Chapter 2Clays and Clay bodies

LEACH 1975

IntroductIon

A true craftsperson understands the material of his or her craft. The Song Dynasty potters knew the properties of the clay that formed their pots and the pots of their ancestors through generations of hands-on experience. This knowledge was handed down to their children along with the clay deposits, and they in turn, passed it on to their children. Today, some 800 years later, there are still some potters who experience this kind of physical and spiritual union with their clay, particularly among the fast-disappearing indigenous cultures of the world.

“The pueblo potter has a true understanding, a harmony with his materials. Europe may have had this once, but no longer does. The Indian people are still making pottery in their own way—their intuition for their mate- rials is still alive.”

(Peterson 1977, 92)

Many of us, however, are not so blessed. We neither quarry our clay nor live near its source and are thus cut off from a concrete, visible connection with the primary material of our craft. Ceramic clays are shipped to us from unknown regions, mined by unknown hands and machines, and tested by strangers for but a brief period of time. We fill abstract formulas with these unfamiliar clays in order to obtain our claybodies. These modern conditions are bemoaned by the great Japanese potter Hamada in his extraordinary conversations with Bernard Leach (Leach 1975, 100–101).

“Today potting is like a department store; clay is bought by sending a postcard to any area of the world and asking for a particular clay and getting it in a few days.”

(Leach 1975, 101)

If our craft is to thrive under these modern conditions, we must fill the void of distance with a different kind of knowledge. Recent findings of clay mineralogists tell us much about the nature of clays. Once we open our minds to the ever-changing formations of the clay minerals as they leave the earth to enter the fire, we find a new and exciting world that stretches the boundaries of our imagination and restores our connection with clay.

WHAT IS CLAY?

Clay is an astonishingly simple four-letter word that describes the most complex and significant material known to humans. It is an essential mineral for the ceramics, refractory, petroleum, paper, adhesives, radioactive waste disposal, cement, fabrics, fertilizer, food, wine, beer, soil, medicine, pharmaceuticals, cosmetics, paint, leather, plastics, rubber, soap, polishing compounds, and water clarification indus- tries (Fyre 1981, 78–79). In the last 10 years, clay minerals have played an increasingly important role in advanced space and computer technologies that have revolutionized our modern age. Thus, clay supports and nourishes all life on the earth’s surface.

The dictionary describes clay as a mineral, a rock, a par- ticle size, and lastly, a “symbol of the material of the human body. . .” (Webster’s New Universal Unabridged Dictionary 1979, 335). This last usage appears over and over again in the great literature and poetry of the ages, including biblical writings, and the poetry of Rudyard Kipling, Lord Byron, Thomas Love Peacock, and John Dryden, to name but a few

“Lo, I have wrought in common clay Rude figures of a rough-hewn race . . .”

(R. Kipling, Soldiers Three1888 Dedication Stanza 2.)

“This is the porcelain clay of humankind”

(John Dryden, Don Sebastian1690 Act I, sc. i.)

Scientific data supports the connection  between  life and clay.

“. . .[t]he kind of relatively mild, watery conditions that were presumed required for the origin of life on Earth would inevitably  have  generated  clay  minerals in abundance. In this respect at least, life and clay go together.

(Cairns-Smith and Hartman 1986, 80)

The Clay-Making Process1

All forms of matter on our earth must adapt to their ever-changing environment or be doomed. They search for the most stable form of existence that will put them into a temporary state of equilibrium with their environment. Both the living and nonliving share this goal. Their mutual strivings bear witness to the driving force of perpetual change that underlies our universe, and shapes the destinies of everything contained within it (see Stevens, Wm. K. “New eye on nature: the real constant is eternal turmoil.” New York Times, 31 July 1990, 1–2(C)).

Sedimentary clay minerals are transformations of igneous rocks existing at or near the earth’s surface. Some of these igneous rocks are formed quickly when deeply buried magma flows upwards close to the earth’s surface; other igneous rocks, such as granite, originate from magma that slowly solidifies beneath the protecting top layers of the earth’s crust under conditions of low oxygen, high heat, and pres- sure. Movements within and upon the earth force magma and igneous rocks upwards into the cooler, less-pressurized

environment of the earth’s surface. Here they are first exposed to air, rain, snow, ice, winds, rising magmatic gases, fire, climatic changes, plant growth, and animal behavior— the forces of perpetual change and disruption that restlessly shape and reshape the earth’s surface. In their search for sta- bility in this new, ever-changing environment, igneous rocks change into clay minerals. The geological term for this trans- formation process is weathering. The weathering process is highly complex, and some aspects are still being debated by geologists. The end result of the weathering process, the clay minerals, may require thousands or even millions of years for completion; the sheer length of this process makes it impossible to duplicate in a laboratory. Fortunately, many clay minerals (particularly kaolinite) form more quickly in reaction to high-temperature gases and water vapor. Thus, chemists have created certain clay minerals in the laboratory by subjecting finely ground feldspars to simulated hydro- thermal weathering conditions. In this way, much has been learned about the formation of clays.

