Is Heat Soak Test Effective? »»»

When glass is heat treated the nickel sulphide inclusions are modified into a form that grows or transforms with time and temperature. If the inclusions are near the thickness center of the glass, the transformation of the inclusions may create the sufficient stresses in the glass to cause a spontaneous breakage. This is more likely to occur in fully tempered glass then heat strengthened glass.

The nickel sulphide inclusions are actually a chemical group having varying ratios of nickel to sulphur. The effect of this ratio is that some of the inclusions transform much more rapidly then others for the same combination of time and temperature.

To reduce the possibility of breakage from nickel sulphide inclusion in the field, accelerated exposures at high temperatures are sometimes conducted. The inclusions that transform at at a rapid rate may fail in the test.

The accelerated exposure or “heat soak” test may then reduce the likelihood of field breakage in some cases. Those inclusions that transform at a slow rate, however, may be transformed to the point of causing imminent failure. In the latter case, the incidence of breakage in the field may be increased from the affects of the “heat Soak” test. Since the manufacture has no means of identifying the nickel to sulphur ratio in the inclusions that may be present, the “heat soak” test cannot be relied upon to result in reduced field breakage of tempered glass.