"Glazes on pottery are a form of glass and the colours in the glaze are created using metal oxides. Cobalt oxide gives a blue, tin oxide white, iron oxide browns and so on. Copper oxide usually gives a green but under certain firing conditions, may give a red. If you starve a fuel-burning kiln of air (oxygen) during the firing, the carbon in the fuel which is looking for oxygen takes it away from the copper oxide leaving almost pure copper in the glaze. This creates a red colour, the tone and intensity of which are influenced by the type of the base glaze used and the strength of the reduction firing. Since no two firings are the same, there is a certain elusiveness to copper reds and as a result, potters prize good copper coloured glazes." ~ Rynne Tanton
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