Glazes in the Chun glaze family are lovely light blue high fire glazes that feature opalescence caused by micro bubbles that refract light. Interestingly, Chun glaze is 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.
Chün or Jün glazes are opalescent bluish stoneware or porcelain glazes (celadon) 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.
Chün 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.
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.