In theGlass industry, the quality of the final product depends on many parameters: glass formulation, melting conditions, furnace atmosphere, but also – and this is sometimes forgotten – the quality and durability of the refractory bricks. The choice of refractories and their condition over time have a direct impact on the performance of the furnace and the purity of the glass produced.
In this article, we offer an overview of the glass defects most frequently linked to refractories, classifying them by type of refractory material. The aim: to better understand these interactions in order to prevent them more effectively.
1. AZS refractories: robust but not without risks
Refractories of the AZS (Alumine – Zircone – Silice) sont les plus utilisés dans les zones en contact direct avec le bain de verre fondu, notamment dans les fours de fusion. Leur bonne résistance à la corrosion et leur faible solubilité les rendent indispensables. Cependant, leur vieillissement ou leur mauvaise mise en œuvre peut engendrer plusieurs défauts :
Refractory inclusions (corundum, zirconia)
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Defect observed : small opaque white or black spots in the glass.
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Origin : poorly dissolved alumina (corundum) or zirconia crystals resulting from corrosion of the refractory.
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Common cause : progressive dissolution of the core of the AZS bricks, especially when the temperature exceeds 1350 °C, or in the presence of aggressive fluxes.
Striae or cords
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Defect observed : blurred areas in the glass, visible after tempering or annealing.
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Origin : heterogeneity in the glass-refractory mixture, when the leaching of the material is not uniform.
Bubbles and seeds
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Defect observed : presence of isolated or clustered bubbles.
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Origin : excessive porosity in the AZS bricks, or secondary reactions with the components of the glass bath.
Good practice:
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Choose high-density fused-cast AZS bricks (type AZS 36 or 41).
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Avoid temperature fluctuations that encourage internal cracking.
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Set up regular monitoring of wear on sill blocks and side walls.
2. Silica refractories: thermal fragility and acid corrosion
Silica-based refractories are widely used in the upper areas of the furnace (crowns, arches), in particular for their resistance to thermal shock. But their crystalline structure can cause problems in the event of thermal variation or the presence of acids.
Devitrification
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Defect observed : whitish areas in the glass, fragile or more brittle.
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Origin : release of crystallised free silica, especially in the event of overheating or condensation.
Silica dust or fallout
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Defect observed : contamination of the bath with fine particles.
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Origin : effritement des voûtes ou des joints mal protégés, surtout si les températures dépassent les seuils critiques (>1700 °C).
Risks linked to hydration
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Silica is sensitive to moisture, which can weaken the refractory bricks before or during commissioning.
Recommendations:
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Store silica bricks away from moisture.
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Ensure gradual heating when bringing the furnace up to temperature.
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Plan regular inspections of elevated areas.
3. Alumina refractories: between mechanical strength and chemical dissolution
High-alumina bricks are often used in areas subject to high mechanical stress (wall bases, gantries). Although they resist abrasion well, they are sensitive to certain glass components (particularly alkalis).
Alumina cords
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Defect observed : pale yellow to brown colouring of the glass.
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Origin : release of alumina or iron oxides through slow corrosion.
Crystalline inclusions
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Defect observed : microcrystals visible under the microscope or to the naked eye.
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Origin : partial melting of the brick and solidification in the bath.
Usage advice:
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Favour dense, low-porosity aluminas.
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Avoid using them in direct contact with the bath in highly alkaline areas.
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Reinforce wear areas with combined solutions (AZS + alumina, or mechanical anchoring).
4. Chromium or spinel refractories: beware of colouring
Less common, chromium-based refractories or magnesia-based refractories are sometimes used in special furnaces (coloured glass, borosilicates, glass fibre). They are robust but present risks of unwanted colouration.
Green / brown-green colouring
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Defect observed : greenish tint of the glass.
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Origin : diffusion of chromium oxides into the bath.
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Particularly visible in clear glass (flint glass).
Preventive measures:
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Reserve these materials for areas not exposed to the bath (bottoms, external areas).
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Monitor oxidising/reducing atmospheres that can encourage ionic migration.
Conclusion: refractory bricks to be monitored as critical components
Energy efficiency, thermal stability and the quality of the glass produced depend largely on the refractory brick used in each zone of the furnace. In theGlass industry, preventing defects therefore requires perfect mastery of the physico-chemical properties of refractory materials, but also their proper maintenance and continuous monitoring.
At SXE-Consulting, we support glass manufacturers in the selection of refractories and theanalysis of defects, in order to guarantee optimal production levels while reducing scrap linked to glass quality.
Xavier Schuster
Consultant at SXE Consulting. Industrial consulting firm based in Luxembourg, 25 years of experience in operational excellence.
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