Thermal Properties of Glass
The strain point represents the extreme upper limit of serviceability for annealing glass. The maximum service temperature will always be below this point. The annealing point is the temperature, at the upper end of the annealing range, at which the internal stress is reduced to a commercially acceptable value over a short period. In an annealing operation, the glass is slowly cooled from the above the annealing point to somewhat below the strain point. The softening point is the temperature at which a small diameter fiber of the glass will elongate under its own weight. As one moves above this temperature, the glass becomes more workable. As a general rule, the coefficient of expansion indicates the thermal shock resistance of the glass. The lower the expansion, the greater the resistance of the glass to sudden temperature changes.