What is reduction firing?
Reduction firing is the stage of the firing process in which bricks or roof tiles are kept in an oxygen-starved kiln atmosphere, so that the red colour of the clay changes to blue-grey or anthracite. In Dutch the process is called smoren and the result blauw gesmoord (blue smoked); in German it is Dämpfen or Reduktionsbrand.
How it works
Red-firing clay contains a lot of iron oxide, which gives fired clay its red colour. When the air supply is shut off towards the end of firing, the burning fuel draws oxygen from the iron oxide. The fired body then turns blue-grey.
- Traditionally: in the Netherlands the fired bricks were left for several days in the smoke of smouldering (alder) wood, or in the fumes of coal, in the closed kiln.
- Today: manufacturers shut off the air while the gas keeps burning, sometimes adding nitrogen. This is how blue smoked roof tiles, grey and grey-green bricks and blue papensteen bricks are made.
Unlike an engobe or glaze, the colour is not applied as a coating; it develops in the clay itself.
Application
- Heritage work. Blue smoked tiles suit many historic Dutch roofs. On listed buildings, agree colour and tile profile with the conservation officer first.
- Expect variation. Reduction-fired products have a lively, uneven colour. Mix tiles or bricks from several packs as you lay them.
- Matching repairs. Take a sample when reordering; shades vary between manufacturers and batches.
- Ask what the finish is. Check whether a dark tile is reduction-fired, engobed or glazed; in photos they often look alike.
Common mistake: treating smoked and engobed tiles as the same thing. If an engobe gets damaged, the red body can show through.
Related terms
- Roof tile production
- Brick manufacturing
- Papensteen brick
- Engobe
- Hoffmann kiln
- Roof tiles
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