Curb Spontaneous Breakages with Heat Soaking

Over the last year, we have read about spontaneous glass failures in the field, believed to be associated with nickel sulphide inclusions. I have not heard of this issue for many years. But, with the emergence of global supply sources, I believe architects and project engineers need to be very concerned with this spontaneous glass breakage and should take steps to minimize the risks.


So what is causing these breakage problems, and what is the possible solution?

I do not claim to be a ceramic engineer; however, I will attempt to express my opinion in layman's terms. Please understand I have no direct knowledge of the projects that are having problems, but offer my experience of working with glass for more than 30 years.

The ingredients used in making float glass consist mainly of silica sand (SiO2), in addition to other ingredients of soda ash, limestone, dolomite, salt cake and cullet. Normally, the raw materials are carefully screened and tested for any contaminants. Without proper screening, nickel contaminants are present and combine with sulphur to form nickel sulfide (NiS) inclusions. When glass containing nickel sulfide inclusions is tempered, the risk of breakage increases. Once installed in the field, the glass will normally expand due to solar heating. If the tempered glass has nickel sulfide inclusions, the glass will develop small fractures around the inclusion. Once these fractures expand into the other compression layer, spontaneous glass breakage will occur.

How do we minimize the risk of spontaneous breakage? Heat soaking of the tempered glass is the best possible solution at this time.

Heat soaking is a process in which tempered glass is baked at approximately 280 degrees Celsius in a furnace for a two-hour period of time, and then cooled. During this controlled heating process, expansion of any nickel sulfide inclusions should occur, thus causing the glass to fail in a controlled environment as opposed to in the field. Heat soaking is not 100 percent effective, but it does minimize the risk of failure in the field. Currently, a U.S. standard for heat soaked glass does not exist, but many companies are following the draft of the European standard (EN 14179).

Heat soaked glass is strongly recommended for any tempered glass project in which spontaneous breakage will present a risk to the general public. 

Louis G. Merryman is president of Consolidated Glass Corp., New Castle, Pa.,


I may not have the knowledge of how to make a float glass, but I do believe that they should know the outcomes or affects of having a huge spontaneous glass breakage to the general public.

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It's unfortunate that lower quality glass from a limited number of sources would affect the entire industry.
This action increases costs, lead-times, and still does not alleviate the problem.
Why don't we just hold the questionable suppliers to a higher standard?

I don't think I understand to what Mark Wade is making reference. Which suppliers of tempered glass are questionable? Currently, the cost of tempered glass is a very small premium over annealed glass. Can you imagine what the price would be if each lite of glass had to remain in a furnace for 2 hours during this heat soak cycle? This would price the use of tempered glass right off the market. Is this logical? I don't think that would fly for use in most of the Northwest.

I just read some statistics that say heat soaking only reduces the incidence of spontaneous breakage from 8 in 1000 for regular tempered glass to 5 in 1000. Doesn't seem to be enough of a change to justify the doubling of the cost for the process. Better to go to a tempered laminated product for safety in my opinion.