Physical and chemical weathering forces of the earth’s environment change igneous rocks into clay minerals. Physical weathering breaks up the igneous rock into its mineral grains of feldspar, mica, and (if a granitic rock) quartz. The mineral grains are then pulverized into smaller and smaller particles and eventually become a primary ingredient of soils, rivers, lake beds, and ocean basins. The increased surface area of these fine-grained particles makes them highly susceptible to chemical weathering.

In comparison, chemical weathering changes the chemical composition and the crystalline structure of the mineral grains. This change is activated by the powerful, dissolving force of water. Water is one of the most powerful solvents on earth and is the essential medium for the birth of all clay minerals. Fourteen percent of a clay mineral is estimated to be water. One hundred pounds of dry clay releases 11 pints of water (U.K. measure), which, in turn, becomes 370 cubic feet of steam during the firing process (Hamer and Hamer 1986, 62). Free water molecules, polarized with negatively charged oxygen and positively charged hydrogen atoms, attract molecules of silica, potash, soda, and other elements contained in the feldspar and mica grains. Water molecules draw these molecules away from their three-dimensional feldspathic and micaceous bonds. Other water molecules, attracted by the remaining silica and alumina molecules, slip into the spaces formerly occupied by the departing silica and potash molecules and realign themselves to form the two- dimensional, layered structure of the clay mineral.

Clay minerals are found in three kinds of watery environments: hydrothermal, continental (soils, rivers, and lakes), and oceans. Mention has already been made of the hydrothermal environment, which creates the most ordered form of the kaolinite clay minerals. Heated water and steam, together with other gases, accompany rising magma and create pure kaolin deposits, such as are found  in Cornwall, England.

Waters of the earth’s atmosphere shape clay minerals found in soils. Atmospheric water, in the form of rain or snow, flows downward into the soil layers. As described previously, the dissolving action of the water removes silica and various metallic ions from the feldspathic mineral grains of the topsoil layers and weaves the remaining molecules into the tightly bonded structure of kaolinite. The watery solu- tion enriched with the highly charged molecules drawn from the feldspathic mineral grains seeps downward toward deep- er layers of soil to form yet different kinds of clay minerals. Among the clay minerals so formed are the extraordinary montmorillonites. The key to life, the transition between the living and the nonliving, is said to begin with this type of clay mineral (see Cairns-Smith and Hartman 1986).

Our earth is mostly water—ocean waters cover 70.8% of the earth’s surface and are the final burial grounds for fine- grained rock particles, volcanic ash, and cosmic dust, which are carried by winds, melted glaciers, and rivers to their oceanic grave. Rivers flowing into the ocean enrich ocean waters with various kinds of dissolved elements and min- erals, such as potassium, sodium, and calcium carbonate. Volcanic eruptions under the ocean floor spew forth a rich assortment of magma and minerals, accompanied by hot reactive gases. Thus, ocean waters teem with myriad assortments of electrically charged atoms (ions), which react with the drifting particles to form various mineral deposits. These particles, together with indigestible remains of plants and animals, drift about ocean waters in accordance with their natural cycle—beginning, ending, sinking, settling. As part of the sinking process, they may float in deeper and deeper

layers of ocean waters for periods of weeks, months, or even years, depending upon their size and weight. Eventually, they will settle on the ocean floor and, after millions of years, build up deep layers of sediments, some of which are deep- sea muds containing rich deposits of clay minerals. These minerals will surface on the earth’s crust whenever ocean waters recede or when “mountain building” tectonics raises the seafloor above sea level, as has happened so often in the earth’s history.

It is believed that the actual alteration of igneous rock particles to clay minerals occurred prior to ocean deposition due to the inhibition of the weathering process in the ocean. The absence of frost, currents, and variations of temperature in the ocean retards physical weathering processes. Chemical weathering is reduced because the high concentration of potassium, magnesium, and sodium ions in seawater pre- vents these elements from leaving the rock particles. Hence, the majority of clay deposits of deep ocean muds are com- posed of previously weathered clay particles. On the other hand, substantial montmorillonite clay deposits form near deep ocean trenches as a result of heated seawater interacting with oceanic basalt and gabbro.

Among the clay minerals found in deep-ocean sedi- ments are the complex, unstable, high-iron clays known as illites, which appear throughout the deep-ocean sediments of midlatitude Pacific and  Atlantic  waters.  In  contrast, the stronger-bonded, stable kaolinite clay minerals, which require heat for formation, are found in marine sediments of warm, tropical waters (Gross 1987, 89–91).

