The Dutch word kleisteen carries two meanings that should not be mixed up.
1. Claystone as a rock. In geology, claystone is a sedimentary rock formed from compacted clay. It is somewhat less dense than slate and related to shale, which has a pronounced layered structure.
2. Claystone as refractory brick. In building practice the Dutch term is another name for refractory brick: chamotte or firebrick. That is made from fired, ground clay (chamotte) bound again with clay and fired once more. The result withstands high temperatures and temperature swings without cracking.
Use as refractory brick
- Fireplace and wood stove — the lining of the firebox
- Bread oven and pizza oven — the floor and vault of a wood-fired oven
- Chimney — the fire-exposed part of the flue
- Industrial kilns — the classic origin of the material
Refractory bricks are not laid in ordinary masonry mortar, which breaks down at high temperature. Use refractory mortar and thin joints — the thinner the joint, the better the construction copes with heat.
Points to watch
- Refractory is not insulating — a solid firebrick conducts heat well and stores it. Where insulation is required (behind the firebox, against a timber wall) an insulating board or an air gap belongs there
- Temperature swings — firing a cold, damp oven hard causes cracking; break a new oven in gradually
- Claystone as a rock is of little use in building: it is soft, splits along the bedding and is not frost-resistant enough for external use
- Flues — altering a chimney or connecting a stove is subject to safety requirements; keep clearances to combustible materials and have the flue inspected
Related terms
- Firebrick
- Clay
- Slate
- Brick
- Open fireplace
- Sedimentary rock
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