In silica refractory bricks used in glass furnaces, controlling the amount of residual quartz is critical. This is why technical specifications generally require less than 1% residual quartz.
1. A phase transformation that generates dangerous volume changes
Quartz undergoes a crystallographic transformation from α-quartz to β-quartz at 573 °C.
This transformation produces a sudden volume change (≈0.8%).
In a brick already installed in a glass furnace, this can:
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generate significant internal stresses
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cause microcracks or structural cracking
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disturb the stability of the furnace crown
In glass furnaces, where dimensional stability is essential, this phenomenon can become a major operational risk.
2. Bricks must already be stabilized before installation
During manufacturing, firing must convert almost all quartz into:
These phases are thermodynamically stable in the operating temperature range of glass furnaces (typically 1400–1650 °C).
If a significant amount of quartz remains, it means that the mineralogical transformation is incomplete.
As a result, phase transformation may still occur during furnace operation, which is clearly unacceptable.
3. A risk factor for furnace lifetime
In critical areas such as:
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the crown
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the superstructure
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the upper furnace walls
mechanical stresses are already high.
Excess residual quartz may lead to:
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crown deformation
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joint opening
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weakening of the bricks
Ultimately, this can significantly reduce the campaign life of the furnace.
4. Why the industry standard is <1%
Refractory manufacturers have established through experience that:
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<1% residual quartz ensures that critical transformations have already occurred
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the microstructure is dominated by tridymite and cristobalite
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the refractory exhibits excellent dimensional stability in service
For this reason, <1% residual quartz has become an industry benchmark for silica bricks used in glass furnace applications.
In summary
Maintaining less than 1% residual quartz ensures:
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no dangerous volume change at 573 °C
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high dimensional stability
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mechanical safety of the furnace crown
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maximum furnace campaign life.