Our in house production factories in St. Austell holds a Toughening Plant Furnace which can toughen glass up to 1700mm. Toughened glass, also known as safety glass, is up to 5 times stronger than glass that has not been through the toughening process. When toughened glass is broken it shatters into small fragments rather than large sharp shards of glass that can cause serious injury. Toughening is a process which annealed glass goes through a thermal tempering process where the glass is heated up to 700 °C and then rapidly cooled ensuring the glass physically and thermally stronger.
Our production facilities also include the ability to heat soak glass. Heat soaking encourages the glass to explode under stress. In rare incidences toughened glass can explode for no apparent reason, this could be due to any impacts that the glass has encountered or that the glass contains nickel sulfide particles. Although nickel sulfide forms naturally during the making of the glass, it is when the glass is toughened that the nickel sulfide can affect the glass. The glass toughening process requires the glass to be heated to just below its softening point and then rapidly cooled. Because this heating and rapid cooling process induces substantial tension in the glass, nickel sulfide stone inclusions in the tension core can cause spontaneous breakage, as they are known to change phase (expand) some time after the toughening process.
To prevent this spontaneous breakage of toughened glass, it is placed in the heat soaking test oven where the glass is slowly heated 2 degrees a minute until it reaches 290 degree which is kept at this temperature for a set period of time. This testing process encourages any impurities in the glass to break and therefore prevent any spontaneous breaks. The glass is then cooled down roughly over two hours, where the complete process generally takes about 8 hours. Here at Cornwall Glass we have an electric heat soak test oven which allows us to test all types of glass.
Heat Soaking does not give 100% certainty as some impurities can sometimes survive the heat soaking process and it is expected to eliminate 98% of toughened glass that may have otherwise sustained a spontaneous break.