Thermal Resistance
Since borosilicate glass has a high softening point, it can be used at temperatures in the order of 515°C. Caution must be exercised, however, in cooling down apparatus which has been held at high temperatures. The cooling process must be slow and uniform, particularly in the case of thicker sections (see annealing). If a temperature gradient is applied to the glass, stresses are set up - the hotter glass being under compression and the cooler under tension. The magnitude of the tensile stress depends on the temperature difference which in turn depends on the glass thickness.
Caution!
Abrasions greatly reduce the temperature shock resistance.
Maximum Temperatures
In general it is recommended, that the strain point (515°) be regarded as the maximum safe operating temperature of borosilicate glassware 3.3. For certain shapes and for a relatively short time this limit can be exceeded, but at 580° there is danger of deformation and in case of sintered glassware the porous structure may be altered. At high temperatures the glass may acquire permanent stress on cooling and this may result in subsequent breakage.
If it is suspected that permanent stress has occured, the article should be annealed making references to the annealing process explained below. Permanent stress can greatly reduce the mechanical and thermal resistance.
