What is sintering?
Sintering is the process in which fine particles fuse into a solid mass at high temperature without the material fully melting. The temperature stays below the melting point; only at the grain boundaries does the material become partly glassy. The product shrinks, its pores close up and its strength increases. Sintering is central to ceramics (bricks, tiles), to powder metallurgy and to some 3D printing methods, such as selective laser sintering.
Sintering in bricks and tiles
When clay bricks are fired, the clay minerals become partly plastic at roughly 900 to 1150 °C and bond together. The further a brick is sintered, the denser, harder and less water absorbent it becomes. Highly sintered, dense bricks are often described as vitrified; in the UK, dense low-absorption bricks are used as engineering bricks and for damp-proof courses.
- Flashing and kiln marks: darker, sometimes glossy patches on bricks that sat closest to the heat. They give facing brickwork its varied colour.
- Over-firing: bricks fired too hot can warp, blister or fuse to each other in the kiln.
Ceramic tiles follow the same principle. Porcelain tiles are sintered so densely that they absorb very little water, which is why they are frost resistant and hard wearing.
Application
- Use dense, well-sintered units where water and frost attack: plinths, garden walls, copings, paving and below the damp-proof course.
- Match the mortar. Low-absorption bricks take up little water from the mortar. Follow the manufacturer’s mortar advice and do not wet these bricks before laying.
- Blend from several packs. Colour and flashing vary between batches. Mixing avoids bands and patches in the wall.
- Reject warped bricks. Distorted units lead to uneven joints.
- Common mistake: treating kiln flashing as dirt and grinding or acid-washing it off. It is part of the brick, not contamination.
Related terms
- Brick
- Brick manufacturing
- Hoffmann kiln
- Clay
- Firebrick
- Clay paver
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