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THE PHYSICAL PROCESSES OCCURRING IN THE MELTING AND COOLING OF GLASS
Author(s) -
LITTLETON J. T.
Publication year - 1934
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1934.tb19283.x
Subject(s) - devitrification , thermodynamics , viscosity , mineralogy , materials science , annealing (glass) , crystallization , chemistry , composite material , physics
The thermal treatment of glass is followed from the melting and refining stage downward through the annealing zone. The fusion of the batch constituents is shown to be completed in a short time and the greater portion of the time required to obtain glass of good quality is used in freeing the glass from gas bubbles and in improving the homogeneity. It is postulated that melting is similar to a diffusion process, the constituents being mixed by diffusion aided by convection currents. A review of the evidence for silicate compounds in glasses shows that only electrical conductivity data indicate the existence of such compounds. Dissolved gases probably assist fining by diffusing into the bubbles and increasing their size. This offers one explanation of the action of fining agents. Devitrification is shown to be avoidable by a knowledge of the devitrification emperature and staying above this temperature during melting and fining. Any subsequent cooling through the devitrification zone should be rapid. The earlier annealing equations are based on the experimental observation that rate of release of stress in glass is proportional to the square of the stress. This fact is difficult to explain on a physical basis when it is assumed that viscosity is constant with temperature. However, data are cited which show that viscosity changes with time and when the instantaneous values of viscosity are used it is shown that the rate of release of stress is proportional to the stress divided by the viscosity. The constant of proportionality has the dimensions of the shear modulus of glass and is equal to one‐fourth the value of the shear modulus in glass at room temperature.