# Heat Soak Testing: Reducing Spontaneous Glass Breakage

> Heat soak testing holds toughened glass at an elevated temperature for a set period after toughening, so panes containing nickel sulphide inclusions break in the factory instead of on the building. It reduces, though does not eliminate, the risk of toughened glass shattering spontaneously in service.

## Why it matters

Spontaneous breakage is rare, but on a tall facade or above a lobby a single
unexpected failure is a safety incident and a costly replacement at height. Heat
soaking moves most of that risk into the factory.

## How it works in practice

| Stage | What happens |
| --- | --- |
| Toughening | Glass heated and rapidly cooled |
| Heat soak | Held at elevated temperature for a defined time |
| Screening | Panes with harmful inclusions break in the oven |
| Result | Lower in-service spontaneous breakage risk |

## Common mistakes

- **Not specifying it at all** on high-risk locations.
- **Assuming it eliminates risk.** It reduces risk; it does not remove it.
- **No record of treatment.** Ask for evidence that panes were heat soaked.

## Standards referenced

Heat soaked toughened glass under **EN 14179**. Safety glass under **IS 2553
(Part 1)**.

## Frequently asked questions

### Why does toughened glass break on its own?

Tiny nickel sulphide inclusions from manufacturing can slowly change phase and expand after toughening. In the highly stressed core of toughened glass, that expansion can shatter the pane without any impact, sometimes years after installation.

### When should heat soaking be specified?

Wherever spontaneous breakage would be dangerous or very expensive to put right — facades at height, spider glazing, overhead glazing and glass above occupied areas.

## Sources

- [EN 14179](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Heat soaked thermally toughened soda lime silicate safety glass
- [IS 2553 (Part 1)](https://standards.bis.gov.in/website/published-standards/department-wise) — Safety glass — architectural

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Source: https://hagerstone.com/glossary/heat-soak-testing