Fused cast AZS (Alumina-Zirconia-Silica) refractories are the backbone of modern glass furnace design. For engineers specifying materials for glass contact zones — sidewalls, bottom paving, throats, and doghouses — the grade selection between AZS 33#, AZS 36#, and AZS 41# directly determines campaign life, glass quality, and total cost of ownership. This guide addresses the material science, zone-specific selection logic, and the exudation behaviour that is too often overlooked in procurement decisions.
What Makes AZS Fused Cast Different from Sintered Refractories
AZS fused cast blocks are produced by melting raw materials in an electric arc furnace above 1900°C and casting the melt into molds. The resulting microstructure — interlocked corundum (α-Al2O3) dendrites with baddeleyite (ZrO2) crystals embedded in a silicate glassy phase — is fundamentally denser and more resistant to glass-melt penetration than any pressed-and-sintered alternative. Apparent porosity is typically below 1%, compared to 15–22% in dense sintered alumina bricks. This impermeability is why fused cast blocks dominate glass contact applications in float, container, fibre, and borosilicate furnaces operating between 1250°C and 1600°C.
The three commercial grades reflect increasing ZrO2 content, which is the primary driver of corrosion resistance against soda-lime, borosilicate, and specialty glass melts:
- AZS 33# (Zirconia-Brick-AZS-33): ~33% ZrO2, ~50% Al2O3, ~14% SiO2. Standard grade for upper sidewalls, breast walls, and crown spring blocks where glass contact is intermittent or indirect.
- AZS 36# (Zirconia-Brick-AZS-36): ~36% ZrO2, ~48% Al2O3, ~13% SiO2. Mid-grade for lower sidewalls and working-end walls exposed to continuous glass-line contact and alkali vapour attack.
- AZS 41# (Zirconia-Brick-AZS-41): ~41% ZrO2, ~45% Al2O3, ~12% SiO2. Premium grade for throat blocks, bottom paving, and any zone where molten glass contact is sustained above 1450°C.
Glass Contact Erosion Mechanisms and Zone-Specific Behaviour
Understanding erosion mode is essential before specifying grade. Glass furnace refractories fail through three overlapping mechanisms:
1. Chemical Dissolution
Molten soda-lime glass at 1450–1550°C aggressively dissolves the silicate glassy phase between crystalline grains. As ZrO2 content increases, the volume fraction of this vulnerable glassy phase decreases, reducing dissolution rate. AZS 41# extends sidewall life by 30–50% over AZS 33# in aggressive soda-lime float furnaces operating above 1500°C. For TV panel glass or borosilicate applications where B2O3 accelerates silica dissolution, AZS 41# or Zirconia-Corundum bricks are the minimum acceptable specification.
2. Thermal Convection Erosion
The glass-line zone — the interface between molten glass surface and the sidewall — experiences a concentrated erosion notch. Thermal convection currents create a continuous fresh-glass flush against the brick face. This notch forms fastest in the temperature range 1300–1450°C where glass viscosity is low enough for vigorous convection but high enough to maintain significant chemical aggressivity. AZS 36# is the minimum specification for the glass-line zone in most float furnaces; AZS 41# is preferred where campaign targets exceed 10 years.
3. Alkali Vapour Attack
In the superstructure above the melt — crown, port necks, and regenerator checker crowns — volatile alkali species (Na2O, K2O) condense and react with the silicate phase. AZS 33# performs adequately for upper sidewall and breast wall zones where temperatures remain below 1350°C and direct glass contact is absent. For port area blocks exposed to combined thermal cycling (800–1450°C) and alkali condensation, specifying low-exudation AZS 36# or pairing with Silica Brick (96% SiO2) for the crown itself is the standard engineering approach.
Exudation: The Hidden Risk in Glass Quality
Exudation is the most consequential behaviour that procurement managers frequently underestimate. During initial heat-up and whenever a fused cast block exceeds approximately 1200°C, the residual glassy phase within the block softens and migrates to the exposed surface under internal pressure. This exudate — typically a sodium aluminosilicate glass enriched in iron, titanium, and alkali — can drip into the glass melt and cause inclusions, stones, or seeds that destroy downstream product quality.