In addition to water and temperature, the composition of the parent rock is an important factor in the clay-making pro- cess. Ash from volcanic eruptions alters to form bentonite clay minerals (see pp. 136–138). Granites of the continental crust, low in iron and magnesium and rich in quartz and potassium feldspar, tend to form kaolinites. Rocks high in sodium feld- spar, magnesium, and iron and low in quartz, which occur in both the oceanic and continental crust, decompose into the complex, unstable montmorillonite and illite clay minerals. However, nature refuses to be packed into neat, tidy catego- ries, and we find granitic rocks altering to montmorillonite and other kinds of unstable clay minerals, depending on the particular environmental conditions that surround the gra- nitic rocks during the decomposition process.

In addition to specific clay minerals, most clays contain various amounts of other minerals, such as quartz, mica, iron, magnesium, and calcium, which were present in the parent rock. They remain in the clay mineral because the process of clay-making is on going and is never completely finished. The continuous, ever-changing nature of this pro- cess accounts for the infinite number of mineral variations that can exist in specific clays at any one period of time.

oxide sTruCTure and CeraMiC FunCTion oF Clay Minerals

The oxide structures of most clays, because of their feldspathic parentage, reveal the familiar trinity of glass- maker (silica), adhesive (alumina), and melter oxides. The specific ratio of these oxides in a particular clay mineral will ultimately determine the ceramic function of the clay and depends on the extent of the clay-making process. For example, the kaolinite clay mineral has undergone a more complete clay-making process and, therefore, contains larger amounts of alumina and lower amounts of silica, iron, potash, soda, and/or calcium than exist in either the parent feldspar rock or in other kinds of clay. This in turn means that kaolinite clays will function as an economical source of alumina in glazes, and as an important ingredient of white, or light-burning, claybodies.

On the other hand, if the kaolinization process is not completed, the oxide structure of the resulting clay mineral will more closely resemble the parent rock. If, in addition to feldspar, the parent rock contains iron and/or magnesium minerals, iron and/or magnesium oxides will also appear in the oxide structure of the clay mineral. Clays that contain predominant amounts of such clay minerals (such as Albany Slip and Red Art clays) function as dark-colored glaze cores, or claybody colorants at the higher firing temperatures and as iron-bearing claybodies at the lower firing temperatures.

solutions, enriched with highly charged hydrogen ions and metallic ions drawn from prior clay minerals, percolate through the soil layers and change pre-existing clay minerals into different kinds of clay minerals. This fact has important consequences for the potter, because it means that no matter how stable they were in the past, all clay sites are highly vul- nerable to change, as deeper and deeper layers of the earth’s crust are mined.

“Even when taken from two closely-separated areas, a particular clay may show considerable variation. The difference between a deep-mined clay and the  same seam under shallow overburden is so well marked that it is often impossible to use the two materials for the same purpose.”

(Grimshaw 1980, 311).

The more stable kaolinitic clay minerals can also change into the unstable montmorillonites by means of this watery process. Thus, although clay minerals remain stable for a long period of time, the transforming pow- ers of seeping, mineral-enriched waters may eventually change all of them into completely different clay miner- als. In addition, climatic changes of increased humidity together with land erosion can force deeply buried clay layers upwards toward the surface, where once  again they will change in response to the demands of a differ- ent environment. And finally, cultivation of the soil will change the kinds of clay minerals that exist in the soil layers of the field.

“A striking change in the plastic behavior of a clay often occurs when it is taken from the land on which there has been extensive cultivation. The fertilizers used in the soil are sufficient to alter the exchangeable cations even in clays at a considerable depth and so change their character.”

insTabiliTy oF Clay Minerals

Over a period of time, the driving forces of nature transform one kind of clay mineral into another. Watery, acidic

(Ibid., 1980, 311).

Thus, clay, the foundation of the potters’ craft, is as ephemeral and transient as human life itself.

CharaCTerisTiCs oF Clay Minerals

Plasticity

The first and most important characteristic of clays is plasticity. The word clay comes from the German verb kleben, to stick to, and that is, of course, what immedi- ately comes to mind when we think of clay. It is a material that “sticks to” the hand and “sticks to” itself in response to the touch of the hand. In other words, the identifying characteristic of plasticity is intrinsic to the meaning of the word clay.

Plasticity refers to the ability of a material to form and retain the shape directed by an outside force. “This is one of the most important of the properties of clay and one which is least understood” (Parmalee 1937, VIII–16). The plastic- ity of most clay minerals derives from the unique crystal structure of their molecules, which are minute in size and platelike in shape. These crystals form flat, two-dimensional sheets that touch each other on only two sides. There is a disproportionately large ratio of surface area to mass in these platy crystals. When water floods these two-dimensional sheets, it creates a strong bond between them in much the same way that a wet, flat plate bonds to a table surface. The water also acts as a lubricant and causes the platy crystals to slide over one another. The strongly bonded, sliding sheets will take on whatever shape they are directed toward by an outside force.

