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Celadons at Six

Celadon glazes are some of the most popular glazes in ceramics. In particular, transparent blue celadons have a very delicate, beautiful color that shows carving very nicely. But celadons aren’t just blue; they can range from blue to blue-green to gray-blue to gray-green to green to amber, and even to white. They often have distinctive crackle patterns that are sought after but can also be craze-free.

Celadons originated in China thousands of years ago and were meant to mimic jade. The Lung-chuan (Longquan) satin green celadons were important Chinese exports for over 500 years. The term “Celadon” is a European name thought to have derived from a character in a French play who wore gray-green ribbons over his cloak. However, there are several competing theories of its origin.

Technically, celadons are feldspathic transparent high-fire glazes that are colored with iron and fired in reduction. This differentiates them from transparent copper greens known as Oribe, but both glaze names denote a type of ware as well as a color of glaze. Celadons were thought to have been made from the local clay body and ash, but as the glaze traveled to Korea and Japan, potters began using porcelain stone (a naturally occurring decomposed feldspathic rock) in the glaze.

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 (better chances for blue), 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; 18316 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 (0.75–0.80 oxygen probe reading) beginning at cone 012–010 (1582–1657°F; 861–903°C), then hold moderate reduction (0.70–0.75 oxygen probe reading) to the end of the firing. Theoretically, this should be simple, but in order to melt a glaze at cone 6 (2232°F, 1222°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, leaving the glaze unmelted. 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 might 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; or 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 we’ll cover later), 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 molecular 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 1–6% to get a celadon color you like (see tiles below).

Use an Existing Cone 6 Recipe as a Celadon Base

Since you have to do a lot of experimenting and testing to move a cone 10 glaze down to cone 6, I find that you can make an iron celadon pretty easily by just taking one of the hundreds of workable cone 6 glazes already in use with a surface you like and then, after taking out the colorants and opacifiers, running an iron progression. This will take you through a range of iron colors from blue to blue green to green to amber to tenmoku to iron saturate. Remember, always use Grolleg kaolin as the clay to keep the titanium as low as possible. The tone of the colors is dictated by the oxides that predominate in the base and the amount of iron. So a high calcium base will give different colors than a high sodium base, regardless of firing temperature.

Use Stains to Imitate a Celadon

Using stains in a cone 6 base allows you to fire in oxidation (particularly useful for all of the folks out there firing electric kilns) even though some stains work in reduction as well. As I said earlier, zinc oxide in oxidation makes a wonderful cone 6 flux and can produce some nice colors with both oxides and stains. Many stain manufacturers recommend using 8% stain, but that makes the glaze flat and uninteresting to me, so I use very small amounts (1–3%) to keep the color delicate and transparent. 

Clay Body Considerations

Finally, clay body must be considered when making cone 6 celadons. To get delicate iron blue celadons you will need to use a Grolleg clay body. Some suppliers only produce cone 10 Grolleg clay bodies, which only start to mature (less than 1% absorption) at cone 7, so you may need to fire them to cone 7. If you want a blue/green celadon, there are many cone 6 domestic porcelains that will work, as well as light and dark stonewares. It is important to mention that different clay bodies have different CTE (coefficients of thermal expansion) and that will affect the crazing or the crackle pattern. Also, expansion and contraction is affected by firing temperature, and mid-range firing (cone 5–7) spans a large temperature range (2167– 2262°F; 1186–1239°C) so maintaining  consistent firings is essential.

Firing at cone 6 is a great way to both save energy and still get outstanding celadons. You can knock about three hours off of a typical cone 10 firing time, and save about a third of the fuel by firing celadons to cone 6. No one will know unless you tell them!

the author John Britt is a frequent contributor to CM, and is the author
of
The Complete Guide to High Fire Glazes: Glazing and Firing at Cone 10. To see John’s work and writing, go to
www.johnbrittpottery.com.

This article was excerpted from the September 2011 issue of Ceramics Monthly.

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