Toughened glass, also called tempered glass, starts as ordinary float glass. What makes it stronger is not an ingredient but a process: heat followed by very fast cooling.
How it is made
- Cut. The glass is cut to the final size.
- Edge worked and drilled. Edges are ground or polished, and holes and notches are made now. This is the last chance to change the shape of the glass.
- Heated. The panel travels through a furnace at around 620 to 700 degrees Celsius.
- Cooled quickly. Jets of air cool both faces in seconds.
Why it is stronger
The outside of the glass sets first. When the middle cools, it shrinks and pulls on the hard outer layers, locking them in compression. A crack has to get through that compression before it can grow. Toughened glass is commonly quoted as four to five times stronger than ordinary glass of the same thickness.
How it breaks
Hit hard enough, the whole panel breaks at once into small, blunter pieces instead of long shards. That is what makes it a safety glass. It also means the panel does not stay in the opening. If the glass is overhead or holding back a drop, ask about laminated glass as well.
Where UK rules ask for safety glass
Approved Document K covers glazing in critical locations in England. These include glass in or next to doors, and low-level glazing in walls and partitions. Your builder or building control can confirm what applies to your project.
The product standard for toughened glass is BS EN 12150. For glass that has to meet a standard, ask us for the documents for your glass type.
What to watch for
- No cutting afterwards. Trying to cut or drill it will shatter the panel, so sizes and hole positions must be right first.
- Protect the edges. Edges are the most vulnerable part. Do not stand a panel directly on a hard floor, and keep metal fixings from touching the glass.
- Spontaneous breakage. Very rarely, a tiny impurity can break a panel without warning. A heat soak test reduces this risk. Tell us if your project asks for it.
Edge finishes
All edge work happens before the glass goes into the furnace. The usual choices are:
- Arrised edge. The sharp corners are taken off with a quick grind. Safe to handle, and fine where the edge will be hidden in a frame.
- Flat ground edge. The edge is ground flat with a matt finish. A tidy look for edges that show a little.
- Flat polished edge. Ground and then polished clear. The choice for shelves, table tops, splashbacks and frameless screens where the edge is on show.
Toughened or heat strengthened
Heat strengthened glass goes through a similar furnace but is cooled more slowly. It ends up roughly twice as strong as ordinary glass, not four to five times, and breaks into larger pieces that tend to stay in a frame. On its own it is not a safety glass. It is mostly used as panes inside laminated glass, where the larger pieces help the broken panel keep its shape. Heat strengthened glass is made to BS EN 1863.
What toughened glass looks like
Toughened glass is as clear as the glass it was made from, but the process can leave small visual marks that are normal and allowed by the standard:
- Roller wave. A slight ripple from the rollers in the furnace, most noticeable in reflections on large panels.
- Quench marks. Faint patches or stripes from the cooling air, seen at some angles or through polarised sunglasses.
- Slight bow. Large thin panels can have a small overall curve.
If appearance matters, for example in a large frameless screen or a mirror, tell us when you ask for a price so we can advise on thickness and size.
Heat soak testing
Very rarely, a toughened panel breaks on its own, sometimes years after it was fitted. The usual cause is a tiny nickel sulphide stone in the raw glass that slowly grows. A heat soak test holds toughened panels at a high temperature for several hours so that most panels with this flaw break in the oven instead of on site. The test is described in BS EN 14179. It is often asked for on glass that is overhead, at height or hard to replace, such as balustrades, facades and roof glazing.
How thick can toughened glass be?
Toughened glass is made in a range of thicknesses, from thin panes for furniture to thick panes for balustrades and floors. Tell us what the glass is for and we will advise on the thickness.
Can toughened glass go in an oven or fire?
Toughened glass handles heat better than ordinary glass and is used behind hobs as a splashback, but it is not fire-resistant glass. Fire-rated glazing is a separate product.
Why did my toughened glass break for no reason?
Most "no reason" breaks start from a chip on the edge or a fixing touching the glass. A smaller number are caused by nickel sulphide inclusions, which heat soak testing reduces.
Have your sizes ready? Ask for a price on toughened glass, or read toughened or laminated: which do I need?