Why Do We Heat Soak Test?
What is the principal reason for heat soak testing and why do the team at Carey Glass refuse to rely on batch testing of our tempered glass? Mike Carew, North American Sales Director explains.
Reason for Heat Soaking
To minimise the risk of spontaneous tempered glass breakage due to nickel sulphide inclusions within the glass installed in a building.
What is Nickel Sulphide (NiS) and how does it get into the glass?
NiS is a source of metallic nickel found to be largely from steel used in handling the processing of the float batch material(when the glass was being created from sand and other materials). Within the glass it is an inclusion which is barely visible to the human eye and can only generally be seen after the glass has broken (within a laminate panel makeup).
How does something so small break my big glass panel?
Nis only effects Tempered glass (not annealed). This is due to the tempering process of glass. During tempering the glass is heated to 1148F followed by rapid cooling. If there is NiS in the glass it actually shrinks at the high temperature but the cooling phase of tempering is too fast to allow the NiS to revert back to its normal state and ”freezes” the NiS in an unstable state.
Over time the NiS reverts back or expands back to its stable state creating huge stress, without any load or at any time the glass can break. Tempered glass breaks at 3000mph due to the huge internal stresses of 10,000 lbs per square inch.
What is Heat Soak Testing
Heat soak testing is a destructive test. Tempered Glass is put into a heat Soak Oven and brought to and held at a temperature of 555 degrees Farenheit for two hours. Most glass containing Nickel Sulphide will shatter during this process and thus be eliminated from the glass project.
It has been concluded that the incident of critical NiS inclusions is, on average:
1 inclusion in 11,000lbs of glass for ordinary tempered glass EN12150 (nearest equivalent ASTM C1048)
1 inclusion in 882,000lbs of glass for heat soak tempered glass to EN 14179.
Over the years, Carey Glass has recognised the demand by the architectural community to heat soak test glass to ensure a quality glass product. For this reason Carey presently have 4no. Heat soak ovens with another oven purchased and being installed by end of 2015/early 2016.
As an extra quality assurance at Carey Glass we also use a UV pen that we mark all glass destined for the heat soak process. On the corner of every unit beside the heat soak stamp the letters “CG” is hand written. During the heat soak process this mark is baked into the glass. It leaves a permanent invisible mark on the glass that can only be viewed under a black light.