In contrast, the molecules of a nonplastic material, such as the parent feldspar, for example, are larger and touch each other on three points. Water does not create the same kind of bond or lubricating force between the molecules. Add water to a feldspar and try to form a shape with it. Its only response to the hands’ pressure will be to fall apart into a sodden mass. This is the best way to understand the true meaning of the word plasticity, which for the potter is the identifying characteristic of clay.

Particle size

Particle size is an all-important characteristic of clay min- erals and is of crucial importance in the identification of clay minerals. All clay minerals possess a fine-grained, minute particle size. The Encyclopedia of Mineralogy describes this trait as the most significant factor in the identification of a material as a clay.

“Clays are characterized primarily by their small particle size, which is usually taken as less than 2 µm.2 Coarse, medium and fine clays have ranges about 2–0.5, 0.5–0.2, and below 0.2 µm respectively. . . .On this basis any material ground to less than a 2 µm particle size becomes a clay.”

(Frye 1980, 69)

According to the above definition, a material could be classified as a clay even though it does not possess the prop- erty of plasticity. (Note Frye’s reference on p. 69 to Flint clays, which are fine-grained but not plastic. In this clay, the bonds between the crystals are “firmly cemented” so that they do not slide past each other as they do in most other clays.) Although plasticity may not figure in a geological definition, it is everything to a potter. If a material is not plastic, it is not clay, insofar as a potter is concerned. Because this is a book for potters, plasticity is always the first and most important characteristic of a clay mineral.

Subject to the exception noted above, the fineness of the particle size determines the plasticity of the clay mineral. The minute clay crystal contains more surface area and therefore greater water-bonding capacity than the larger particle size crystals of other materials, such as feldspar. Consequently, the bonds between the clay particles and the sliding power produced by water increase in strength as the particles become smaller and more minute. Hence, clays with smaller particle size are more plastic and take on more water than clays with a larger particle size. This has impor- tant consequences for the potter, as we shall see when we consider the characteristics of various ceramic clays.

chemical and crystalline structure

A third identifying characteristic of clays is their chemi- cal and crystalline structure. All clays contain significant amounts of silica (SiO2), alumina (Al2O3), and water (H2O). More important is the fact that the atoms are bonded together in flat, two-dimensional sheets of two or more layers. Sheets of silica, alumina, and other metallic oxides are interspersed with sheets of hydrogen-oxygen molecules (HOH). The top layer consists of silicon and oxygen atoms; the second layer is alumina and (in the case of clay minerals other than perfectly ordered kaolinite) other metallic ions such as magnesium, calcium, iron, sodium, and potassium. The third layer contains the hydrogen and oxygen atoms. It is the second layer of aluminum and other metallic atoms that holds the most interest. This is the layer in which exchanges of aluminum are made with other metallic atoms. The exchange property of the aluminum layer creates the complexity and disorder of some clay molecules, for here, many different atoms can appear. This fascinating exchange property includes the ability to store, exchange, and transfer energy, and it is the reason certain clays are thought to be a possible link between inorganic and organic matter (see Cairns-Smith and Hartman 1986).

metamorPhosis by Fire

A fourth unique feature of clays is their metamorphosis by fire into a stronger material. Most earth materials are weak- ened and broken by the heat of the fire. Clays, on the other hand, exchange their fragile and perishable existence for a hard, durable form that is capable of lasting for thousands of years. They become hard, water-impermeable materials with a new crystalline structure and different physical properties. Though clays lose plasticity in the heat of the fire, they gain permanence in its stead.

conclusion

A layered, hydrous, fine-grained silicate of  alumina with the properties of plasticity in the raw state and hard strength in the final, fired state constitutes the identifying

characteristics of clay minerals; this is what is meant here by the term clay.

kinds oF Clay Minerals

Clay minerals are classified by geologists according to the symmetry of their crystalline structure, and although this approach means little to most potters, there are in fact certain relevant factors that contribute to an understanding of clays.

There are six general groups of clay minerals, and numerous subgroups within each group. (These groups are worth mentioning, if only because of their exotic titles!)

Classification of Clay Minerals3

Kaolinite-Serpentine Group Kaolinite minerals kaolinite, halloysite

(kaolins-ball clays, fireclays) Serpentine minerals lizardite, chrysolite (fibrous)

Clay Micas—Illites

Similar to muscovite and biotite in composition (Albany Slip clay, Cedar Heights Red Art clay)

Smectites

Montmorillonite

Fine-grained, important swelling properties (bentonites)

Vermiculites

Chlorites

(Sheffield Slip Clay)

Allophane

Amorphous clay

Imogolite—may be an early stage of crystallization of allophane.

