The state in which the structure of glass exists when quickly cooled and the action of annealing might be explained. When glass is quickly cooled, being a bad conductor of heat, insufficient time is allowed for the middle or interior portions of the glasswares to settle down and assume their normal state of solidification. The outer portion, or crust, will first cool and contract with an enormous strain upon the hot interior. This difference in the state of tension between the outer and interior portions gives a want of uniformity, and stresses of tension and thrust are developed, which cause the whole to collapse with the slightest external scratch or heat change. In annealing, this strained or forced condition in the structure of the glass is relieved by subjecting the glass to a pre-heating, and gradually diminishing the temperature, allowing a sufficient time for the different layers mutually to adjust themselves to their comparative normal positions, and thus relieve the strains within the mass. Much depends upon the pre-heating temperature and the rate at which the diminution of the temperature takes place. If this is properly provided for, the best results are obtained in the stability of the resulting glass. The presence of any stress can be determined by using a polariscope. The average British glass manufacturer has little knowledge of the value of a polariscope, or stress viewer, in ascertaining the physical state of his glasswares, and until he adopts its use there is little prospect of an improvement in his annealing methods. Much faulty annealed glass is being turned out which the glass manufacturer is not aware of, and which could be avoided by the intelligent use of such a simple instrument, which detects badly annealed glass at once by the aid of crossed nicols and a selenite plate. Owing to the unequal densities of the various silicates present in the heavy lead and barium glasses, they are more subject to striation and require more careful annealing than the soda-lime glasses, in which the silicates present are of more equal density. However, much depends upon the proper “founding” and melting of such glasses. The use of a larger proportion of cullet assists in breaking up striation. The presence of striae or cords in glass disqualifies it for most purposes, as it is usually found that, apart from their defective appearance, they tend to produce stresses within the glass. Transparency, brilliancy, stability, and homogeneity are important factors in producing perfect glassware, and the proper development of these distinguishing properties requires considerable skill on the part of the glass manufacturer, alike from a technical, physical, and practical standpoint. ------------------------------------------------------------------------ CHAPTER XI THE MANIPULATION OF GLASS GLASS MAKERS’ TOOLS AND MACHINES