Safex Tempering process increased the strength of glass many folds, offering a greater resistance to breakage. On rare occasions, solid glass shatters due to the presence of microscopic additions, Nickel-Sulphide. NiS stones, which occasionally and randomly form in glass during primary glass manufacturing, become unstable when the glass is tempered. Over a period, these unstable stones could expand inside the glass due to temperature variation at site, causing the entire glass to break. Therefore the concept of heat-soaking is defined as the transfer of potential on - site breakage to breakage in the factor under a controlled process of accelerating the inversion of the Nickel-Sulphide inclusions, will break during this heat soaking process. Chance of breakage in solid glass is greatly minimised, if the glasses are head soak tested after toughening.