Tempered glass can break with nobody touching it. The National Glass Association says unstable inclusions trapped during tempering, most notably nickel sulfide, can break a panel if their form changes after installation, and that edge damage can negate the entire benefit of tempering. Accurate Glass & Mirror, a Bergen County NJ glass shop, cuts and edges its shower glass in Lodi before it is tempered.
The first documented cause of a tempered door breaking on its own is inside the glass. The National Glass Association's technical paper on heat soak testing says fully tempered glass "may infrequently spontaneously break" for a variety of reasons, including imperfections or inclusions too small to see. Tempering heats the glass and cools it rapidly. Most inclusions are stable, but the paper explains that during that rapid quench some unstable inclusions, most notably nickel sulfide, can be trapped in the tension region, and can cause spontaneous breakage if their physical form changes with heating and cooling once installed.
Nobody can pick those panels out in advance. The Glass and Glazing Federation, a UK trade association that uses the British term toughened glass, calls unexpected fracture a remote possibility, says the inclusions responsible are so small it is currently impossible to detect them in production, and says there is currently no method to calculate the likelihood. Any figure for the odds of a particular door breaking is a guess.
The second cause starts at the edge. The National Glass Association says "glass strength is significantly reduced by glass edge damage or poor edge quality," which on heat-treated glass can offset or negate the entire benefit of heat treating, so care must be taken through fabrication and installation to keep the glass free of damage. One shower door maker's guidelines add that tempered glass can break if handled with unequal pressure or if an unprotected edge strikes a solid surface. The tempered safety glass answer explains why edges and corners are the weak point and how to read the stamp.
Heat soak testing is aimed at nickel sulfide. The association describes it as a batch process in which fully tempered glass is tested in an oven some time after tempering, to make nickel sulfide inclusions break the glass during the test rather than in the field. It applies only to fully tempered glass, it does not change the look or performance of glass that passes, and it adds processing time. The association says there is currently no North American standard for heat soak testing. Whether a tempering plant offers it is a question a shop can put to its plant.
A tempered door that fails breaks across the whole panel rather than along one crack. The National Glass Association says that "when broken by impact, fully tempered glass breaks into small particles, thereby reducing the potential for serious personal injury," and the Glass and Glazing Federation notes that whenever toughened glass fractures, for any reason, it has the potential to fall from the pane. Pieces still held in the hinges or clamps are part of that same broken lite.
There is no repair for a shattered tempered door; new glass is the only path, and the replacement answer lays out the scope, including whether the existing hardware can carry new glass. The replacement starts with a measure of the opening, then the glass is cut and edged in the Lodi shop, with every hole and notch made before it is sent out for tempering.
No. The National Glass Association says a heat soak test "will reduce the risk but will not completely eliminate the potential" of a spontaneous break from a nickel sulfide inclusion, and the UK Glass and Glazing Federation likewise says heat soaking cannot rule it out. The test applies only to fully tempered glass, and whether it is offered is a question for the tempering plant.
The fixed panel is a separate piece of tempered glass, and it is judged on its own edges. The National Glass Association says edge damage can negate the entire benefit of tempering, so the fixed panel's edges, clamps and anchors are checked before a new door hangs beside it, and a panel chipped at an edge is the case for replacing the pair.
Text Jessica photos of the empty opening, the hinges and the edges of the fixed panel for a fast quote from photos. The free in-home measure follows, and the fixed panel gets looked over at the same visit.
