Today, we live in an age of super abundance of ceramic raw materials. Innumerable clays and glaze materials offer us a bewildering array of choices. Far from understanding these materials as familiar rocks, feldspars, and clays, each with unique personalities of their own, we know them only as white, gray, or brown powders neatly packaged in uniform bags. Consequently we beg, borrow, and steal glaze and clay body recipes that “work.” Ceramic raw materials come to us from every corner of the earth in a purified and refined state. Fortunately, it is not necessary to have an intimate understanding of these hundreds of ceramic materials. Ancient potters created their masterpieces from three or four ceramic materials, and, if we similarly narrow our choices, we can also achieve extraordinary results. Ceramic Raw Materials: Understanding Ceramic Glaze Ingredients and Clay Making Ingredients offers access to that knowledge, including how to formulate a glaze using a glaze core, charts for clays and feldspars found in most clay and glaze recipes, a glossary of common ceramic raw materials, and the primary functions of those materials.
Check out this excerpt from Ceramic Raw Materials: Understanding Ceramic Glaze Ingredients and Clay Making Ingredients:
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).
Download your free copy of Ceramic Raw Materials: Understanding Ceramic Glaze Ingredients and Clay Making Ingredients to read the rest of this article and those below!
Ceramic Raw Materials: Understanding Ceramic Glaze Ingredients
and Clay Making Materials also includes the following:
Understanding Glazes Through Raw Materials: Using Glaze Cores
By Mimi Obstler
There are so many materials available to ceramic artists that it can be somewhat overwhelming. But if you understand the concept and function behind glaze cores, the process becomes far more manageable.
By Dave Finkelnburg
Mixing the right raw clay materials, in the right order, affects clay body performance more than you may think. Follow this expert advice to get it right!
Clay Body Plasticity
Find out what makes clay materials do what they do—like bend and stay, smoosh and stick.
Clay Materials We Use
Because clays and recipes can change over time, it is good to know specifically what your clay contains. If you need to substitute one material for another, you’ll want to get as close as possible, so you’re changing as little as you can.
By Dave Finkelnburg
This abundant ceramic raw material, once you understand it, can be the perfect natural frit for glazes, as well as a great flux for clay bodies.
Glossary of Common Ceramic Raw Materials
This quick reference to the most common North American raw materials will come in handy when formulating clay bodies as well as glazes.
Primary Functions of Common Ceramic Raw Materials
A companion to the glossary of common materials used in studio ceramics, this chart allows quick identification and understanding of the main uses of our materials.
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