The Viscosity and Working Characteristics of Glasses. Part III. Some Observations on the Rate of Cooling and Viscosity of Glasses During Manipulation by Hand

JSGT 1943 V27 T207-T237

Determinations of the temperature by thermocouples have been made both at the surface and in the interior of 1 oz and 3 oz parisons and 4 oz and 8 oz bottles at the various stages of manipulation by a skilled hand-worker. The temperatures at which it was possible to gather from the pot (50 lb capacity) on the blow-pipe the different kinds of glass investigated were also ascertained, in all cases by an optical pyrometer, in a few instances by a thermocouple. The glasses tested included the following types: a potash–lead oxide–silica; colourless soda–lime–silica for bottles; a cobalt blue soda–lime–silica; bottle glasses coloured amber by iron-manganese and carbon-sulphur, respectively; and green by iron-manganese. The precise compositions of the glasses were stated by the authors in Part I of the investigation (this .J., TRANs., 1942, 26, 215). Cooling curves were derived for the glass parisons, and comparison of the results was made with those based on the cooling, under closely controlled conditions, of glasses in a small laboratory crucible (see Part II, this J., TRANs., 1943,27, 94). From the temperature measurements the corresponding values of the viscosity (in poises) were ascertained from the viscosity determinations set out in Part I. The basic factors which determine the most favourable conditions for the shaping of molten glass are temperature and viscosity; and these are related to the composition of the glass, its colour (diathermancy), weight of glass gathered and shape of article produced. Nunerous tables and curves show the interrelation of these factors and of the compositions and gathering temperatures of the glasses with the times occupied by the successive stages of manipulation. Glass for 1 oz parisons could be gathered between the temperature limits 1300° and 1075°, and viscosity limits 500 and 5000. The most suitable gathering temperatures and viscosities were, for 4 oz colourless and carbon-sulphur amber bottles 1125° and 2500, respectively; and for 8 oz, 1105° and 3000. The mean viscosity values during the marvering of 1 oz parisons of a colourless soda–lime–silica glass were as follows: (a) at the surface, start of marvering 13,000 poises, at end, 1,300,000; (b) at the interior (see text), at start of marvering, 3200 poises, at end, 16,000. The considerably greater difference of temperature between surface and interior of the coloured as compared with the colourless glasses during manipulation is demonstrated fully, and the modifying influence of the ''skin effect'' on manipulation is discussed.

James Boow & W. E. S. Turner