GLAZES

 

 

 

 

 

The colouring possibilities of crystalline glazes fired in oxidation are usually rather limited. They are obtained in great majority by transitional metal oxides (those from block " d ") such as iron, manganese, nickel, copper and cobalt... A wider range of colourings is nevertheless possible for those who fire in a gas kiln by the reducing action of the gaseous atmosphere (production of carbon monoxide during firing).   The main transitional metals that react to the reducing effect of carbon monoxide are: copper, iron, manganese, titanium... Transitional metals can form many ions whose colouring can vary depending on their state of oxidation. Thus for example MnO4- (Mn has a state of oxidation +7) is purple, whereas Mn2+ is pale pink.

Cobalt colour (CoCO3, 2 %) obtained in an oxidation firing

Reduction firing , by producing a gaseous atmosphere containing a certain amount of carbon monoxide, is the mechanism which makes it possible to modify the metal ions by lowering their state of oxidation. Thus, Fe3+ ions coming from red iron oxide (Fe2O3) colour glazes yellow. Under the effect of reduction firing, ferric oxide Fe2O3 loses a part of its oxygen and is transformed into ferrous oxide FeO, this state of lower oxidation of iron will then produce Fe2+ ions which will colour the glaze green or blue (green colouring generally results from the simultaneous presence of Fe3+ and Fe2+, yellow + blue = green).

Colour obtained in a reducing atmosphere (Carbon Monoxide) Glaze by John Tilton (USA)

To get round the difficulty of carrying out a reducing atmosphere in an electric kiln, I imagined the development of glazes containing the elements likely to reduce or maintain reduced certain metallic oxides. Elements which must mainly act on one or more oxides and be compatible with the glaze without harming the required effects for crystallization (nucleation, growth of the crystals, weak flow of the glaze).

While being inspired by techniques used in glass-making, a first result of this kind was obtained in 2005 with the realization of a "Titanium Blue" glaze with crystals. The blue colouring being due to the reduction of TiO2 to Ti2O3 under the reducing effect of antimony trioxyde (Sb2O3) added to the base of the crystalline glaze also containing barium. Sb2O3 oxidizing itself to Sb2O5 to the detriment of titanium dioxide which yields to it a part of its oxygen.   The Ti3+ ions coming from Ti2O3 then produce a violet colouring of the glaze, a color very characteristic of this oxide.   This result was the proof that the process of "in vitro" reduction was possible in an electric kiln, under certain conditions met in the composition of the glaze, and that the need for a reducing gaseous atmosphere (containing carbon monoxide) was not impossible to circumvent any more.

Colour obtained by the reduction of "in vitro" titanium Glaze "Titanium Blue" - September 2005

Recently, in the continuity of this process, I obtained an intense " Iron Blue" with two superimposed glazes, one potassic and plombous, rich in alumina and ferrous oxide (FeO, black iron oxide), the other rich in alkali and zinc, containing barium and relatively little alumina. The auxiliary colourants used for the crystals are cupric oxide and green nickel oxide (see picture).   The first glaze is intended to preserve the FeO by quickly locking it up in an alkaline and plombous flow while preventing it from running, thanks to alumina. The second glaze is a crystalline base revealing the blue color of iron by a lower partial dissolution of the glaze while preventing the oxidation of FeO. After firing at cone 8 (1255-1260°C) this combination of glazes does not produce any flow to the foot of the pots.   This colouring due to Fe2+ is usually obtained in gas reduction firing, after a not very fast rise in temperature, and a moderate reduction starting at 900-1000°C. The firing being able to end in oxidation at the moment when the glaze is completely molten.   Denis Caraty / November 2007     Translated by my friend Edouard Bastarache, ceramicist in Quebec - http://www.sorel-tracy.qc.ca/~edouardb/

Iron Blue" colour obtained by ferrous oxide, FeO, maintained in its state of oxidation by the glaze throughout an oxidizing firing. November 2007

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