The Viscous Flow of Glass at Low Temperatures
JSGT 1952 V36 T164-T180
It is suggested that the majority of commercial glasses, which are incompletely melted and then rapidly chilled by blowing, pressing, drawing or rolling, have a chemical heterogeneity which is undetectable by ordinary chemical methods, since it applies to domains of some millions of molecules. As normally manufactured, the surface of bulk glass and glass in the form of fine fibres and thin-walled bubbles, is so rapidly chilled that a high-temperature (or "depolymerised ") structure persists. Commercial annealing does not remove this structure, although fine annealing (or stabilisation), in which the glass is held for long periods in the annealing range, does so. The effects produced by such rapid chilling (or "super-toughening") are illustrated by reference to the viscous flow of fibres rapidly chilled by water during the drawing process. Permanent set was obtained for chilled fibres heated under constraint at temperatures ranging from 18° to 450°, whereas no such effect was observed with fineannealed fibres at 350° and only a slight curvature resulted after 30 minutes at 450°. Other experimental evidence for such flow is : (I) the lowering of the strain point in the thermal expansion curves of rapidly chilled fibres; (2) the lower refractive index compared with massive glass; (3) the looping and curling of fibres by asymmetrical cooling; (4) the flow taking place during the scratching of rapidly chilled glass surfaces; (5) the increase in the electrical resistance of molten glass with time at melting temperatures; and (6) the changes in bulk electrical resistance of chilled fibres in the range 350°–550°. The important conclusion is drawn that many routine measurements of physical properties of manufactured products are vitiated by the thermal treatment applied in the laboratory.
I. Peyches
