Small Defects, Big Breaks

Nickel sulfide (NiS) inclusions are a hidden yet significant cause of spontaneous glass breakage in buildings with tempered glazing systems. These inclusions are microscopic impurities that form during the glass manufacturing process, when trace amounts of nickel and sulfur combine in the molten batch. Contamination may come from raw materials, furnace components, or fuel. Once trapped inside the glass, the accidental inclusions become potential weak points that may not reveal themselves until long after installation, making them both a safety hazard and a source of complexity in forensic investigations.

In an article published in the March/April issue of IIBEC Interface, Exponent's Misael Rojas, AIA, RRO, NCARB, explores how nickel sulfide inclusions affect building glass. 

Mr. Rojas states the danger of NiS inclusions arises from their behavior during the tempering process. When glass is heated and rapidly cooled, the inclusions enter a high‑temperature crystalline phase called alpha‑nickel sulfide. Ideally, slow cooling would allow it to transform into the beta phase, which is slightly larger in volume. Because tempering traps the inclusion mid‑transformation, it remains unstable. Over time, the inclusions gradually complete the shift to the beta form, expanding within the rigid glass matrix. In tempered glass — where the surface is in compression and the core in tension — this tiny expansion can raise localized stress enough to initiate cracking and in come cases, sudden shattering.

In his article, Mr. Rojas explores ways to reduce NiS-related hazards in building glass. These include using emerging nondestructive evaluation methods  —  ultrasound, laser imaging, and X‑ray diffraction  —  to more accurately detect inclusions before glazing is installed. New glass compositions, including NiS‑resistant borosilicate options, could also offer further protection. 

Read more about how this increasingly common glass defect is affecting architecture and look for the second installment in the series in an upcoming issue of IIBEC Interface.

From the publication: "While NiS inclusions are rare, occurring in roughly 1 out of every 1,100 tonnes (1,200 tons) of raw glass, they pose a disproportionately high risk."