Manufacturing and Installation issues regarding toughened glassWed, 21 Sep 2016
Exploding glass is a phenomenon by which toughened glass (or tempered) may spontaneously break (or explode) without any apparent reason. The most common causes are:
- Internal defects within the glass such as nickel sulfide inclusions
- Minor damage during installation such as nicked or chipped edges later developing into larger breaks normally radiating from point of defect.
- Binding of the glass in the frame, causing stresses to develop as the glass expands and contracts due to thermal changes or deflects due to wind
- Thermal stresses in the glass
- Inadequate glass thickness to resist wind load
Strata Managers, Facility Managers, Insurers and the general public should be aware that toughened glass has the potential to explode for a variety of reasons including poor manufacturing AND incorrect installation. Fitting panels too tightly, or knocking or putting too much pressure on corners, which are a pane’s weakest point, can result in toughened glass exploding.
Causes of exploding glass:
Incorrect Installation: While glass is being moved and installed, it is easy for the glaziers to nick or chip the edges of the glass with various tools. These small nicks or chips may not result in immediate breakage. However, over time, as the glass expands and contracts, stress concentrations can develop around the nick, leading to breakage. In the case of tempered glass, the entire unit usually breaks.
Binding in the frame: Glass expands and contracts with changes in temperature, so almost all modern glass is set on resilient blocks at the bottom and with space for expansion at the sides and top. If no space is provided at the perimeter of the unit, the glass will bind against the frame, causing internal stresses to develop in the glass which can exceed the strength of glass, resulting in breakage.
Manufacturing faults: Nickel sulphide inclusions can be present in the glass. The most common cause of these inclusions is the use of stainless-steel machinery in the glassmaking and handling process. Small shavings of stainless steel containing nickel change structure over time and grow, creating internal stresses in the glass. This type of breakage is almost always found in tempered glass.
What is nickel sulphide? Sodium sulphate is added during float glass manufacture to promote bubble removal from the molten glass during the melting process. When combined with nickel contamination, sodium sulphate forms nickel sulphide (NiS). Nickel contamination can be caused by: an impurity in the raw materials; contamination during the storage and handling of raw materials; or contamination from the float line equipment, e.g. firebricks and burners.
Improving performance: Heat Soaking Toughened Glass
Heat soaking is a test process that attempts to eliminate nickel sulphide inclusions, which can cause spontaneous fragmentation in toughened (tempered) glass.
What effect does nickel sulphide have on glass? During the manufacture of float (annealed)glass, the raw glass materials are heated to around 1100°C and the nickel sulphide consequently reduces in size. When the glass is slowly cooled during the annealing process, the nickel sulphide expands back to its original size. This expansion does not interfere with the properties of the glass. However, an issue arises if the glass is toughened. Glass is heated to around 600°C during toughening, and the nickel sulphide consequently decreases in volume. To create toughened safety glass, stress and tension is induced in the hot glass by rapidly cooling it. Unlike the slow cooling of annealed glass, this rapid cooling arrests the transformation of the nickel sulphide. The nickel sulphide will expand to its original size over time. If this expansion occurs in the area of the toughened glass that is under tension, it will cause the glass to fragment.
The presence of nickel sulphide is quite rare: Approximately one ‘stone’ of nickel sulphide is present per 8 tonnes of raw glass (although it can come in batches). The incidence of nickel sulphide varies from manufacturer to manufacturer with estimates ranging from one stone per 8 tonnes of glass to one stone per 13 tonnes of glass and some suppliers having a more frequent incidence than this. Nickel sulphide can cause glass to fragment at any time in the product’s life – from a few moments after thermal treatment to years after glazing installation. Nickel sulphide can also affect some types of heat strengthened glass.
Essentially, heat soaking artificially ages the glass and although not 100% effective, will significantly minimise the risk of glass exploding at a later stage. This process is an ‘extra’ and can be completed by a glass manufacturer upon request. Due to this extra processing, Heat Soaked panels are significantly more expensive.
Solutions: Using Glass that meets Australian Standards and Accredited Glaziers
To ensure toughened glass fulfills its residential or commercial objectives long-term, it’s important that those choosing to buy and install toughened glass do so knowing their glass meets Australian standards. Make sure your glass has the logo on it.
Australian Standard 2208:1996 - Imported glass, may or may not meet Australian Standards, and is increasingly being used as it is often a cheaper alternative to toughened glass made in Australia. For this reason, manufacturers may not be as stringent about quality as local Australian producers. Local and international glass manufacturers approved to use the Australian Standards stamp are subject to random inspections to ensure compliance – the cases of exploding toughened glass are reason enough for increased inspections of manufacturing processes, both locally and internationally.
Toughened glass should meet AS/NZS 2208:1996 that covers safety and functional properties of glass and glazing materials in buildings – including toughened glass.
In order to prevent installation faults causing a toughened glass incident, it is advisable to always use an accredited glazier for glass installation. A list of accredited glaziers can be found on the Glass and Glazing Association of Australia’s website.
Summary:Essentially, heat soaking artificially ages toughened glass and although not 100% effective, will significantly minimise the risk of glass exploding at a later stage. This process is an ‘extra’ and can be completed by a glass manufacturer upon request. Due to this extra processing, Heat Soaked panels are significantly more expensive.
The chemistry behind toughened or tempered glass ensures it crumbles or explodes into tiny pieces, as opposed to breaking into larger, sharper pieces. This ultimately keeps users safe. Australian businesses and consumers must place their trust glass industry professionals.