Ceramic Raw Materials: Understanding Ceramic Glaze Ingredients and Clay Making Ingredients

Learn the fundamentals of clay and glaze materials in this FREE PDF!

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:

 

Understanding Glazes Through Raw Materials: Using Glaze Cores

by Mimi Obstler

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 glaze cores 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.

2 Rock: Calcite: Calcium Carbonate (Collection of Department of Earth and Environmental Sciences, Columbia University, New York).
Tests: Feldspar and Whiting (Calcium Carbonate) on stoneware fired to cone 9–10 reduction.
Left: Potash feldspar 100%.
Center: Potash feldspar 90%, Whiting 10%.
Right: Whiting (Calcium Carbonate) 100%.

Origin

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.

3 Left: Granite. Slow-cooled, coarse-grained, igneous rock containing 25% quartz, 50% feldspar (mostly potash in this sample), some muscovite, biotite, and/or amphibole.
Right: Rhyolite. Fast-cooled, fine-grained igneous rock with the same chemical composition as granite.
(Collection of Department of Earth and Environmental Sciences, Columbia University, New York).

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.

4 Cone 5–6 oxidation. Porcelain claybody. Left: Satin-matt surface: Nepheline Syenite 80%, Wollastonite 20%. Back: Gloss surface: Jacky’s Clear: Nepheline Syenite, 50; Colemanite, 10; Wollastonite, 10; Flint, 20; Zinc oxide, 5; Ball Clay, 5; Bentonite, 2. Front: Matt surface: Ron’s White Matt #5: F-4 Feldspar, 55; Whiting, 15; EPK, 16; Zinc oxide, 14.

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.

 

Clay Making

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.

Feldspar

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.

Feldspars Used In Ceramic Glazes and Clay Making

These handy materials, used as the core of glazes as well as in most clay bodies, appear in lots of recipes. Some recipes may be so old that the feldspars are no longer available or their names have been changed. If this happens to you, this guide will help you identify the best possible substitute.

Feldspars We Use

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.

Download the free guide right now, and become a better ceramic artist tomorrow. That’s our promise to you from Ceramic Arts Network!

Best regards,

Jennifer Poellot Harnetty
Editor, Ceramic Arts Daily

PS: Remember, the artists featured on Ceramic Arts Network are among the top ceramic artists in the world today, who excel in everything from functional pottery to abstract ceramic sculpture. When you download one of our free guides, you get the best possible advice available and you become a part of our community – enjoying our artists’ stories, gaining inspiration from their work and finding confidence to try new techniques every day!

PPS: Even if you’re not brand new to clay, this guide is bound to have some tips in it that you’ve never heard before – and remember, it’s absolutely FREE, so why wouldn’t you read it today?

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