The first group is known as the Kaolinite-Serpentine group and contains mostly kaolinite. Kaolinite is the major clay mineral in the kaolins, or china clays, which are the basis of the porcelain claybodies, and the main source of alumina for glazes. The purest kaolinites are formed by the hydrothermal weathering of granitic rocks and, conse- quently, have a well-formed, large crystalline structure, low

What is Clay?

capacity of ion exchange, and lower plasticity and iron impurities than the other clay minerals. Also included in this first group is the clay mineral livesite, which has an identical chemical composition to kaolinite, but is finer grained, and consequently more plastic. Livesite is a major clay mineral in the white-burning kaolin-ball clays, which make up the essential core of the porcelain claybodies. Livesite appears in many other sedimentary clays, such as ball clays and fire- clays.

The Serpentine portion of this group of clay minerals results from the weathering of basaltic rocks that are high in magnesium and iron. The clay minerals lizardite and chrysolite are formed by the same hydrothermal processes as kaolinite.

A second group of clay minerals bears the name of Smectites. Smectite comes from the German word smektis to wipe off, or clean, and refers to the cleansing property of removing oils and grease, which is typical of these clay minerals (Webster’s New Universal Unabridged Dictionary 1979, 1713). This group contains the extraordinary mont- morillonite clay minerals, which are said to possess the secret of life. Their name derives from the place of discovery, Montmorillon, France (Webster’s New Universal Unabridged Dictionary 1979, 1166). Unlike kaolinite, their crystalline structure is disordered and subject to innumerable varia- tions. They freely exchange their molecules for those of other materials and are, therefore, the most variable and exciting of all the clay minerals.

A startling characteristic of some clays in this second group is their tendency to swell and increase in size when immersed in water. Bentonite, which frequently appears in low percentages in both glazes and claybodies as a suspend- ing agent, provides a prime example of this feat. I will never forget the day I mixed up some 900 grams of Bentonite with 100 grams of Whiting, added water to the mixture, and watched in amazement as one container of liquid swelled to a volume of almost three containers!

A third group of clay minerals consists of clay micas and are known as Illites and Glauconites. These clay min- erals, like the Smectites, contain a disordered crystalline structure and are a complex mixture of minerals. They are often found in combination with the Montmorillonite clay minerals. Although related to muscovite and biotite micas, they are finer grained, and contain less potassium and more

water. Albany Slip clay, Red Art clay, and other similar iron- bearing lower-fire clays contain major amounts of Illite clay mineral.

A fourth group of clay minerals includes the Vermiculites, which again are similar to Smectites. Vermiculites possess an even higher exchange property, and also swell in water, although their rate of swelling is lower than the Smectites. Potters use Vermiculite to achieve a lighter-weight clay- body.

Chlorites, fibrous Palygorskite, and gel-like Allophane, which appears in some sedimentary clays, make up the fifth and sixth groups. Sheffield clay, which is an iron-bearing, lower-firing clay, contains chlorite minerals.

This classification of clay minerals is impressive in its awesome complexity and creative nomenclature; yet, for a potter, it is more important to know how a particular clay will behave in a glaze or claybody. Thus, the only meaning- ful classification of clay minerals for a potter is one based on the working properties of the clays. There can be many differences, depending upon the kinds of clay minerals con- tained in the clays. That is the reason why most claybodies are a balanced formula of different clays. Each clay contrib- utes one or more of the following qualities needed to make up a good claybody:

l.Plasticity:              How does the clay hand-build and

throw on the wheel?

2.Wet strength:      Does the clay slump when it

takes on water?

3.Dry strength:       Does the clay crack during the

handling or drying process?

4.Firing strength:   Does the clay shrink excessively,

slump, or crack when subjected to the heat of the fire?

5.Firing range:        Is the clay mature or immature at

the desired firing temperature? Will water seep through its pores? Can the clay tolerate a range of firing temperatures?

6.Color/Texture:     How does the clay contribute to

the desired color and texture of a glazed or unglazed piece after firing?

7.Thermal shock:   Does the clay crack or dent

during the firing or cooling cycle?

8.Glaze fit:              Does the glaze crack or shiver off

the fired clay form?

Chapter 2 Clays and Claybodies

In contrast to the requirements for glaze surfaces, it is preferable for a claybody to include many different kinds of clays. The workability and resulting surface of the clay- body is usually better if this approach is followed. Bear in mind that if one of the clays used in the claybody is highly unplastic, it may interfere with the general plasticity of the claybody, despite the presence of other plastic clays. Red Art clay is not as plastic or workable as A. P. Green fireclay, Jordan clay, or Gold Art fireclay. Thus, we suspected that the workability and plasticity problems of the Portchester 5–6 claybody were due to the presence of 25% Red Art clay, notwithstanding the fact that the balance of the claybody was made up of plastic clays, such as A. P. Green fireclay and Jordan or Gold Art Clay.

