Float glass is chemically inert: it doesn't corrode, doesn't react with oxygen, and isn't degraded by UV rays in its basic vitreous structure. And yet glass in facades does fail — and when it does, it fails through very specific mechanisms that anyone designing with this material needs to recognise before they even show up, because in most cases the defect isn't visible to the naked eye until it has already happened.
Thermal breakage: the crack that starts at the edge
Thermal breakage is the most insidious failure mechanism in untempered float glass. It happens when one part of a pane heats up much faster than another — typically the centre of the pane, exposed to direct sun, against the edges, shaded inside the frame — generating a temperature difference that produces tensile stresses at the edge of the pane, the structurally weakest point of a glass panel (because of microscopic cutting imperfections, invisible but always present). If this stress exceeds the edge's strength, the pane fractures, often with a single crack that starts perpendicular to the edge and runs across the pane. The typical conditions that trigger thermal breakage are partial shading (an awning, an architectural element, even the pane's own supporting structure casting shadow over part of it while the rest sits in full sun), glass with high solar-absorption coatings (dark Low-E or screen-printed) installed without the correct heat treatment, and climates with wide daily temperature swings. The standard prevention is using tempered or heat-strengthened glass in cases at high risk of thermal gradient, since the heat treatment itself introduces surface compressive stresses that offset the subsequent thermal shock.
Nickel sulphide: the spontaneous shattering of tempered glass
Tempered glass, paradoxically the safety glass par excellence, has a rare but spectacular pathology: spontaneous breakage from nickel sulphide (NiS) inclusions. During glass melting, traces of nickel from contamination in the raw materials or the plant can form microscopic nickel-sulphide crystals scattered through the glass mass. These crystals exist in two crystalline phases with slightly different volumes: the high-temperature phase, which forms during tempering, and the low-temperature phase, toward which the crystal tends to slowly transform over time — with a minimal volumetric expansion that's nonetheless enough, if the crystal sits in the tensile zone at the core of the tempered pane, to trigger a sudden fracture of the entire pane, even months or years after installation, with no external load at all. The Heat Soak Test (EN 14179-1) drastically reduces this risk: tempered panes are heated to around 290°C for eight hours in a dedicated oven, artificially accelerating the transformation of unstable crystals and making defective panes break at the factory before they're installed — a test that doesn't eliminate the risk 100% but reduces it to statistically marginal percentages.
Delamination and interlayer ageing
Laminated glass owes its safety function to the perfect adhesion between the glass panes and the polymeric interlayer (PVB or SGP) that holds them together. Delamination is the loss of this adhesion: bubbles or cloudy areas form between the glass and the interlayer, typically starting from the edges of the pane, where ambient moisture can seep in more easily over time if the perimeter sealing of the glazed unit isn't perfect. Typical causes are manufacturing defects (an autoclave with insufficient temperature or pressure during the laminating stage), prolonged moisture exposure at unprotected edges, or chemical incompatibility between the interlayer and the sealants used in the frame. PVB, in particular, is hygroscopic — it absorbs moisture from the environment — which is why manufacturers specify storage and installation conditions with controlled relative humidity. PVB yellowing, a phenomenon distinct from delamination, is linked to prolonged UV exposure: after fifteen to twenty years of direct exposure, especially in climates with intense solar radiation, the interlayer can develop a perceptible yellowish tint, an aesthetic issue more than a structural one, which SGP (chemically more stable than PVB) tends to show to a lesser degree.
Fogging: when double glazing mists up from the inside
Fogging, or internal misting of the glazing unit, is the most common and least serious pathology of contemporary glazed facades: it shows up as persistent condensation or a cloudy film visible inside the DGU's sealed cavity, impossible to clean from outside because it sits, precisely, in the airtight space between the two panes. The cause is always the same: failure of the perimeter sealant (silicone or polyurethane), which loses its sealing capacity over time — typically lasting twenty to twenty-five years before the risk becomes significant — letting atmospheric moisture seep in and condense on the pane's cold inner surface. There's no repair for a unit with active fogging: the only remedy is full replacement of the glazed unit, a maintenance cost that every continuous-facade project should already plan for at the building's life-cycle planning stage.