Digitalfire Ceramic Glossary

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Celadon Glaze


A green or blue-green reduction fired glaze that has been stained using iron oxide. Celadons were first developed by the ancient Chinese. The celadons that potters are accustomed to firing today are glossy transparent whereas the ancient versions were more waxy and opaque. Thus there is dispute among practitioners and purists about what exactly a celadon really should be or what glaze can truly be labelled 'Celadons'. There are many books and webpages on the subject.

Typically celadon glazes are employed on porcelain but can also be used effectively on stonewares. Modern Celadons usually possess their high gloss because of high amounts of sodium and potassium, these oxides also cause the crazing often seen. However this problem can be solved by substituting some of the Na2O with lower expansion MgO or CaO and increasing the SiO2 (using ceramic chemistry calculations of course). Celadons have traditionally been fired at cone 10 but lower temperatures are possible with the addition of more flux (e.g. Gerstley Borate).

Blue celadons typically have high sodium/potassium, high silica, not too much iron, and low titanium (Grolleg kaolin is a good option to minimize the TiO2). Some claim that a little tin oxide and/or barium carbonate will help with the blue color. Some people are investigating creating celadons for cone 6 oxidation using stains to impart the color.


By Tony Hansen

Out Bound Links

  • (URLs) Celadon on Wikipedia
    http://en.wikipedia.org/wiki/Celadon
  • (Glossary) Crazing

    Crazing refers to small hairline cracks in glazed ...

In Bound Links


Pictures

Celadon cone 10R glaze (about 3.5% iron oxide) on a buff firing reduction stoneware with G1947U transparent liner glaze

Crazing in cone 10 reduction celadon glazes, especially on porcelain, is common because they are high in K2O/Na2O. However this problem can be solved by increasing the SiO2 and substituting some of the KNaO for lower expansion fluxes like CaO.

Close-up of cone 10R celadon bubbles suspended in the glass. This is happening because this glaze lacks flux. In the upper half they are more evident (double thickness).

GR10-E Ravenscrag:Alberta Slip (50:50) celadon with 10% added calcium carbonate produces an overly melted glaze.

Ravenscrag GR10-E celadon glaze (50:50 Ravenscrag Slip:Alberta Slip) at cone 10R on porcelain (right) and stoneware (left).

GR10-E (50:50 Alberta Slip:Ravenscrag Slip) celadon at cone 10R on a white stoneware and a porcelain.

50:50 Alberta Slip:Ravenscrag Slip cone 10R celadon on iron stoneware, buff stone and porcelain.

Iron oxide is an amazing glaze addition in reduction. It produces celadons at low percentages, then progresses to a clear amber glass by 5%, then to an opaque brown at 7%, a tenmoku by 9% and finally metallic crystalline with increasingly large crystals past 13%. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

The outside glaze on this cone 10R mug (made of Plainsman H550) is simply an Alberta Slip:Ravenscrag Slip 50:50 mix with 5% added Ferro Frit 3134 (the Alberta Slip is calcined). This produces a stunning celadon and great working and application properties. Inside glaze: Ravenscrag Slip 90%, talc 10% (a matte having an extra ordinary silky texture). Learn more at ravenscrag.com.




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