Tiny nickel sulfide inclusions (NiS) can cause spontaneous glass breakage

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Video: This large sheet of tempered glass was broken on purpose to demonstrate the appearance of spontaneous glass breakage due to a nickel sulfide inclusion. Spontaneous glass breakage might happen even when nobody is near the glass. Even though tempered safety glass breaks in small pieces, the pieces can remain interlocked with one another and come down in dangerous clumps.

Video: Here are some views of glass sinks, glass stove doors and glass doors breaking. The two clips at the end of the video shows glass doors experiencing actual spontaneous breaks captured by a security camera. The building staff cleaned up the broken glass, so there was no evidence to examine to determine of the spontaneous glass breakage was a result of a nickel sulfide inclusion.
People using tempered glass products such as safety glass in windows and doors, shower enclosures, glass sinks, stove doors or tabletops sometimes find themselves shocked when these glass products shatter spontaneously and unexpectedly, even with nobody in the vicinity. More shockingly, in some cases the glass was totally destroyed by a tiny speck they can barely see. This speck might be a nickel sulfide inclusion.

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Spontaneous tempered glass breakage results from the convergence of two separate phenomenon:

1. Permanent stress in tempered glass
Tempered glass is heated and rapidly cooled to create a permanent equilibrium of tension/compression between layers of the glass. This internal tug of war is at least 10,000 pounds per square inch (psi). The tempering process turns glass into “safety glass” because when it breaks, it breaks in small, less-dangerous cubes.
2. Nickel sulfide inclusion in the glass

Nickel sulfide inclusion that caused spontaneous glass breakage
Nickel sulfide inclusion, which was the cause of spontaneous glass breakage in a tempered glass sliding door. The diameter was measured at .113 mm = 113 ?m = .004 in

The nickel sulfide inclusion, a tiny metallic ball in the glass, actually grows over time. If it is positioned in the center 60% of the glass, known as the “tension layer”, the growth can exert enough localized pressure to disrupt the tension/compression equilibrium and shatter the glass. This spontaneous glass breakage happens all by itself, however sometimes a hapless person, like the woman in the video above, provides just enough additional force to tip the scale and break the glass. This force might be the simple opening or closing of a shower door, the cleaning of the glass, or the grasping of a handle.
How can you tell the glass broke from a nickel sulfide inclusion?
If all of the glass has fallen into a pile of rubble, it won’t be easy. But if the broken glass cubes stayed in the opening, the overall breakage pattern can be seen. Look closely at the focal point of the break.
Butterfly pattern

Focal point of spontaneous tempered glass break. Butterfly pattern in glass pieces. Nickel sulfide inclusion may be found on the border between the two pieces
At the focal point of the spontaneous glass breakage, sometimes a butterfly pattern can be found. If the break was caused by a nickel sulfide inclusion, it will be found on the border between the two polygons

There is a good chance you will see a pair of polygons bordering one another at the focus of the spontaneous glass break. The pair of polygons is called the “butterfly pattern”. If the polygons are nearly identical in size, a nickel sulfide inclusion might be the culprit. Look closely at the border between the two polygons with moderate magnification. If you see a tiny spec on the border between the polygons, that is probably the nickel sulfide inclusion.
Where do nickel sulfide inclusions come from?
Nickel sulfide inclusions are made of nickel and sulfur. These elements are undesired impurities that occur in the glass melt in the float glass plant where glass is made. In the float glass plant, tons of powdery ingredients are fed into a furnace and melted into glass. Nickel is a component of stainless steel, and this is present in the glass-making process in nozzles, mixers and other furnace components. Nickel is also a component of nichrome heating elements. If combustion fuels are used to melt the glass, sulfur can be found in the fuel or gasses.
With trace elements of nickel and sulfur floating in the mix, they team up to make a beautiful little ball that is full of trouble.
How big is the nickel sulfide inclusion?

Small NiS Inclusion (dia)Large NiS Inclusion (dia)
Millimeters (mm).05 mm.1 mm
Micrometres (μm)50 μm100 μm
Inches (in).002 in.004 in

1 mm = 1000 μm = .0394 in
1 in = 25.4 mm = 25400 μm
μ = Greek letter pronounced “Mu”

Composite photo showing relative sizes of two nickel sulfide inclusions next to a grain of salt.
This composite photo shows relative sizes of two nickel sulfide inclusions at left next to an average-sized grain of table salt on the right. The upper NiS inclusion measured .076 mm and the lower NiS inclusion measured .113 mm in diameter, making it what some Wisconsinites might call a “lunker”

How often do nickel sulfide inclusions occur?
If your building is plagued by NiS inclusions, you will say “way too many”, but in actuality, the incidence of nickel sulfide inclusions is quite low. Here are some estimates given by various sources:

  • High estimate: One 5/1000 gram inclusion per 1 tonne of glass (1 tonne = 1012 kg = 2204.6 lbs)
  • Low estimate: One 5/1000 gram inclusion per 8 tonnes of glass

But the presence of nickel sulfide inclusions does not necessarily equate to spontaneous breaks. Here are some estimates regarding breakage:

  • One inclusion breakage per 450 tonnes of tempered glass
  • One inclusion breakage per 8.7 tonnes of tempered glass, found in heat soak testing
  • One inclusion break per 4-12 tonnes of glass (not just tempered glass)
Tonnes of silica sand and other ingredients are fed into the float glass furnace
Tonnes of silica sand and other ingredients are fed into the float glass furnace.

