The short answer is no: fully tempered glass should not be cut, drilled, notched or edge-worked after tempering. All fabrication must be completed before the glass enters the tempering furnace. Attempting to alter a finished tempered panel normally releases the internal stress pattern and can cause the entire sheet to fragment.

Tempered glass and toughened glass are two names for the same type of heat-treated safety glass. “Tempered” is common in North America, while “toughened” is widely used in Europe and other export markets.

Why tempered glass cannot be cut afterwards

During thermal tempering, the glass is heated close to its softening temperature and then cooled rapidly with controlled air. The surfaces enter compression while the interior remains in tension. This stress balance gives tempered glass higher resistance to mechanical and thermal loads than ordinary annealed glass and produces its characteristic fragmentation behaviour.

Cutting a finished edge, drilling a hole or grinding deeply into the surface disturbs that stress balance. The panel may break immediately, during fabrication, in transport or later after installation. For this reason, post-tempering alteration is not treated as a normal production process.

What must be completed before tempering?

What if an existing tempered panel must be resized?

Do not plan to resize it as if it were annealed glass. First verify whether the panel is genuinely tempered by checking permanent markings, production documents or a suitable stress inspection method. If it is tempered and the dimensions are unsuitable, record the existing make-up and order a correctly fabricated replacement.

Where future site adjustment is unavoidable, discuss the requirement before specifying the glass. Depending on the application, an annealed or heat-strengthened component, a laminated construction or a redesigned framing detail may be more appropriate. Product selection must still comply with the applicable safety and structural requirements.

Can toughened glass break on its own?

Tempered glass can occasionally break without an obvious impact at the time of failure. Possible contributors include damaged edges, installation stress, contact with metal or masonry, excessive point loading, severe thermal gradients and, more rarely, nickel sulphide inclusions. A breakage investigation should consider the fracture origin, edge condition, support system, clearances and recent thermal or mechanical events.

Heat-soak testing can be specified for selected projects to reduce the risk associated with certain inclusions, but it cannot eliminate every possible cause of breakage. Good edge quality, appropriate clearances, correct setting blocks, compatible hardware and careful installation remain essential.

Information to provide when ordering tempered glass

• Finished width, height, thickness and quantity.
• Glass type, colour, coating and coating-surface position.
• Dimensioned drawings for every hole, notch and cut-out.
• Required edge finish and identification of exposed edges.
• Application, support condition, design load and applicable standard.
• Whether heat-soak testing, ceramic printing or additional processing is required.
• Packing method, delivery destination and installation sequence.

Approved fabrication drawings are especially important for doors, façades, balustrades, staircase glass and panels with point-fixed hardware. Changes made after production normally require a new panel, so resolving details before tempering prevents delay and avoids unnecessary cost.