It is well known that quickly cooled glass is in an unstable condition due to stresses acquired in cooling. Such glass is said to be "strained," and if the strain is excessive, the glass may crack or even fly to pieces when handled, or when heated or cooled. If the strained glass be heated to a sufficiently high temperature and then slowly cooled, it may lose a considerable part of its internal stress. This operation of removing or diminishing the strains in glass is called annealing. It should be noted that this is not the only use of the word annealing, which is often applied to other kinds of transformations. A metal such as silver when drawn, hammered or otherwise deformed, undergoes a physical change and acquires increased hardness which it may lose when heated. It is then said to be annealed. "Annealing" is also used to characterise the chemical and physical changes taking place in the complex mixture, steel, when it is heated to certain temperatures. In the present discussion, however, we use the term annealing only in connection with the relief of internal stress, a process which has no necessary connection with the other above-mentioned physical processes.
