What is tempered glass and where is it mandatory?

What is tempered glass? In short, it is float glass that has been heated to about 650 °C and cooled rapidly, which makes it 4–5 times stronger than ordinary annealed glass and causes it to break into small, blunt granules rather than sharp shards. Below we explain the process, how it compares with annealed and laminated glass, where it is required and how to recognise it.

How is glass tempered?

Cut to size and edge-finished float glass is heated in a furnace to about 650 °C and then cooled rapidly with air. The surface layers end up in compression and the core in tension. This “stressed” structure makes the glass 4–5 times stronger than annealed glass. At Magna this step takes place in our tempering furnace. In Europe, thermally toughened safety glass is described by the EN 12150 standard.

How does tempered glass differ from annealed glass?

Property	Annealed (float)	Tempered
Bending strength	1×	4–5×
Thermal shock	~40 °C	~200 °C
Breakage	Large sharp shards	Small blunt granules
Cutting afterwards	Possible	Not possible

Tempering does not change how glass looks or how much light it lets through; it changes its strength, its resistance to sudden temperature changes and the way it breaks.

Tempered or laminated glass: what is the difference?

Both are called safety glass, but they protect in different ways. Tempered glass is a single pane that becomes stronger and breaks into harmless granules. Laminated glass consists of two or more panes bonded with a plastic interlayer: when it breaks, the fragments stay stuck to the film and the pane usually remains in its frame.

Criterion	Tempered	Laminated
Construction	One heat-treated pane	Two or more panes with an interlayer
Strength	4–5× annealed glass	Depends on the panes used
After breaking	Falls out as small granules	Cracks but generally stays in place
Typical role	Impact and thermal resistance	Holding the opening closed after breakage

The choice depends on the risk: where a person might walk into the glass, tempered glass is the usual solution; where a broken pane must not fall out — overhead glazing or a balustrade with a drop behind it — building rules in many countries call for laminated glass. Laminated glass is not part of our range, so if your project needs it, confirm the requirement with your designer first.

Where is tempered glass typically required?

Application	Why tempered	Typical thickness
Glass doors and frameless partitions	Impact, frequent use	8–10 mm
Shower enclosures	Wet floor, temperature changes	6–10 mm
Floor-to-ceiling windows, low sills	Risk of walking into the glass	4–6 mm pane in an insulated unit
Schools, nurseries and public buildings	Large numbers of people, children	By design
Balustrades and stair infills	Body impact	By design and local rules
Furniture: table tops, shelves, cabinet doors	Load, knocks, edges at hand height	4–10 mm

A few notes on the list:

• Where privacy is needed in doors and partitions, frosted glass can be tempered too.
• Shower glass has its own requirements for thickness, hardware and measuring — see our guide to choosing shower enclosure glass.
• In windows the tempered pane is usually part of an insulated glass unit; unit formulas mark it with a “T”, for example 4T-16-4.

We temper glass 4–10 mm thick; heavier structural applications fall outside that range.

Why can’t tempered glass be cut or drilled afterwards?

The most common mistake: the customer has the glass tempered first and then asks to “trim it a little” on site. Tempered glass cannot be cut, drilled or polished — once the stressed surface is breached, the whole pane shatters. All dimensions, hole positions and cut-outs must be specified in the order beforehand.

Checklist before tempering

1. Final width × height of every pane in millimetres, measured after finishing works.
2. Holes: diameter and the position of each centre measured from two edges.
3. Cut-outs and notches for hinges, locks and handles, shown on a drawing.
4. Edge finish for every pane.
5. Hardware chosen in advance, with the manufacturer’s drawing to hand.
6. For frosted glass, the side the satin face should be on.

As a general guide, a hole’s diameter should not be smaller than the glass thickness, and its edge should be at least about twice the glass thickness away from the pane edge; holes near corners need even more distance. The hardware manufacturer’s drawing always takes priority.

Which edge finish should you choose?

Before tempering every edge must at least be seamed, because tiny chips on a raw cut edge can cause the pane to break in the furnace. Where the edge stays visible or within reach — frameless doors, shelves, table tops, showers — a polished edge is the better choice: it is smoother to the touch and looks finished. Edges that will be hidden in a frame or an insulated unit generally do not need polishing.

How can you tell if glass is tempered?

• Marking. Manufacturers typically mark tempered panes in a corner; the mark cannot be removed without damaging the surface.
• Polarised light. Through polarised sunglasses, tempered glass often shows faint darker spots or stripes — a trace of the cooling pattern.
• Reflections. Slight waviness in reflected lines may be visible, caused by the rollers in the furnace.
• Edges. A tempered pane never has a raw cut edge, since the edge is always finished before tempering.

None of these signs is conclusive on its own, and never test glass by striking it; the reliable answer is the supplier’s documentation.

How to order tempered glass

At Magna we temper clear, frosted, tinted and reflective glass from 4 to 10 mm; tempered panes can also be assembled into insulated units. Thicknesses and typical uses are listed on our tempered glass product page, and our guide to window glass thickness helps with sizing. Send us the dimensions, holes and cut-outs for each item — our software calculates the cutting plan and we send a quote for tempered glass within one business day.