What is clamp firing?
Clamp firing is a traditional method of firing bricks in which dried, unfired bricks are stacked into a large pile, the clamp, with fuel placed under or among them, so the bricks themselves form the kiln. There are no permanent walls. The Dutch term is ‘veldbrand’; in German it is ‘Feldbrand’ or ‘Meiler’. Do not confuse it with a brick clamp in the sense of a hand tool or attachment for lifting bricks.
How it works
- Drying: green bricks are air-dried first; poorly dried bricks crack in the fire.
- Setting: the bricks are stacked with gaps and fire channels so heat can travel through the pile.
- Casing: the outside is closed off with fired bricks and sealed with clay or earth, leaving draught holes at the top. A clamp packed with earth or mud is sometimes called a scove kiln.
- Firing: fuel such as wood, peat or coal burns in the channels. In the London area, ash and cinders were mixed into the clay itself, so the bricks partly fuelled their own firing.
- Cooling and sorting: once cool, the clamp is taken down and the bricks are graded.
Uneven results
Bricks on the outside are often underfired and soft, while those near the fire can be overfired or vitrified. Sorting is therefore part of the process, and clamp bricks vary noticeably in colour, size and hardness.
Clamp, kiln and Hoffmann kiln
- Clamp: temporary, no fixed walls, often built close to the building site.
- Permanent kiln: fixed walls keep in more heat and use less fuel.
- Hoffmann kiln: a continuous ring kiln from the nineteenth century with heat recovery and far more consistent quality.
Clamp firing has not disappeared. Traditional London stock bricks, with their typical pock-marked faces from burnt-out ash, are still made in small quantities, mainly in Kent and Sussex, for heritage work.
Tip for repairs
When matching old clamp-fired brickwork, a perfectly uniform modern brick will stand out. Reclaimed bricks or a handmade stock with natural variation usually blend in better.
Related terms
- Hoffmann kiln
- Brick
- Hand moulded brick
- Lime kiln
- Clay
- Listed building
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