Cords, Surface Condition, and the Quality of Glassware

JSGT 1942 V26 T177-T196

A comparison has been made of the direct shadow, polariscopic and photo-optical methods of estimating the degree of cordiness of glass by the examination of numerous samples of cordy glass obtained from eight experimental melts made under controlled conditions. For routine comparison and control the direct shadow method is an extremely sensitive one, and very satisfactory estimates of cord can be made by it and a “figure of merit” obtained. The polariscope method is less sensitive for direct inspection, and its most useful employment is in the examination of ring-sections, by means of which the distribution of glass throughout the article can be checked. A photo-optical method has been developed which gives a measure of the cords present, depending only on the accuracy and sensitivity of the apparatus employed. By its means the direct shadow prints may be graded. A variant of the thermostatically controlled centrifuge method of Turnbull has been devised and built in order to separate, from finely powdered samples, glass of differing densities. An estimate of inhomogeneity may be obtained in this way by giving the total density spread of a sample, or the mass-density distribution in the form of a grading curve. A general comparison of surface defects can be made by a shadow method, but optical or photographic methods are to be preferred. Examples are given of the general surface condition of pressed, blown, and blown and spun ware made by automatic machinery. A numerical value for surface condition is more difficult to obtain even than for cord, but satisfactory ”figures of merit” have been derived from photographs of the surface at magnifications between 50 and 100 under conditions of oblique illumination. The dependence of surface condition on that of the mould is illustrated by photographs taken of samples of a small panel bottle taken at regular intervals throughout the mould life. It is suggested that the methods described should be more widely adopted in the routine control of glass quality.

A. J. Holland, M.Sc., Ph.D., A.R.C.S., Eric Preston, Ph.D., D.Sc., F.I.C., F.Inst.P., and Professor W. E. S. Turner.