Glassatelier Hebing Efteland's

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What is Thermal Shock?

By HotPot Norge, Jan 16 2013 01:06PM

Most Hotpotters know that glass can break due to rapid, uneven temperature changes.

Mostly during cooling down and sometimes while firing.

This is known as thermal shock.

Why this happens may not be obvious.

When solid glass is heated or cooled unevenly, the part of the glass that is heated will expand (or contract if cooled). The glass that isn’t changing temperature stays the same size.

This uneven expansion creates a lot of stress inside the glass. If the stress is strong enough the glass will break. And that is called thermal shock.

A good example of this you can see in the picture.

Here I mixed glass with different thickness and in combination with rapid firing, it gave me an S shaped crack. The thinner glass got hotter faster than the thicker glass and causes thermal shock.

I repeated the experiment with less heat over a longer firing time up to 10-12 minutes and the results were much better. The pieces came unbroken out of the HotPot, that was already a big improvement.

When I came back to the studio the next morning still some thermal shock had occurred during the night.

Overall I can say that with lower heat and longer firing times, it is very possible to fuse glass with different thicknesses and from different suppliers together with a success rate of +80%.

All the glass used in this test had the same COE.

S shaped crack result of thermal shock.
S shaped crack result of thermal shock.
Different thickness of glass fired with a lower wattage.
Different thickness of glass fired with a lower wattage.
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