... considerably in the post-war period The writer fully acknowledges his debt to many sources of information: to books on glassblowing; glass-tube manufacturers' information circulars; laboratory furnishers'... on the composition and varies (from 3.3 x 1 0-6 per °C for ordinary borosilicate glass (widely used for laboratory glassware) to 7.2 x 1 0-6 per °C for special types used in graded seals The annealing... for about 18 months The existing stock should be used before the new stock Buying stock calls for considerable experience and foresight, and some skill in making an informed guess as to the glassblowing