Tempering: Why Glass Becomes Safe

Glass seems like a fragile material — and it is, when we are talking about ordinary annealed glass. But after the tempering process it turns into a completely different material: 4-5 times stronger in bending, resistant to temperature swings and, most importantly, safe when it breaks.

What Happens During Tempering

Tempering is a heat treatment process in which the glass is heated to about 620-650 C and then rapidly and evenly cooled by a stream of air. The process takes mere seconds.

The key point is creating a difference in stress between the surface layer and the interior of the sheet. The surface of the glass cools faster and "sets" in a state of compression, while the inner layer remains in a state of tension. It is precisely this surface compression that makes the glass incredibly strong.

The Numbers

The difference between ordinary and tempered glass in figures: - Bending strength: 40-45 MPa for ordinary glass versus 120-150 MPa for tempered — a 3-4x difference - Impact resistance: tempered glass withstands the impact of a 227 g steel ball dropped from a height of 2 m - Temperature range: ordinary glass cracks at a 40-50 C swing, tempered glass withstands up to 200-250 C

Safe Breakage

When ordinary glass breaks, it forms large sharp shards — and it is those shards that cause serious cuts. Tempered glass disintegrates into small granules with blunt edges of about 10-15 mm. This effect is called a "safe breakage pattern".

That is why tempered glass is mandatory in: - Shower partitions — an accidental impact will not cause injury - Balcony railings — wind loads and accidental impacts are much higher there - Door glazing — door glass is broken most often - Staircase railings — safety requirements are the strictest

The Limitations of Tempering

Tempered glass cannot be cut, drilled or worked after tempering — any attempt will disturb the stress balance and the glass will shatter. All holes, cutouts and chamfers must be made before tempering. This is important to take into account at the design stage.

Tempering can also cause slight surface waviness (roll wave distortion) — a consequence of the glass moving on the furnace rollers. In most cases it is invisible, but for critical optical applications it is worth bearing in mind.

Quality Control

Every sheet of tempered glass must be tested for compliance with the EN 12150 standard. The main test is the fragment check: the glass is broken with a controlled strike, and the fragments must meet the requirements for size and count. At Nova Glass we work only with tempered glass from certified manufacturers, and every product undergoes incoming inspection.