The fact that the claybody does not depend solely on one or two clays is a good thing in view of the ephemeral nature of clays. East Coast potters have recently had a bitter dose of the perils in overdependence, when Jordan clay sud- denly became unavailable. Suppliers had difficulty finding an acceptable substitute, and as a result, many high-Jordan claybodies were put on hold. A number of years ago, depos- its of A. P. Green fireclay contained sizable amounts of limestone (calcium carbonate), which caused bloats in the fired ware. Pine Lake fireclay was substituted until it too became contaminated with limestone nodules, and once again, bloated ware was the result. In both of these cases, the fireclay constituted a sizable proportion of the claybody. Changes in the mineral content can easily occur in any kind of clay, because all clay deposits are continually subjected to ongoing weathering forces. The use of different clays in a single claybody is one way to minimize the consequences of unavailability and changed mineral content.

tHe FIrIng ProceSS And ItS eFFect on A cLAYbodY4

The question “What is a stoneware glaze?” is answered as follows: A stoneware glaze is a prescribed ratio of glassmaker (silica), adhesive-glue (alumina), and melters. When suf- ficient heat is applied to this trinity, a layer of melted glass is bonded to the walls of the clay form.

The primary mineral source for a stoneware glaze is feldspar, whose chemical oxide structure reflects a similar trinity. The question now to be asked is “What is a stoneware claybody?” A claybody, once again, is a trinity of glassmaker, adhesive-glue, and melter oxides. The ratio of these materials is now altered, because the focus here is on structural form and workability, rather than on surface effects. Alumina—a primary ingredient of the mullite crystal, which provides the strength of the fired clay form—increases in proportion to the melter content in the ratio.

The high-alumina clay minerals are the primary mineral source for a claybody, because workability and fired strength are now the paramount considerations, and only the clay minerals provide these properties. Stoneware claybodies usu- ally contain about 75%–80% clay. The remainder is made up of small quantities of nonplastic quartz, feldspar, mica, and other melter minerals that help the claybody achieve proper fusion and glaze fit at the desired firing temperature. Thus, we are in fact replacing some of the potash and silica that have been removed from the clay minerals by the clay- making process. And because the kiln fire removes the water produced by the clay-making process, it could well seem as though we have recreated the feldspar that gave birth to our clay minerals. However, it takes millions of years of slow cooling at high temperatures to recreate the feldspathic crys- talline structure. Although the fired claybody is in fact stony and rocklike, it has now become a manufactured creation of mullite, partially dissolved quartz particles, and glass. Most important, although the kind of oxides and minerals in the claybody is the same as exist in the feldspar and glaze, the proportion of each has altered because of the changed needs of the potter.

A functional potter needs a claybody that is strong and intact after firing. The accompanying glaze must fit the body; it must be neither too tight (craze or crackled), nor too loose (splinters off and/or cracks the claybody in its effort to remain on the clayform). This optimal result depends on the interaction of four kinds of minerals: clay minerals, feldspars, micas, and quartz. First and foremost are the clay minerals, primarily kaolinite (found in kaolins, ball clays, and fireclays), but also livesite (primary mineral in ball- kaolins), montmorillonite (primary mineral in bentonites),

the Firing proCess and its eFFeCt on a Claybody

chlorite (Sheffield Slip clay), and illite (Albany Slip clay and Red Art clay). Ceramic clays contain some or all of these clay minerals. They also contain varying amounts of feldspar, quartz, and/or mica. Separate amounts of these latter three minerals are often included in the claybody. How much of each is added depends on the amount already present in the clays of the claybody. Each mineral makes a unique contribution toward the creation of a claybody that fits the glaze surface and does not crack or break apart before, dur- ing, or after the kiln firing. Before firing, a ceramic clay is a mixture of clay minerals, quartz, feldspar, and/or mica. After firing, the ceramic clay is a combination of mullite crystals (two molecules of silica bonded with three molecules of alumina; named after the Island of Mull, Scotland5), which gives the fired ware its strength, and three kinds of silica-free quartz, cristobalite (finely divided, highly reactive silica), and silica glass (fast cooled, nonshrinking, noncrystalline, melted silica).

The strength of a claybody lies in the formation of as many mullite crystals as possible. Its weakness lies in an excess of any of the three kinds of silica. An excess of either cristobalite or free quartz will crack the claybody and/or cause pieces of the glaze to shiver off the pot. An excess of nonshrinking silica glass may result in slumped ware and/ or crazing of the glaze, which weakens the ware. Thus, the essence of a good claybody lies in the right proportion of mullite crystals and the three kinds of silica. The achieve- ment of such a claybody requires knowledge about the kinds and grain sizes of the minerals contained in the clays of the claybody. This information appears in the mineralogical analysis of a clay and is more important than its chemical analysis (which gives only the bare bones of oxide percent- ages), because irrespective of amount, the same oxide will behave differently in a claybody depending on its min- eral source and consequent particle size. For example, the silica in kaolinite, montmorillonite, quartz, and amorphous silica each have a different particle size. In addition, the free silica contained in certain kaolinite clays may have a finer particle size than in other kaolinite clays. (Kentucky Stone is an example of such a clay; see pp. 122–123.) Montmorillonite silica and amorphous silica are finely divid- ed and more reactive than kaolinite or most other forms of silica. They both convert more easily to the cristobalite form

of silica, which has such serious consequences for the glaze fit and fired strength of the claybody. On the other hand, there may be enough mica minerals in the montmorillonite clay to inhibit large amounts of cristobalite formation. Only mineralogical analyses give this kind of information.

