An analysis of certain beautiful Song Dynasty porcelain glazes revealed that a single feldspathic rock material (Petuntse) provided the core of the glaze. This single material contained nearly the right proportion of glassmaker, adhesive, and melter oxides. Only small amounts of wood ash and limestone materials were added to improve the color and melt of the glaze. I believe that this is still the most meaningful way to approach the stoneware glaze, or any glaze or clay body for that matter. The objective is to locate one single earth material that alone almost provides the desired surface, and then to add as few additional materials as possible. I call this primary material, which almost achieves the desired glaze surface, a “glaze core.” The list of glazecores is long and disparate and includes feldspars, mica, granitic rocks, some clays, volcanic ash, wood ash, boron minerals, and the artificial manufactured frits. The key characteristic of these materials is their combination of glassmaker, adhesive, and melter functions.
Feldspars and feldspathic rocks contain a complex structure of silica, alumina, and the melter oxides of sodium, potassium, and calcium. This structure makes them ideal glaze cores at stoneware temperatures. Mix powdered feldspar with water, apply this mixture to a clay form, fire it to stoneware temperatures, and there will appear a glossy, white surface on the clay. Thus, feldspars and feldspathic rocks with their complex chemical structure of silica, alumina, and melter oxides of sodium, potassium, and calcium possess the unique ability to form an “almost” acceptable glaze surface at stoneware firing temperatures.
Throughout earth’s history, violent upheavals have forced silica-rich magma up toward the earth’s outer layers. Under these outer layers, the magma cooled slowly for thousands of years to form the large-grained crystalline rocks known as granite. When exposed on the earth’s surface, granites are subjected to two types of weathering. Mechanical weathering (physical disintegration of granites by expansion of water, tree roots, groundwater, animal footsteps, etc.) causes the granites to be broken down into their various minerals—mainly feldspars, quartz, and micas. Chemical weathering (chemical reaction of the granites to the air, living beings, earth, and water on the earth’s surface and atmosphere) causes some feldspar and mica minerals to further decompose into clay minerals. Granites are the basis of most of our ceramic materials and make up 75% of the earth’s crust. They are rocks, which by definition are mixtures of one or more minerals. Granites consist of over 50% potash and soda feldspar and up to 25% quartz. They also contain as much as 20% mica and lesser amounts of magnesium-iron minerals. Some granites, if crushed to a fine particle size, will make exciting glaze surfaces at high stoneware temperatures.
General Characteristics of Feldspars
Feldspar includes an assortment of minerals of varying composition. Despite this range, the feldspars commonly used by potters tend to follow a fairly recognizable pattern when fired to stoneware temperatures.
1. The most striking characteristic of a feldspar that is fired to stoneware temperatures is the formation of a glassy, white surface. The heat of the stoneware kiln fire, combined with the feldspar’s soda and potash melter oxides (14%–15%) have transformed its considerable silica content (60%–70%) into glass. The white color is a happy consequence of the selection of atoms by size—the atoms of the coloring minerals such as iron and copper are too large to fit into the feldspathic structure. The result is a relatively pure white material to which colorants can always be added.
2. The melting action of the feldspars has a very long range: 2138°F (cone 4) to 2381°F (well beyond cone 10).
3. Melted feldspars possess a high surface tension because of their considerable alumina content (17%–25%); they crawl and flow unevenly. This is especially noticeable with a thick coat of feldspar.
4. The surface of melted feldspars contains an intricate network of fine cracks alternately described as “crazes” if considered a glaze defect and “crackle” if considered aesthetically desirable. Melting oxides, contained in the oxide structure of the feldspar, are responsible for the craze/crackle network. These melting oxides are for the most part sodium and potassium, which undergo a high rate of expansion when heat converts them from a solid into a liquid state.
5. Feldspars do not remain evenly suspended in the liquid glaze mixture. The feldspathic powder settles at the bottom of the glaze bucket, forming a dense, rock-like substance that defies even the most vigorous attempts at disbursement.
It must now be apparent that although feldspar provides the basic core of a stoneware glaze, it does present certain problems for the potter. We can solve these problems by adding small quantities of three or four minerals to the feldspathic glaze.
Additions of limestone or calcium minerals will increase the melt at stoneware temperatures and thus quicken the flow of the feldspathic glaze.
Additions of the glassmaker (silica) will eliminate the craze/crackle network, should this be desired. Silica, unlike the sodium and potassium melters, has a minimal rate of contraction upon cooling, and thus inhibits the high contraction rate of these melters.
Physical suspension of the feldspar in the liquid glaze may be improved by adding 10% or more of clay materials such as kaolin or ball clays. The addition of the clay materials will also toughen the raw glaze coat and help it withstand the handling that takes place when the kiln is stacked. Suspension will be further improved by the addition of 2%– 3% superplastic clay (bentonite) or even smaller amounts of soda ash or Epsom salts (magnesium sulfate).
Minerals, such as copper, iron, or cobalt, may be added in oxide or carbonate form to achieve color.
This combination of materials spawns a broad range of standard stoneware glazes. Although a specific stoneware glaze formula may show four or even five ingredients in its recipe, in most cases the core of the glaze is the feldspar. The rest of the materials are present in order to cure the problems contained in the feldspar.
At the cone 5/6 oxidation temperatures, 70% F-4 feldspar and 30% Wollastonite creates a creamy, satin-matt surface. See also the example piece with Nepheline Syenite 80%, Wollastonite 20% above.
The oxide structure of a feldspar explains why it constitutes the central ingredient core of a stoneware glaze. Most feldspars contain about 60%–70% silica (the glassmaker), 17%–25% alumina (the adhesive), and 10%–15% sodium, potassium, and/or calcium oxide (the melters).
This text was excerpted from Out of the Earth, Into the Fire: A Course in Ceramic Materials for the Studio Potter, by Mimi Obstler.