WHY DO IMPURITIES IN GLASS CAUSE SPONTANEOUS BREAKAGES?

WHY DO IMPURITIES IN GLASS CAUSE SPONTANEOUS BREAKAGES?

Glass by its very nature is a brittle material. One of the realities that the construction sector has come to accept in recent years, is that glass br

Glass by its very nature is a brittle material. One of the realities that the construction sector has come to accept in recent years, is that glass breakages, particularly spontaneous breakages appear to have become a more common phenomenon. Whether this is as a consequence of the manufacturing processes, source materials, or quality control is undetermined, however, this issue is having a direct effect on the longevity and robustness of the glass produced.

One particular concern relates to impurities in the glass, such as, nickle sulphide (i & ii) and the reason why this is becoming more of an issue is as a result of the widespread use of toughened glass as safety glass, throughout the construction industry.

Toughened glass, also known as tempered glass, is up to five times stronger than regular glass. It is considered a safety glass because when it breaks, it will break into small, blunt pieces under pressure, making it much safer than shards of normal glass and less likely to cause injury.

 

So why is toughened glass more likely to spontaneously break?

Unfortunately, whilst the glass is stronger than untreated glass, it is more rigid and less capable of accommodating the same levels of movement within a pane. Therefore, in instances where there are impurities in the glass and during atmospheric temperature changes, the impurities will expand and contract at a different rate to the glass.

As glass is heated during the toughening process these impurities change state.  The high temperature a-state of the impurities may be frozen when the glass is quenched, and recovery to the low temperature b-state may then take several years.  Spontaneous breakage of the glass may follow, as the low temperature of the nickel sulphide impurities occupies a slightly greater volume and so generates a local stress concentration.

 

How can nickel sulphide breakages be identified?

Nickel sulphide breakage of glass exhibits a characteristic fracture pattern. Assuming that the fractured pane remains in place, at the origin or epicentre of the fracture there should be located two fragments which are larger than the rest and which form a ‘figure of 8’ or ‘butterfly’, image.  In the centre would be located the inclusion which is a small, round, shiny, yellow-black particle which may be as small as 0.05mm in diameter.

Are there any other factors that can cause to spontaneously fracture?

I should also point out that impurities are not the only reason why glass spontaneously fractures and without a laboratory test it can be impossible to determine the true reason why a piece of glass has broken. Other potential reasons include:

  • Minor damage during installation such as nicked or chipped edges.
  • Excessive torque of the glass in the frame, restricting expansion and contraction movement.
  • Internal defects within the glass such as nickel sulphide inclusions
  • Thermal stresses in the glass
  • Inadequate glass thickness to resist wind load

So, what can be done to avoid spontaneous breakages?

We would recommend considering alternative types of glass for any overhead glazing, such as laminated, toughened-laminated glass, or Heat Soaked Tested glass (HST). HST glass is becoming much more common and whilst it’s more expensive than toughened glass, it is far less likely to spontaneously break.

 

How is HST glass made?

The process for producing HST glass involves heating the toughened glass to approximately 280oC, and once at this temperature, it is held for 2 hours. This heating will cause the NiS inclusion to phase change more rapidly and so expand in volume, causing failure of the panel.

As part of the results of the normal float glass process, one large reputable manufacturer has reported that only 1 critical inclusion occurs in every 13 tonnes of glass that they produce, and that 90% of the inclusions will cause premature breakage during their heat soaking process.  This means that only one nickel sulphide inclusion will ‘get through’ for each 8,470 m2 of 6 mm heat soaked toughened glass.

 

Does HST glass still spontaneously break?

Fully Tempered-HST glass, that has been manufactured in accordance with European regulations is practically safe from spontaneous breakage. That means that the remaining breakage risk is minimal, but it is not zero i.e. HST glass does not completely eradicate the chance of Nickle Sulphide contamination and consequential glass breakage (iii).

For a building with 10,000m2 of glass, the remaining breakage risk depends on the thickness of the glass – the thicker the glass, the greater the mass and therefore the more probable a breakage. If we use an average glass thickness of 8mm and fully tempered HST in a curtain wall screen, there will still be an annual remaining breakage risk of 1%. This means that from 100 buildings, each with 10,000m2 of HST glass, only one single spontaneous breakage will occur in the year. On a building with only 100m2 of glass, the remaining spontaneous breakage risk is therefore extremely small.

Untested glass has nearly 100 times the risk of breakage compared to tested glass!

 

So what needs to happen?

The issue of spontaneous breakage is particularly pertinent when glazing at height, or dealing with glazed barriers and the importance of selecting the right type of glass for each application, all the more important. We would strongly recommend that thorough risk assessments are carried out for all glass project specifications,  with consideration of the location of the glass, environmental factors, human traffic, usage of the area and impact requirements. The selection of glass should also consider the manner in which glass breaks and whether the risk of injury posed to people can be reduced, or mitigated by selecting one glass specification over another. For example:

  • Annealed glass: Annealed glass is a non-safety glass and there are not may instances that this type of glass should be used for construction projects. You will typically find annealed glass usage in greenhouses and garden sheds. When annealed glass breaks, it can crack, or break into large, sharp jagged pieces. Glass shards from broken annealed glass are dangerous and must be handled with care.
  • Toughened/Tempered Glass: Toughened glass gets its strength from the tempering process which sees it subjected to intense heating followed by rapid cooling during manufacture. When toughened glass breaks, the entire pane has the potential to shatter into small fragments, centred around the impact zone, although in some circumstances, when a toughened glass pane is included in the outer pane of a vertically installed insulating glass unit, the glass will often remain in place until further forces are applied i.e. additional impacts and/or loads.
  • Heat Soak Tested Glass: Although significantly more robust than toughened glass and far less likely to break, when it breaks, it will display the same breakage characteristics as toughened glass breaks.
  • Laminated glass: When broken holds together as a result of a thin plastic type layer called a Polyvinyl Butyral (PVB) that is sandwiched and adhered between two sheets of annealed glass, during the manufacturing process.

When conducting glass risk assessments, we would recommend referring to CIRIA 2005 Guidance on glazing at height. One of their safety tables is detailed below and it should be used with  the objective of reducing or mitigating the potential to injure someone in the event of glass failure. Glass type selection should be made on the basis of a ‘Tolerable’ classification:

Notes:

  • i) Nickel Sulphide is an inorganic compound with the formula NiS. It is a black solid that is produced by treating nickel(II) salts with hydrogen sulphide.
  • ii) Other impurities include; dust, metal oxides.

References:

  • CIRIA.pdf: Guidance on Glazing at Height
    • page 121 for risk assessment table
    • page 121 guidance note for using the glass risk assessment table
    • page 74 to 75 for Nickel Sulfide Inclusions
    • page 78 for Toughened Glass (paragraph 2)
    • page 79 for Heat-Soaked Toughened Glass (paragraph 2 – reduction in NiS breakages)
    • page 167 recommendation for HST glass to be used for any glass at height.
  • CWCT TN61.pdf: Glass Types
  • CWCT TN62.pdf: Specification of Insulated Glass Units
  • CWCT TN63.pdf: Glass Breakage
  • CWCT TN68.pdf: Overhead Glazing
  • CWCT TN69.pdf: Selection of glass to prevent falls from height
  • Approved Document N: Glazing impact opening cleaning
  • BS EN 356 2000: Glass in building, security glazing testing and classification of resistance against manual attack
  • BS 6262 part 4 2018: Glazing for buildings – Code of practice for safety related to human impact
  • Exkelt Glas GmbH report on the spontaneous breakage of fully tempered glass

 

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