The Beautiful Variations of Chun Glazes

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Chun glazes are lovely light blue high fire glazes that feature opalescence caused by micro bubbles that refract light. Interestingly, Chun glazes are very similar chemically to the opal gemstone.

In this post, an excerpt from his book The Ceramic Spectrum, the late Robin Hopper explains what makes Chun glazes so lovely. And he shares a couple of Chun glaze recipes. – Jennifer Poellot Harnetty, editor.


 Chun Glazes Explained, Plus 3 Gorgeous Chun Glaze Recipes

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Chün or Jün glazes are opalescent bluish stoneware or porcelain glazes originating in Song Dynasty, China. These high fire glazes are typically fired to Cone 8 or 10. Their color is primarily an optical illusion stemming from light refracted off the inside of bubbles trapped in the glaze. The glazes are usually high in silica. The color comes from small percentages of iron often enhanced with minute amounts of copper.

chun4Chün glazes are often used in conjunction with copper red slips underneath to develop a range of opalescent purples and blues. Similar opalescent effects may by made by covering a high-iron content temmoku glaze with a fluid ash glaze. The glaze bases here are very similar, 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.

 

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Chün or Jün glazes are usually very subtle opalescent pale blue to pale grey-blue. They usually have very small iron colorant additions, usually below 1.5%. In larger amounts the iron gets continually darker until a cloudy temmoku-like glaze is achieved at 5% to 10%. In the test tiles shown here, a brush stripe of copper carbonate in solution with water and a little gum arabic shows the potential of traditional red-purple opalescent glazes. Opalescence is caused by micro-bubbles trapped in the glaze that refract light from the inner bubble surface. They seem to be generally caused by decomposing carbonates, phosphates, or borates. Technically and chemically, they are very similar to the opal gemstone.

This series of glazes were developed for North American materials from formulae found in Nigel Wood’s excellent book, Chinese Glazes.


**First published in 2014
Comments
  • Chun are beautiful, but the glazes in example are very similar. I’d wish for more recipes 6-8 cones in ox.

  • Interesting glazes and article.
    Nigel Wood states (pp120-121) that the blue colour of Jun glazes are due to liquid-liquid phase separation in the solidified glaze giving ‘droplets’ that are about one sixth of the wavelength of light and thus are good at scattering blue wavelengths. He notes that that slivers of Jun glaze appear ‘straw-coloured’ in transmission, the blue colour being an effect seen only in reflection with thick glaze applications and enhanced by a diffusely reflecting white underlayer at the glaze-body interface. In his table on p124 all the glazes contain iron but this is not thought to contribute to the blue colour. Indeed, if the glazes are yellowish in transmission then they could have been made in oxidation!
    In your glazes, iron is present and, interestingly, the blue-ness of the glazes shown in the photographs increases with decreasing iron. Have you tried any of the glazes with zero added iron? And have you tried oxidative rather than reductive firing?

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