Naturally occurring wood ash has created an ash-glaze trickling down from the shoulder of this floor vase, covering its surface in drips and drops; wood-fired at 1300°C, Claude Varlan, Prissac, France 1982 (photo by Wolf Matthes).

As opposed to a classic ash-glaze, this type of wood- or coal-ash deposit occurs naturally and usually unplanned when a vessel is fired in a solid-fuel-burning kiln. The draught in a wood-burning kiln (vertical or horizontal potter’s kiln) causes ash from the fuel to settle on the vessel. At temperatures of between 1200°C and 1350°C (depending on the composition of the ash), the calcium oxide CaO, magnesium oxide MgO, potassium oxide K₂O and phosphorus pentoxide P₂O₅ in the ash can react with the silica SiO₂ and the aluminium oxide Al₂O₃ in the body of the clay to form a flux that is of low viscosity and high mobility, and that corresponds in its composition to common stoneware glaze. This means that it takes ash and a ceramic body to create a glaze, since most wood ash is low in SiO₂ and Al₂O₃.

Translation Sandy Haemmerle

German: Ascheanflugglasur

French: Glaçure aux traces volatiles de cendres (grès aux cendres)

References:

Wolf Matthes, Keramische Glasuren 2, Koblenz 2012, 21–34.