Digitalfire Ceramic Glossary
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.
Celadon cone 10R glaze (about 3.5% iron oxide) on a buff firing reduction stoneware with G1947U transparent liner glaze
A down side of high feldspar glazes: Crazing!
This is crazing in a cone 10 reduction celadon glaze, on a porcelain. This is common because such glazes are often high in K2O/Na2O. Where do these oxides come from? Feldspars. Feldspars do produce brilliant glossy surfaces and they produce great colors but a glaze having 40% or more feldspar comes at a cost. Some body manufacturers in the North America produce whitewares and porcelains having extremely high quartz and ball clay contents, these will actually work with such glazes. However if you are using traditional bodies this problem can be solved by substituting some of the KNaO for lower expansion fluxes like MgO, SrO, CaO, Li2O (using glaze chemistry software).
Close-up of cone 10R celadon bubbles suspended in the glass
This is happening because this glaze lacks flux and is not fluid enough to enable their migration. In the upper half they are more evident (double thickness).
GR10-E Ravenscrag:Alberta Slip with 10% calcium carbonate
At cone 10R this produces an overly melted glaze. It also crazes.
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. The glaze is transparent, it has depth and varies in shade according to thickness, breaking to a much lighter shade on the edges of contours.
The multitude of things iron oxide can do in reduction
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.
An incredible glossy celadon recipe
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.
Alberta Ravenscrag Cone 6 Brilliant Celadon
The magic of this recipe is the 5% extra frit, that makes the melt more fluid and brilliant and gives the glaze more transparency where it is thinner on edges and contours. The extra iron in the Plainsman P380 (right) intensifies the green glaze color (vs. Polar Ice on the left). The specks are cobalt oxide agglomerates that were made by slurrying cobalt oxide and bentonite, then crushing it to sizes large enough to make the specks.
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