Exudation rate is directly related to glassy phase volume and viscosity:
| Grade | Glassy Phase Vol. % | Exudation Severity | Recommended Mitigation |
|---|---|---|---|
| AZS 33# | ~20–22% | Moderate to High | Slow heat-up schedule (<15°C/hr through 1000–1200°C range); avoid direct glass contact at start-up |
| AZS 36# | ~17–19% | Moderate | Controlled pre-heat; barrier coating on cut surfaces |
| AZS 41# | ~14–16% | Low to Moderate | Standard commissioning protocols adequate; preferred for glass-contact bottom paving |
For bottom paving in float glass tanks and television glass furnaces where exudation-induced defects carry the highest penalty, AZS 41# produced by the oxidising casting process (as opposed to standard or void-free casting) delivers the lowest glassy phase volume and correspondingly reduced exudation risk. Always request the casting process specification alongside the composition certificate when sourcing bottom blocks.
Practical Recommendations by Furnace Zone
Melting Tank Sidewalls (Glass-Line and Below)
Specify AZS 36# as the baseline for soda-lime container and float furnaces with a design life of 8–10 years. Step up to AZS 41# for the bottom 300–500 mm of the sidewall below the glass line where convection erosion is most intense, and for any furnace targeting a 12+ year campaign or processing high-alkali or specialty glass compositions.
Throat and Passage Blocks
The throat is the highest-erosion zone in the furnace. Glass velocity through the throat creates severe mechanical and chemical attack simultaneously. AZS 41# is the only appropriate specification. Throat block thickness should be sufficient to accommodate a minimum 80 mm wear allowance before the block requires replacement; standard 250–350 mm thickness is appropriate for most float furnace designs.
Bottom Paving
Bottom paving blocks rest under a static head of glass at temperatures of 1300–1500°C throughout the campaign. Low-exudation AZS 41# is standard. Joints should be minimised — large format blocks (600 × 400 × 250 mm or custom) reduce joint count and infiltration risk. Where budget constrains the full-bottom AZS 41# specification, prioritise AZS 41# for the melting zone bottom and AZS 36# for the conditioning zone.
Upper Sidewalls, Breast Walls, and Crown Spring Blocks
AZS 33# is cost-effective and technically adequate for zones above the glass line where operating temperatures are 1100–1350°C and glass contact is absent. For the crown itself, Silica Brick (96% SiO2) remains the standard for oxy-fuel and air-fuel float furnaces due to its superior creep resistance above 1600°C and lower thermal mass.
Summary: Grade Selection Decision Framework
The correct AZS grade is determined by three parameters: operating temperature, glass composition aggressivity, and required campaign life. AZS 33# covers superstructure and upper sidewall applications below 1350°C. AZS 36# addresses glass-line sidewalls and working-end walls in standard soda-lime furnaces. AZS 41# is non-negotiable for throat, bottom paving, and any sustained high-temperature glass-contact zone where campaign life and glass quality are the primary engineering constraints. Complement these with Zirconia-Corundum Brick in specialty glass applications where mixed-oxide corrosion resistance is required, and Silica Brick 96% for the furnace crown.
Always obtain material test certificates verifying ZrO2 content, bulk density (>3.70 g/cm³ for AZS 41#), apparent porosity (<1%), and — for glass-contact bottom and throat applications — confirm the casting process type and exudation test data at 1400°C.
ThermalEast supplies fused cast AZS 33#, AZS 36#, and AZS 41# blocks, Zirconia-Corundum bricks, and Silica Brick 96% to glass manufacturers across Europe, the Middle East, and Southeast Asia. Our technical team provides zone-by-zone material specifications, custom block dimensions, and full material certifications to support your furnace design and procurement process. Contact ThermalEast to request a technical quotation — provide your furnace type, glass composition, and campaign target and we will return a detailed grade recommendation with pricing within 48 hours.