SOAKING IT IN - Why Heat Soak Test

Heat Soak Testing, the contentious glass industry topic.?Is it useful? Why are Nickel Sulfide (NiS) inclusions even a problem??Let us explore the why’s.?

First, what is spontaneous breakage and what do NiS inclusions have to do with it?

Spontaneous breakage of tempered glass is the result of Nickel Sulfide (NiS) expansion. NiS inclusions are small particles that are formed in the manufacturing process of glass from the firing agent and surrounding metallic surfaces.

An estimated 1m2 for every 10,000 m2 (10 ft2 / 100,000 ft2) of glass produced contains an inclusion sizable enough to cause failure. (GGF)

These inclusions undergo contraction in the tempering process resulting in an unstable, smaller structure. Over time, the NiS inclusions will expand back to their original state, exerting immense force on the glass and eventually causing breakage.

Next, why is this an issue?

In 2016, the estimated worldwide production of glass totaled 9.2B m2 (91B ft2). (Glass Canada, 2018, Report on GPD 2018)

Using the 1/10,000 m2 estimation, this production volume results in nearly 1M m2 (10M ft2) of glass that potentially contains problematic NiS inclusions.

Granted, many lites are not tempered and NiS is NOT guaranteed to cause breakage, but this illustration is meant to show the pervasiveness of NiS worldwide.

This might not seem like a lot of glass, but it becomes increasingly important for high-quality and jumbo-sized glass that requires tempering for structural reasons.?Coupled with the fact this glass is often large, there is a far greater potential for NiS breakage.

ABOVE: The tell tale sign of NiS breakage: The "Butterfly"

Why Heat Soak Test (HST)?

The HST process allows NiS inclusions that may be in the tempered glass to expand back to their original size in the safety of a chamber. AGNORA uses the newest ISO 20657 standard (replacing EN14179-1:2005) which aims to react all forms (there are many) of NiS within a tempered lite, within the safety of AGNORA’s facility.?

What is the expense of Heat Soak Testing?

Once glass is installed, the cost of repair and replacement of a unit that spontaneously breaks is dramatically more than the cost of the HST process itself.? NiS breakage is not covered under a typical warranty. Additional fabrication and labour costs associated with the removal and re-installation of the problem unit will largely outweigh the small price of HST.

Application and glass type are also important considerations.?In projects involving large lites, custom IGUs, or laminated glass which have tempered elements HST makes sense.?The cost of performing HST is relatively low and factors into roughly?3%?of the overall price of the unit, dependent on size and the overall volume of the order.

The Real World Example

NiS induced breakages got so prevalent in Toronto Ontario, Canada that the Ontario Building Code updated its requirements in 2012 to address the continuous shower of glass fragments falling from newly built condo towers.?

The growing use and popularity of glass only balcony enclosures were the driving issue.?

Now all glass panels 4” on either side of the slab edge must be heat strengthened laminates.?From 4” to 6” back of the slab heat strength laminated OR fully tempered WITH heat soak testing, and anything further than that, simply tempered.

You may notice that heat strengthened glass does not require HST.?That is because annealed glass, and heat strengthened glass are not prone to NiS breakage. However, due to load requirements (in Toronto’s codes) the glass must be thickened to achieve the same PSF value as the fully tempered piece, thus requiring lamination.?

The last note, ISO and EN standards

Currently ISO 20657 and EN?14179-1:2016 is the industry benchmark for HST, an evolution of EN14179-1:2005.?Both standards reduce the temperature to below 260°C to encourage all manner of NiS to break.??