Techno File: Iron in Celadons Robin Hopper

Appears in the January 2024 issue of Ceramics Monthly.

Iron-colored glazes fall into many groups: celadon, chün, temmoku, kaki, teadust, and shino. Here we focus on celadons and chüns, and how to use the many variations of iron available to develop a glaze. 

Defining the Terms 

Opalescent: The visual property of something having a milky brightness and a play of colors from the surface. 

Reduction Firing: Firing where the kiln atmosphere has insufficient oxygen for complete combustion (more fuel than air to burn it). At high temperatures, this produces carbon monoxide gas, which will steal loosely-bonded oxygen from other materials, in ceramics mainly iron and copper. 

Developing Glazes with Iron 

Iron oxide is the main colorant used in many of the early Asian stoneware and porcelain glazes. The colors that come from iron are greatly affected by small amounts of titanium, manganese, and phosphorous that may be found as impurities in many glaze ingredients. Ball clays are particularly likely to have trace amounts of titanium, an impurity that causes iron to produce green rather than bluish celadon colors. Material analysis sheets on clays are usually available from suppliers, and for the development of celadon colors, care should be taken to select clays that have as little titanium as possible. Manganese will tend to make the iron colors muddy looking. Phosphorous, when used in conjunction with small percentages of iron, can give opalescent qualities, and in larger amounts, 10% to 20%, may give iron-red colors. 

The glaze bases listed below cover a range of glazes with varying additions of iron oxide, generally from 0.25% to 10% (the iron amounts added to each base are noted under each tile shown on page 59). The addition of up to 5% iron will give celadons very pale greens to pale blues, through gray-greens to olive-greens. Fired in oxidation, celadon glazes are usually pale yellow to amber. For blue celadons fired in reduction, occasionally referred to as “Ying Ching” or “Ying Xing,” it is preferable to use black iron oxide instead of red. For other variations it doesn’t seem to matter which form of iron is used, although they will all have slight variations. Yellow iron oxide has the benefit of not staining hands or clothes as red iron is prone to do. From 5% to 10%, iron will give dark olive celadons but may also go as dark as a black temmoku. 

Celadon/Chüns 

Celadon and chün glazes are usually known to be a very subtle pale blue to pale gray-blue. They usually have very small iron colorant additions, usually below 1.5%. They are usually fired in reduction between cone 8 and 10, and originated in Song Dynasty, China. Their color is primarily an optical illusion stemming from light refracted off the inside of bubbles trapped in the glaze. The bubbles seem to be generally caused by decomposing carbonates, phosphates, or borates. Technically and chemically, they are very similar to the gemstone opal. The glazes are usually high in silica. 

The ten base glaze below are very similar to one another since this form of opalescence only occurs within narrow parameters. Besides the glass-forming high-silica content in all chün glazes, the carbonate materials, calcium carbonate and dolomite are next in importance. They help to produce the bubbles these glazes need in order to refract ambient light. 

Celadon/Chün Base Glazes 

Celadon/Chün Base Glaze Tests with Added Iron 

This article was excerpted from The Ceramic Spectrum by Robin Hopper, published by The American Ceramic Society and available in the Ceramic Arts Network Shop, mycan.ceramicartsnetwork.org/s/shop.