A Traditional High Fire Pottery Glaze is Well Within the Reach of Cone 6 Potters

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 above).

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 Ceramics Monthly archives, John Britt explains one way: converting an existing cone 10 recipe to cone 6.  – Jennifer Poellot Harnetty, editor.


Celadons at Cone 6

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.

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Pinnell Celadon (rev.) with Nepheline Syenite and Gerstley borate on Grolleg Porcelain fired to cone 6 in reduction, 2011.

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.

**First published in 2011

Comments
  • Thanks for giving those cone 6 recipes, they’re really usefull to fill up the places of my wood firing kiln where the temperature is not quite high enough for cone 8 and above glazes.

  • But where is the recipe for that beautiful amber colour celadon shown on the top photo ?

  • What is “Synthetic” Red Iron Oxide? I though RIO is RIO.

  • I was hoping for a possible oxidation cone 6 recipe? Any chance?

  • Since I am trying to stay away from using Barium Carbonate, can I use Barium Frit or Strontium Carbonate instead? I have in most of my glaze recipes that call for Barium Carbonate to replace them with less toxic chemicals.

  • After 25 years as a high fire reduction potter and hard-core iron-based glaze addict, I was curious to read what John Britt had to say and am now excited to start running some test firings. I will so happily let go of an extra 2 – 4 hours of firing and the expense of all that fuel! Not to mention reducing my carbon footprint. Three cheers to the mid-range pioneers!

  • That’s funny Genevieve because that doesn’t look like any celadon I’ve ever seen.

  • I second Alina, please let us know of any oxidation versions for these recipes!!

  • Oh I third Alina, I would love an oxidation version too- pleeeeasse! 🙂

  • Glaze name: Amber Shaner Celadon ARAC no albany
    Cone: 9-10
    Color: Brown
    Testing: Tested
    Surface: Gloss
    Firing: Reduction
    Glaze type: Celadon
    Transparency: Transparent
    Recipe:
    EPK 14.00
    Dolomite 5.00
    Whiting 19.00
    Silica 29.00
    Custer feldspar 27.00
    Frit 3134 6.00
    Yellow ochre 8.00
    Comments:
    Good functional glaze as liner, etc. Greenish-brown thin, brown thick
    Thanks http://lindaarbuckle.com/

  • this looks crazed to me am i seeing things lets see what we can come up with.
    and havent we had global warming thousands of years ago not every country has the concerns of a lot of us

  • Do a triaxial blend in whatever your oxidation base glaze is with corner point additions of 2% copper oxide, 2% sky blue stain and 2% Iron Oxide and you will get faux celadon glazes from amber through green to blue

  • Do all celadons craze? Does that make them unsafe for food and drink?

  • I’m all about the oxidation versions , as well ! Just sayin ‘.

  • Richard L, the chart clearly shows substitutions you require.

  • It’s probably worth making the comment that to consider glaze colour in isolation from fired clay body colour is a bit risky. I would emphasise firing your tests on the lowest iron content porcellaneous body at whatever temperature – cone 6 of cone 10. Especially if you are after very subtle celadon like colours that are substantially affected by body/glaze interaction.

  • About 7 years ago MudFire Clayworks converted all of our studio cone 10 glazes to cone 6 and have been firing over 50 glazes in reduction and oxidation, interchangeably. The results are wonderful. We get shinos, copper reds, celadons… all at cone 6. It’s very much worth doing, especially since you then have the option of firing each glaze in the gas OR electric kiln.

    Check our Erik’s cone 6 shinos here: http://www.mudfire.com/erik-haagensen.htm
    The recipe for the shino (fired in reduction of course) is:

    Neph Sy 54.5
    Spodumene 22.8
    OM4 14.9
    Gerstley Borate 4.9
    Soda Ash 2.9

  • Richard L., You can probably sub strontium for the barium but it won’t look the same and since there is nothing to back up potter’s fears concerning barium handled in a proper manner I don’t see why you would.

  • Luba,
    Do you share your Cone 6 glazes shinos, copper reds and celadon?

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