Despite the importance of the mineralogical analysis, it is rare for a supplier to provide it. The customary data that accompanies a clay shipment from a supplier on the request of the customer usually contains only a chemical and particle size analysis and rarely provides information about the minerals that make up the particular clay. Of the more than 20 analysis sheets I obtained, only one contained a mineralogical analysis, and even this one did not provide percentages. (It is possible to compute the approximate mineralogical structure of a material from its chemical analysis by following prescribed formulas; see Robinson 1981, 9:79; Tichane 1990, 55–57.) In addition, a comparison of the actual chemical analysis of a material with its theoretical chemical analysis would indicate the presence of certain minerals. Thus, 1%–2% potash in a chemical analysis suggests the presence of 15%–30% mica (Tichane 1990, 14–15).

The Firing ProCess

The importance of accurate mineralogical information becomes obvious when we look at the reactions of the clay- body in the fire.

The metamorphosis of a fragile claybody and powdery glaze surface into a durable, hard, permanent form provides a fascinating account of the unseen action within a firing kiln. Just as the toys in the Nutcracker Suite ballet leap and dance after the toy maker closes the door, so too, do the clay, feldspar, mica, and quartz molecules spring to life after the kiln door is shut and the heat rises. Though the exuberant reactions of these materials are hidden from view, the final fired result is proof of the extraordinary transformations catalyzed by the fire.

When the temperature of either a bisque or glaze firing reaches 1063°F, the first important silica reaction takes place. Free silica, or quartz, expands about l% as it converts from alpha quartz to its beta form. This change will reverse itself

Chapter 2 Clays and Claybodies

during the cooling process; as the kiln cools down to 1063°F, free silica will contract 1% as it converts back to its alpha form. At this point in the glaze firing, the glaze is already rigid and set. Hence, the contraction of free silica in the clay minerals at 1063°F will affect the glaze fit of the fired ware. If the glaze is too tight, as evidenced by craze marks on the glaze surface, this would mean that there is insuffi- cient quartz in the clay minerals of the claybody. Additional free silica in the form of Potters’ Flint can be added to the claybody so as to increase the amount of body contraction at the 1063°F cooling temperature. The particle size of the added free silica or of the free quartz contained in the clay minerals is of crucial importance, because a fine-grained quartz will more easily change into noncontracting silica glass or sudden-contracting cristobolite6 and thus will not provide the necessary contraction for the proper glaze fit. As mentioned previously, some kaolinite clays (e.g. Kentucky Stone) possess sizable quantities of fine-grained, highly reactive free silica, which easily converts to cristobalite and thus plays havoc with the glaze fit. Ordinary oven heat does not reach the temperatures of silica conversions; thus, free silica expansion and contraction will occur again only if the ware is refired.

Mullite continuously forms from about 2000°F and upwards. Once this initial temperature is reached, the clay minerals in the claybody release their excess silica in the process of forming mullite. The kaolinite clay mineral, which is the primary mineral in kaolins (EPK kaolin), ball clays (Kentucky Stone), fireclays (A. P. Green), contains six molecules of silica joined with three molecules of alumina. It releases four reactive silica molecules in the process of form- ing mullite. The remaining two molecules of silica, joined with the three molecules of alumina, make up the strong, needlelike mullite crystal, which provides the strength of the fired ware.

Other clay minerals, such as Illites (e.g. Albany Slip clay and Red Art clay) or Montmorillonites (bentonites), would release even greater amounts of highly reactive silica mol- ecules during the firing process.

Both the released silica molecules and the fine-grained particles of free silica are highly reactive. In the absence of sufficient feldspar or mica minerals, they will convert

to alpha and then beta cristobalite silica. The conversion to alpha cristobalite results in a 3% expansion. This expan- sion does not stress the claybody for the following reasons:

1.It is offset by the shrinkage of the claybody due to the ongoing contraction of the clay pores.

2.The body is still pyroplastic.

3.The conversion to alpha cristobalite is a continuous, gradual process that continues up to the end of the firing and does not occur all at one time.

