The Use of Standard Discs in the Strain Testing of Glassware

JSGT_1940_V24_T186_T196

Glass discs with standardised degrees of strain (described shortly as standard strain-discs) have been used at eleven bottle factories and one other glass factory. The results and comments of the observers are correlated in this report. Four different types of strain-viewer have been used, and a large variety of glassware with respect to its weight, thickness and contents capacity has been examined. The main conclusions can be summarised briefly. The method is most easily employed in conjunction with a strain-viewer which has a large, uniformly illuminated field of view, with uniform polarisation over a relatively large area. In general it is agreed that the strain-disc method of comparison is simple and trustworthy, is useful for training personnel, and that good agreement between different observers is obtained. When the degree of strain is small, observers rarely disagree by more than one disc, colourless or pale green bottles being used; but when the strain is greater than normal, agreement between observers is not so good. The discs are not particularly suitable for use with amber or dark green bottles unless a suitable colour tint plate be superimposed on them. In general, it is agreed that for bottle examination no revision of the degree of strain of the discs is necessary. Only about 1.5 per cent of the ware examined had a strain grading greater than three discs.

E. J. Gooding, B.Sc., Ph.D.