What is a Hoffmann kiln?
A Hoffmann kiln, also called a ring kiln, is a continuously operating kiln for firing bricks, consisting of one uninterrupted covered space around a usually central flue. The distinctive point is that the bricks do not move through the kiln; the fire moves from chamber to chamber while the bricks stay where they are.
The ring kiln became the basis of industrial brick production in the nineteenth century and made year-round firing possible. The kiln was sometimes round, but often oval or elongated; you could walk, and sometimes drive, right around inside it.
How it works
The efficiency comes from heat recovery. Combustion air is drawn in through the chambers that have just been fired and are still glowing: those bricks cool in a controlled way, and the air reaches the fire preheated. The flue gases then pass through the chambers ahead of the fire, drying and preheating the bricks still waiting their turn. Almost nothing is wasted.
The cycle
A full circuit takes about fourteen days, with a firing temperature in the order of 1060 °C:
- Mixing and forming — the clays are blended and pressed in moulds, or hand-moulded for special bricks
- Drying — the green bricks dry for at least six days in drying chambers, using warm air recovered from the kiln
- Setting — the dried bricks are stacked in the coldest chamber
- Firing — day and night, coal is fed in through holes in the crown; the firemen move the fire from chamber to chamber over some ten days
- Cooling — gradually, to avoid cracking
- Drawing and sorting — the bricks are sorted by quality, hardness and colour
Heritage
Many ring kilns are now industrial heritage, and some survive as museums. They are recognisable by the long curved body with rows of bricked-up wicket openings and a single tall chimney.
Related terms
- Mortar
- Listed building
- Restoration
- Facade
- Compressive strength
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