Estimates vary widely, but for very good reason. Not all glass is the same. Spontaneous glass breakage due to nickel sulfide inclusions tend to come in batches, and not all batches are the same. Some carloads of source material are less pure than others. Some float glass plants are less controlled than others. Some plants utilize scanning equipment to remove infected glass before it goes out into the market. Some float glass plant use electrified lances that attract metallic impurities while the glass is still molten. Some vary their ingredients mix to reduce NiS. So the variation in the incidence of breakage can be great from one project to the next.
When will the breakage occur?
The growth of the nickel sulfide inclusion is associated with temperature cycling it experiences when in the field. Window glass in the northern hemisphere tends to break first on the south side, then on the west and east side, then finally on the north side. First breaks might occur in the first one or two years of service and can continue for 10 years. I personally know of buildings that suffer an occasional break in the 7-10 year timeframe. One especially infected building was still having spontaneous breaks over 15 years after installation.
Why does the nickel sulfide inclusion grow?

Nickel sulfide inclusion that caused spontaneous glass breakage
Nickel sulfide inclusion that caused a spontaneous glass break

NiS growth is associated with a phase change. Water is a useful example. When water does a phase change from liquid to solid (ice) it expands, as anyone who left a glass water bottle in the freezer will testify. Nickel sulfide does something similar, but here are the differences:

    • The nickel sulfide inclusion phase change temperature is 379 °C (714 °F)
    • The nickel sulfide inclusion is actually a solid above and below the phase change temperature, it just changes size when that temperature is crossed
    • The high temperature phase – the alpha state – is crossed into when the glass is heated up to 650 °C (1202 °F) during tempering, so the inclusion shrinks
Nickel sulfide inclusion and the glass fracture pattern that emanates from it
Nickel sulfide inclusion (top left) and the glass fracture pattern that emanates from it when catastrophic breakage occurs.
  • When the glass is rapidly cooled during the quenching stage, the denser nickel sulfide inclusion is unable to cool as rapidly as the surrounding glass. The glass hardens, entrapping the nickel sulfide inclusion in its alpha state dimension even though it soon cools and “wants” to expand to its beta state.
  • The nickel sulfide inclusion exerts intense localized pressure where it meets the glass. Some have estimated this pressure at 125,000 pounds per square inch. But the area involved is a tiny fraction of a square inch, so the localized force is accordingly less. Let’s take the two NiS inclusions found in the picture above with the grain of salt. At 125,000 psi, the small inclusion would be exerting a total of 14 lbs outward on all of its surface and the “lunker” will exert 31 lbs.
  • The localized pressure creates a “vent” (a microscopic flaw) which eventually turns into a flaw. The stage is set for a catastrophic failure. It is now just a matter of time.

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What should I do if I have a spontaneous glass break?

  1. Take good pictures immediately. Include a wide shot showing the full piece of glass from both sides of possible. If the focal point of the break is still intact, zoom in and get lots of good photos.
  2. Record the date, time and outside temperature when the break occurred.
  3. If the glass is still remaining in the opening, stabilize it with carpet protection film available at Home Depot, or with lots of duct tape.
  4. If you want to keep the origin for evidence analysis, place clear tape or carpet protection film on a 3″ x 3″ area surrounding the focal point. Use good duct tape to tape a border around that area, then chip away the border with a screwdriver and hammer to free up the focal point and the tape.
  5. Package the sample well and send it to me. I will photograph the glass under a microscope and issue a report for a nominal fee.
  6. Even if this is the first spontaneous break experienced in your building, start keeping track. If it turns into a repeated problem, you will have lots of information to work with in seeking redress.

Nickel Sulfide Inclusion – A World Inside Glass

R.C. Ropp, Handbook of Glass Fractography, AuthorHouse 2008
Dr. Leon Jacobs, A Review of the Nickel Sulphide Induced Fracture in Tempered Glass, Glass Processing Days, June 2001
Dr. John Barry, The Achille Heel of a Wonderful Material: Toughened Glass
Ballantyne, E.R., C.S.i.R.O. Division of Building Research, Melbourne Australia Report No. 061-5, 1961
Wasylak, Reben, Bielecki, Behaviour of Nickel Sulphide Inclusions in Glass Melts, AGH-University of Science and Technology, Faculty of Materials Science and Ceramics, Kraków, Poland, 2010
Dr. Andreas Kasper, Saint-Gobain Glass Deutschland, Herzogenrath, Germany, Fundamentals of Spontaneous Breakage Mechanism Caused by Nickel Sul?de
Napier & Blakeley. (September 8, 2008). “The Shattering Truth About Glass.” Napier & Blakeley
Nickel sulfide, Wikipedia

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2 thoughts on “Nickel Sulfide Inclusion and Spontaneous Glass Breakage”

  1. mark
    one of the most missed facts on NiS is that the inclusions are allowed in the raw glass (ASTM 1036 ) and pose no risk or hazard. it is not until the glass is tempered that the risk appears. where does that leave the fault ?

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