By the end of this lesson, you should be able to:

2. Distinguish among primary, colluvial, and secondary clay.

3. Describe the properties of clay.

4. Identify three drying stages of clay.

5. Describe the changes that occur during firing, and distinguish bisque firing from glaze firing.

Introduction:

Clay is probably the primary material most people think about when they hear the word, "ceramics." Although it is just one of many ceramic materials, it continues to be important today, as it has been historically.

---

Formation

Clay is a naturally occurring material. It typically forms due to the weathering of feldspar:

Potash feldspar + water + carbon dioxide ==>
 kaolinite + quartz + potassium carbonate

K₂O·Al₂O₃·6SiO₂ + 2H₂O + CO₂ ==>
 Al₂O₃·2SiO₂·2H₂O + 4SiO₂ + K₂CO₃
 

---

Clay Minerals

One of the major clay minerals is "kaolinite." This is hydrous silicate of alumina, and some people use this formula for all clays:

Al₂O₃·2SiO₂·2H₂O

However, other clay minerals exist (Nacrite, Vermiculite, etc.)
 

---

Types of Clay

Clay may be classified as "primary," "colluvial," or "secondary," depending on where it located and how it got there.

Primary: clay found at its origin; residual clay; it tends to be white when fired.

Colluvial: clay that has been naturally transported a relatively short distance from its point of formation; it tends to be white when fired.

Secondary: clay that has been naturally transported a relatively great distance from its point of formation; sedimentary or transported clay; many impurities; most abundant.

Illustration from the China Clay Producers Association
http://www.kaolin.com/geology/index.htm

Read about how clay forms, and how clay is processed into a usable material according to the China Clay Producers.

Degritting Kaolin
http://www.kaolin.com/ccpmin.htm

Properties of Clay

Clay is made up of solid particles that tend to be rather flat. When water is added, the clay becomes plastic, that is, it can be easily formed, retaining its shape. Upon drying, it can become porous. "Plastic clay" that lacks enough water to be formed is termed "short" and can develop cracks when deformed.

Throwing clay on a potter's wheel
http://www.nicholasmosse.com/potteryhistory.htm

There are different clays (e.g., stoneware, earthenware, ball clay), with different properties. ("Albany Slip" is a clay that some think is now extinct. It is odd to think of a ceramic material as extinct, but if there is no more left to procure, that's what it is.)

Extruded clay to be wire-cut into bricks
http://www.arch.usyd.edu.au/~graham/pcbw/sec2.htm#2.5

Drying Stages

As clay dries, it shrinks. Typical drying stages proceed from "plastic" to "leatherhard" to "bone dry." Clay that is plastic can be "thrown" to make a pot on a potter's wheel, or can be pressed into shape. Leatherhard clay is a bit drier, and the clay tends to be the toughest in this state. However, prior to firing in a kiln to produce a vitreous, or glassy, bond, the clay is typically dried to the "bone dry" state, where it contains little or no water. Unfired clay is referred to as "greenware."

Before they are fired, clay pieces can be placed back in contact with water to reform plastic clay by slaking, or picking up water; after firing this cannot happen.

Slaking Clay
http://www.glendale.cc.ca.us/~rkibler/clay.html
 

Firing Clay

Clay products are typically fired in a kiln. Here, the clay particles melt together to form a glassy (i.e., vitreous, or noncrystalline) bond. During firing, there may be shrinkage and permanent changes in color, shape, porosity, hardness, strength, and other physical properties. The degree of vitrification varies with the firing temperature.  (Look at a description of pyrometric cones used to regulate firing temperatures.)

Often, there is an initial firing prior to glazing. This is referred to as a "bisque firing." After cooling, a glaze can be more easily applied without damage to the clay body, which might have been deformed if glaze was applied prior to bisque firing. Glaze is a glassy coating fired onto a clay body, and it is often applied as a suspension using brushing, dipping, pouring, or other techniques.

After the glaze suspension dries, the piece is again fired, and the glazing particles form a glassy coat that bonds with the clay body.

Artistic glazeware ready for removal from a kiln
http://www.zms.org/stories/fall98/intofire/intofire.html

---

Please note that many clay products are never glazed. Typically, industrial clay products, such as bricks and pipes fit this category. But flower pots, and many other clay products may be unglazed.

Clay Pipes
http://www3.jaring.my/gbhgroup/vcp/vcp1.htm

Read more about Firing Clay or about historic brickmaking, and bricks.