Over the years, changes in technology, archiving procedures, and content acquisition have meant that many of the 5000-plus recipes from our 66-year archive of books and magazines don’t have images that demonstrate their appearance and fired characteristics. In order to revive a selection of these recipes and make them more useful to readers, we asked Alisa Lisken Clausen to test the base glaze of several recipes as well as color variations. Now finished with the tests and each tile documented, she has reported her findings. —Eds.

This article includes six recipes that I tested. For each, I demonstrated how using various oxides can result in matte surfaces and unique color responses. The oxides included in these glaze recipes are magnesium, barium, calcium, and zinc. Here is a brief description of the properties of these oxides related to matte surfaces and color development in these particular recipes.

Magnesium Oxide

Magnesium oxide in a recipe yields matte and fat matte glazes. Fat or fatty matte generally describes a matte surface that is thick or raised, smooth, and often silky. The colors of glazes containing magnesium are limited to pastels. This is especially evident with cobalt, which can create lavender to purple glazes. Sources for magnesium oxide are magnesium carbonate, talc, and dolomite. Dolomite matte is a common name for magnesium matte glazes with the magnesium oxide sourced from dolomite.

See Leather Glaze, Ball Crawl Glaze, and Dry Sandstone Glaze.

Barium Oxide

Barium oxide as a glaze ingredient develops a satin matte texture similar to magnesium oxide. However, in contrast to the pastel colors of magnesium oxide, barium oxide can intensify colors, yielding vivid blues and a bright, copper-based turquoise. Barium carbonate is toxic in raw form and should not be used on surfaces that will come into contact with food due to potential leaching hazards. The main sources for barium oxide in a glaze are barium carbonate and frits.

See Smooth and Stony Glaze and Dry Sandstone Glaze.

Calcium Carbonate

Calcium oxide has a neutral effect on color, resulting in colors neither pastel like magnesium oxide nor bright like barium oxide. The main sources for calcium oxide are whiting, dolomite, wollastonite, bone ash, wood ash, and Gerstley borate. If the calcium oxide is supplied by whiting at 20–30% and slow cooled, it can make matte surfaces (otherwise they are glossy, fake-ash glazes).

See Cream Cheese Matte Glaze.

Zinc Oxide

Zinc oxide is a strong flux that responds dramatically to color. It can produce vivid cobalt blues and copper greens in low alumina glazes (like crystalline glazes) and may produce muddy greens and browns when combined with chrome oxide and some stains. Zinc oxide can produce dry, matte, and opaque surfaces if added in large amounts and slow cooled (in crystalline glazes with 25% zinc oxide, the glazes are very glossy).

See Dense Waxy White Glaze.

Testing Method

All of the bases (with the exception of Ball Crawl Glaze) were tested on both a light and a darker stoneware clay body. Any color variations included in an original base recipe were made and tested. Additionally, I tested the same amounts of coloring oxides in each base: 2% cobalt carbonate, 4% copper carbonate, and 5% red iron oxide. The color variations were tested on light stoneware.

For the tests, I made a 700g dry batch of each glaze recipe, measured 100g each of the dry mix into 5 cups, and added the appropriate oxides for the three color tests. I kept the extra 200g dry glaze of each recipe for future incremental color tests based on the most interesting results of these first color tests.

In all of the tests, I substituted Forshammer feldspar, a combined soda and potash feldspar, for any other feldspar indicated in the original recipe due to local availability of materials.

The specific gravity of each test is listed with its recipe. The tiles were bisque fired to cone 06, then dipped once into the glaze and then dipped again halfway up to show the difference between a thin and thick application.

All of the tiles were positioned upright and fired to cone 6 in an electric kiln with the following firing program:

176°F (80°C) per hour to 750°F (400°C), no hold

212°F (100°C) per hour to 2156°F (1180°C), no hold

176°F (80°C) per hour to 2224°F (1218°C), no hold

Maximum cool rate to 1652°F (900°C) with a 1 hour hold followed by a natural cool down. Slow cooling is key to creating a matte microcrystalline surface.

Summary

Personally, I liked the Dense Waxy White base because it has an attractive, silky, gloss surface and accepts colors very well with many different oxides. Cream Cheese Matte Glaze, Leather, Smooth and Stony Glaze, and Dry Sandstone Base Glaze are examples of barium, magnesium, and calcium mattes, each with individual characteristics for both surface and color response. There are many color tests to be made from these bases, many of which I have yet to try. Any one of these bases would be a good start for incremental and combinations of colored oxide tests. 

the author Alisa Liskin Clausen is American born and has a BFA in ceramics from Syracuse University. She has lived and worked as a potter in Denmark since 1997, focusing her work on cone-6 oxidation glazes. Her extensive glaze tests are shared on several ceramic forums and databases including the Sankey database, Glazy.org, and Flickr.

Topics: Glaze Chemistry

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