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Celadons at Cone 6: A Traditional High Fire Pottery Glaze is Well Within the Reach of Cone 6 Potters
Posted By John Britt On July 25, 2011 @ 9:30 am In Daily,Features,Glaze Chemistry | 21 Comments
Ah celadons, how I love celadons. These traditional east Asian glazes can produce translucent colors ranging from soft greens and blues, to blue-greens and gray-greens, to amber greens (like the one shown to the left).
True celadons are high fire glazes, but there are lots of ways to get the celadon look at cone 6. In today’s post, an excerpt from the September 2011 Ceramics Monthly, John Britt explains one way: converting an existing cone 10 recipe to cone 6. To see some other ways, check out the full article in the September CM. – Jennifer Harnetty, editor.
Purists would say that a cone 6 celadon is impossible since, by definition, it is high fired, but if we take a more practical approach and widen our definition of celadon to a transparent blue-green glaze colored with iron or other oxides, then we can include cone 6 celadons in reduction or oxidation.
Since I have worked extensively with cone 10 blue celadons, and know the principles necessary to produce that glaze, I assumed that those same principles could be used to make a cone 6 celadon. The idea is to select a glaze with high potassium, high silica, small amounts of iron, and low titanium (to prevent opacifying the glaze and to prevent the iron from going green to brown). Also, a small amount of tin oxide and barium carbonate improve the blue color. Apply it thickly (two to three coats; 1/8 – 3/16 inches or 3 – 5mm) on a clay body also low in titanium. This means that you should use Grolleg kaolin in both the clay body and the glaze recipe. Fire in an early reduction cycle, using heavy reduction (0.75 – .80 oxygen probe reading) beginning at cone 012 – 010 (1582 – 1657 degrees F; 861 – 903 degrees C), then hold moderate reduction (0.70 – 0.75 oxygen probe reading) to cone 6.
Theoretically, this should be simple, but in order to melt a glaze at cone 6 (2232 degrees F, 1222 degrees C), you need to add different fluxes, all of which have different color responses. Boron oxide is an active flux at cone 6, as are sodium, lithium, and zinc oxide, but each have their own characteristics that have to be taken to consideration. For example, zinc oxide is an excellent flux in oxidation, but if fired in reduction it will volatilize from the glaze, leaving the glaze unmelted. But in electric oxidation it makes a wonderful flux. Boron is an excellent flux in oxidation and reduction but can make the glaze cloudy. Because you have to add so much flux, sometimes up to 30% frit or Gerstley borate, it is sometimes necessary to start reduction a bit earlier when firing to cone 6 or the glaze will seal over and the atmosphere will not be able to act on the iron.
So, with these considerations in mind, there are several ways to make a cone 6 blue/green celadon: move a cone 10 reduction celadon down to cone 6 reduction; test existing cone 6 bases with varying amounts of iron; use stains to make blue/green celadons in an electric oxidation firing.
Adjusting a Cone 10 Celadon to Cone 6
Blue celadon is the most difficult color to obtain with iron, so if we start with one of those recipes, then getting a green celadon should be easy. Taking Pinnell Celadon, which is a cone 10 glaze, and substituting Nepheline Syenite for the Custer feldspar should help bring the melting temperature closer to cone 6. (Nepheline Syenite is a feldspathoid that melts at cone 6, while most feldspar starts melting at about cone 9.) If a straight substitution doesn’t cause the glaze to sufficiently melt at cone 6, which it does not in this case, start adding additional cone 6 fluxes, like frits, Gerstley borate, lithium carbonate, or zinc oxide (for oxidation only, which is covered in the full article), running progressions from 1 – 10%. In this case, 10% Gerstley borate worked well. Alternatively, finding the proper glaze melt can be aided by glaze software, in which you get the unity molecule formula of the glaze into acceptable limits for cone 6. You will need to retotal the recipe to 100 if you add additional fluxes.
After you find the surface you like, run iron progressions from 0.5 – 6% to get a celadon color you like.
To learn more about glaze chemistry, be sure to download your free copy of Making the Switch from Cone 10 to Cone 6 Ceramic Glaze Recipes: A Little Knowledge of Ceramic Glaze Chemistry and Raw Materials Goes a Long Way.
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