The reverse of this process, during the cooling cycle, is a different story altogether. As the kiln cools down to about 450°F–500°F, the cristobalite formed in the clay- body undergoes a 3% reversible contraction. As the glaze and other claybody materials have long since become rigid, the sudden movement produced by substantial amounts of cristobalite can cause cracking and glaze-shivering problems. In addition, oven temperatures often reach the temperatures of cristobalite  contraction  and  expansion, at which point the cristobalite in the fired ware will, once again, expand and contract. This sudden movement within a fired, rigid clay form can shatter the ware and, even worse, destroy your dinner!

Hence, it becomes desirable to avoid formation of excess amounts of cristobalite. This can be accomplished by converting the reactive, fine-grained silica into inert, nonshrinking and nonexpanding silica glass. The melting power of the feldspar and mica minerals in the claybody activates this conversion. (See Chapter 1, pp. 38–40.)

Feldspar begins its melt at about 2200°F (cone 4), and thus functions as a melter in the stoneware claybody. It draws some of the glassmaker silica into the melt to create highly desirable silica glass, which, in the right proportion, renders a claybody vitreous and strong. The more silica glass formed, the less silica there is available for the formation of cristobalite, which threatens the strength and glaze fit of the ware. Thus, feldspar, although no longer the star player, is an important part of a claybody. Mica functions as a melter in a claybody in the same manner as feldspar.

Here we see the importance of the ratio of the four min- erals that make up the claybody—clay, feldspar, mica, and quartz. The amount of each that appears in the various

the Firing proCess and its eFFeCt on a Claybody

clays of a claybody affects the strength and glaze fit of the fired claybody. Wherever possible (depending on the avail- ability of mineralogical data) this ratio will be a focal point in the description of specific ceramic clays.

Clays, Claybodies, and Their FunCTion as glaze MaTerials

Most clays result from the transformation of feld- spathic materials, and therefore, it is not surprising that their chemical structure contains that familiar feldspathic trinity of glassmaker silica, adhesive-glue alumina, and melter oxides of sodium, potassium, magnesium, calcium, etc. However, the proportion of these oxides will differ depending on the kind of clay mineral contained in each clay. This proportion is crucial and ultimately determines the ceramic function of the clay. Some clays contain large amounts of the clay mineral kaolinite. Their oxide ratio would reveal a high proportion of alumina and a cor- respondingly low amount of melters and iron impurities. Kaolinite clays become the core of light-burning claybodies and are a cheap source of alumina for glazes. Other clays contain large amounts of illite and similar iron-bearing clay minerals. Their oxide structure contains high amounts of silica and melters and, except for iron and other impurities, bears a close relationship to the feldspars. Although these clays can, and often do, function as iron-bearing claybod- ies at the lower firing temperatures, they make wonderful, dark-colored glaze cores at stoneware temperatures. Figures Intro.3–4, pp13–14.

It is possible to transform any clay, even one with high amounts of kaolinite, into  a  glaze  core  by  adding  melt- ers and quartz. It is important to remember that the terms claybody and glaze core are not absolutely fixed. They do not, once and for all, identify specific  ceramic  materials. They are functional terms, and the materials that they identify often change their roles, and thus their classifica- tion, depending on the firing temperature and the balance of the materials in the formula. For example, the fireclay

A.P. Green is not always just a claybody material. With the right addition of melters and quartz, it can function as

a glaze material (see Fireclay Tests p. 119). A porcelain claybody made up of kaolin clay, quartz, and feldspar may become a fine white slip, and even a glaze, with the additions of water and/or melters. Similarly, most clays can become part of a glaze or claybody if you adjust the temperature of the fire or if you add melters. Figure Intro.4, p14. This is a magical area where all things are possible once you know the oxide ratio and mineralogical structure of your materials. There are no constants—no absolute rules in the ceramic process, other than KNOW YOUR MATERIALS AND TO YOUR OWN SELF BE TRUE.

If you achieve this knowledge, then you can conjure glaze- cores out of claybodies, transform clay-glaze cores into claybodies and have a wonderful time in the process.

Though the discussion of clays that follows stresses their general and most typical function, never forget that these are NOT the only ways to use these materials. Try less- traveled paths and attempt some creative transformations of your own.

ConClusion

Clay minerals are transformations of igneous rocks. These transformations occur in response to their new, ever-changing environment at the surface of the earth. They are as complex and varied as the crust of the earth itself. We can never ponder this fact too long. The chemi- cal reactions that follow, which trace the alteration of actual soda and potash feldspars into three different clay minerals, show the complexity of the clay-making process and the resulting clay minerals (Birkeland 1984, 69; Blatt 1992, 32).

Potters, do not throw up your hands and avert your eyes in horror at these equations. They are NOT given to instruct in technical chemical reactions; on the con- trary, they are repeated here because merely to view them shows more vividly than words the remarkable length and intricacy of the clay-making process. Consider this awe- some process as you handle that humble, unassuming, yet incredibly complex material known as clay.