A refractory lining must survive the furnace’s temperature, its slag and repeated heating and cooling, so choose by all three: fireclay brick to about 1,500 °C for general work, silica to 1,700 °C for acid conditions, magnesia to 1,800 °C for basic slags and alumina to 1,900 °C for strength at temperature. Never put an acid refractory against a basic slag, because they react and the lining dissolves. Heat a new lining slowly, because water and uneven expansion crack it before it ever sees full temperature.
Part of Advanced Ceramics
Refractory ceramics are heat-resistant bricks, linings and shapes used to contain furnaces, kilns, crucibles and molten metal.
Pick The Refractory By Temperature And By Chemistry
The Advanced Materials article’s table gives the working ratings:
| Material | Max temperature | Key property | Made from |
|---|---|---|---|
| Fireclay brick | 1,500 °C | Good general refractory | Kaolin clay, fired hot |
| Silica brick | 1,700 °C | Resists acid slags | Crushed quartz with lime binder |
| Magnesia brick | 1,800 °C | Resists basic slags | Calcined magnesite |
| Alumina | 1,900 °C | Strong at temperature | Bauxite-derived Al₂O₃ |
| Silicon carbide | 2,200 °C | Hard, conducts heat well | Sand and carbon fused in an arc furnace |
The chemistry column matters because slag is a hot chemical. An acidic slag (rich in silica) attacks a basic lining (magnesia), and a basic slag (rich in lime or iron oxide) attacks a silica lining. Match the lining to the slag, or the wall thins from the inside.
Why Grog And Sawdust Go In The Brick
Refractory Materials (kiln bricks) gives a brick body by volume of 40% refractory clay, 30% grog (crushed fired pottery), 20% coarse quartz sand and 10% sawdust or straw, fired 100-200 °C above the intended operating temperature. Refractory Materials adds that fireclay forms beneath coal seams and fuses at 2700-3100 °F against 1800-2200 °F for ordinary clay, with a field test of a clay ball in the hottest wood fire for 2 hours.
The structural reasons for the recipe:
- Grog is already shrunk, so it gives a rigid skeleton that does not shrink again, cutting drying and firing cracks.
- Coarse grain stops cracks, because a crack must turn around each grain.
- Sawdust and straw burn out and leave pores. Pores lower the strength a little and lower the thermal conductivity and stress from heating a great deal, so a porous brick resists shock and insulates.
Silica Bricks Have A Trap In The Middle Of The Heating Range
Quartz changes crystal form at about 573 °C, with a small sudden volume change. A silica brick that is heated or cooled quickly through that region cracks. Take silica linings slowly through the range and do not let a kiln lined with them cool completely if it can be avoided. Fireclay bricks do not share this problem.
Worked Example: Safety Margin On The Firing
A furnace is to run at 1,300 °C. The kiln-bricks article fires bricks 100-200 °C above operating temperature, so fire them at 1,400-1,500 °C. Fireclay rated to 1,500 °C then has a 200 °C margin over the 1,300 °C operating temperature, which is adequate but not generous; a slag attack or a hot spot would use that margin up. Crucibles shows the same logic: bronze at 900-1,050 °C needs a refractory body, iron at 1,538 °C needs graphite-bearing ware.
Wet linings burst, and the dust is a lung hazard
A new or wet lining heated quickly turns trapped water to steam and breaks off in pieces that fly. Dry it slowly, with a small fire first, over days for a large furnace. Crushing and grinding brick and quartz creates fine silica dust, which scars the lungs over time. Wet the work and wear a good mask.
Common Mistakes
- Using a household brick — it spalls and crumbles at furnace heat.
- Dry packing a lining against slag — the slag runs through the joints. Use thin joints with refractory mortar.
- Skipping the slow first firing — the lining fails on first use.
What’s Next
- Learn next: Glass-Ceramics — a more thermal-shock-proof route
- Combine with: Kiln Building — where the lining goes