Terres et couleurs
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Les terres colorantes, comment les produit-on ?
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Coloured earths,
how are they produced ? 		terres colorantes
Why coloured earths ?
Fundamental qualities
  • covering and colouring capacity: can be mixed in all liquids, no reaction with limes, silicates and cements,
  • not altered by ultraviolet rays,
  • non-toxic,
  • relatively low cost.
What are coloured earths used for ?
  • paint, colourwash: 80 %
  • Colouring concretes in the mass: 9 %
  • Colouring of cardboard, rubber, wax: 8 %
  • Denaturising of snow clearance salt, agricultural use, animal feed: 3 %
What were they used for ?
  • Food dye: pork sausage skins, chocolate during the war when there was no cocoa, snuff, Gitane maïs cigarette paper, cheese crusts...
  • As a medicine: desiccative and astringent properties (ferruginous substances), for stomach ache in the Middle Ages.
  • To clean silverware, to polish metals, mirrors, to revive the colour of terra cotta flagstones.
  • To protect wood from water (shutters, hulls of boats...).
  • In the manufacturing of wall papers, linoleums.
  • To colour rubber.
  • Cosmetics.
 
How are coloured earths produced ?  
EXTRACTION  
There are two types of exploitations: in Vaucluse, for example, you can find open air quarries consisting of an ochreous sand cliff which contains 10 to 15 % of pure ochre, while in Burgundy the mines are about thirty metres deep and the ore is rather pure. The seam is limited by a yellow bed of clay that should not be mistaken for ochre.
SEPARATION  

First of all the ore must go through the washing process, which is done by malaxation under running water. Sands and the impurities settle in V shape decantation drains, while the fine colouring particles, finer and lighter, move towards mud tanks.

EVAPORATION AND DRYING  

After drainage, the earth (known as washed earth) is left in the basins to be dried. It is then taken and cut into loaves (or brioches).

MACHINING AND CONDITIONING  
It is fired to obtain various colours (the red is obtained by calcination of the yellow), then crushed, sifted and put into bags (barrels in the old time).