An Introduction to the Production of Clay Ceramics

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This chapter is intended to serve as a general introduction to the process of refining ore into finished clay; as an introductory overview, this chapter lacks nuance, specificity, and details – all of which are covered in later chapters specific to each step in the production of zisha teapots.

A common mistake made by many individuals and scientists, from outside the field of ceramics, is the misguided focus on the total proportion of each compound within the fired clay, such as in the chart below:

Such work is misleading because the physical structure[1] of the compound forms different materials with unique attributes – while chemistry is predictive of the qualities of a glaze, it is the physical structure of the clay that determines its attributes[2]. The transformation of ore into clay is better modeled as a transformation of meta-material or a material of materials: a complex outcome of chemistry, minerology, and processing. Any ore and clay can be analyzed at the level of its processing steps, the separate mineral component, or the compounds that make up each component.

Ore

Earth’s crust is an egg-shell thin layer[3] on which our continents and oceans rest; formed by layers of deposition, erosion, and movement over Earth’s ~4.6 billion years, the crust includes a myriad of rock formations with unique and striking properties – from granite mountains to limestone caves. The interaction of Earth’s early crust-formation with the forces of nature including wind, precipitation, ocean-tides, glacial-retreat, and river-flow have yielded a variety of materials naturally found throughout the world.

The geologic term “clay”[4] refers to a particular type of soil formed from the breakdown via weathering of silicate-bearing[5] igneous rocks (such as granite or basalt) and its subsequent sedimentation either close to its origin (primary clay) or after dispersal by the forces of nature (secondary clay)[6]. Raw Clay is predominantly composed of aluminum and silicon with impurities[7] of other compounds, each of which can yield distinct characteristics in the raw or processed material. Rocks of different composition, weathered in the presence of different organic matter, under different climactic conditions, will produce clay with different and unique properties. Clay may be formed though physical weathering, which changes the particle size and ratio of clay-minerals within the material, or through chemical weathering, which creates new minerals via deposition and recrystallization into a new matrix.

The clay-rock-clay cycle is partially explanatory for the overlapping chemistry within the components of clay-ore; clay is transformed into different rocks under different conditions: with heat it metamorphosizes into argillite, under heat and pressure it metamorphosizes into slate, which under additional heat and pressure will form phyllite, schist, and gneiss. Any of these sedimentary metamorphic rocks formed from clay can melt to form magma, which forms granite when cooled. Granite may then be weathered back into (geological) clay, starting the cycle anew.

While Geologists use the term “clay” to refer to the terrestrial deposits of clay-soil generally, the colloquial usage of the term by ceramic artists render the word confusing; ceramists use “clay” to refer to the processed material inclusive of any modifications ready for shaping into usable wares (“processed clay”). As previously stated, this book will use “clay”, outside of this section, to refer exclusively to the processed zisha material ready to be shaped or fired, and “ore” to refer to geological deposits of all types of zisha material as it is found in the ground. Ore is any natural rock or sediment that contains valuable minerals including metals and clay; ore is rarely pure material, and the valuable components thus must be separated and refined. In this sense, ore refers to the natural state of the clay-soil interspersed with impurities and other materials, until it is rendered into clay through its refining process. This use of terms better reflects the local usage of Yixing artisans and clarifies the materials stage of processing under discussion.

Processed Clay

Ore and its resulting clays are a geological material with unique properties so common as to seem mundane: ultrafine-particle size[8], high levels of plasticity when wet, firm or hard when dry, and strengthened by firing. The level of plasticity, texture of the clay, and reaction to firing are all unique properties that vary depending on the clay-origin, -processing, -additions, glaze (if any), and firing technique used to turn clay-ore into a usable ceramic ware.

Ore is turned into clay through a series of processes (“clay processing”) that yields a homogenous (or near homogenous) material with the desired consistency and texture. The chemical and structural composition of clay is similar to the primary materials from which the clay originates; control of the composition through processing can purify, modify, or introduce new compounds into the clay mix to adjust the resulting attributes of the finished material. While the refining process varies to match the qualities of the ore with the desired ceramic attributes, there are common steps shared though much of the world’s historical ceramics centers.

In summary: ore is cleaned to remove unwanted impurities, dried to prepare the material for milling, milled[9] to turn the ore into powder, and sieved to remove oversized pieces that survived milling (most of which can be re-milled). The powdered ore is mixed with water and allowed to sit[10], before being worked to a cohesive consistency into the first formation of clay. The clay is often aged to increase coherence, allow dissolved salts to rise to the surface, and break down remnant organic matter. The aged clay is re-worked[11] a final time before shaping into a ware; during the final re-working the clay may be mixed with milled-ore, other clays, clay additives, grog[12], or any combination of these to adjust and modify the material’s texture, color, or shapeability.

Glaze

Glazes are a waterproof coating from a large variety of possible materials, fused to the clay body. Glazes can be transparent or opaque, clear or colorful, and smooth or textured – depending on the glaze material and the interaction of the material with the firing atmosphere. While glazes are a focus of most ceramic arts, Yixing is notably unglazed[13] and are thus outside of the scope of this book.

Firing (Sintering and Vitrification)

The firing process hardens clay into a durable and often beautiful material. The control of the firing process includes (at least) the atmosphere, the duration, the temperature, and the fuel – in addition to a specific ware’s placement within the kiln and its exposure to atmosphere and ash.

Ceramics must by fired to maturity[14] to achieve a usable state, such that they will not slack (dissolve) in water or break down naturally due to moisture intrusion. The definition of maturity varies with the intended use of the ceramics – ceramics intended for decorative or light use, such as art and interiors, may be low fired or fired for a shorter time; ceramics intended for use as teaware tend to require a longer and hotter firing-schedule[15] to achieve their high level of impermeability, strength, and glaze effects (if glazed).

The firing process increases the density and strength of clay through sintering, vitrification, or both sintering and vitrification[16]. Both processes can be and often are interrelated, occurring at the same time or in quick succession during the firing process. The distinction of melted versus not melted is important as certain materials (such as glazes) melt while others retain their form in the heat of the kiln (below a certain temperature).

All mature fired ceramic wares are sintered: the state of bonding through densification, without fusion through melting. Before firing, the clay is held together by weak temporary hydrogen bonds, allowing the material to adhere to itself (when hydrated within its range of usable moisture content) while remaining shapable[17]; during firing, water is removed from the clay body and the weak temporary hydrogen bonds are replaced by stronger permanent covalent bonds, strengthening and permanently hardening the material. Sintering can occur at temperatures as low as 1,100°F (593°C) and forms materials of different strength depending on the blend of compounds present and their particle size distribution.

In addition to sintering[18], vitreous wares are bonded via melting and the formation of glass and mullite crystals within the porous matrix of a sintered ceramic; the body of clay wares contain a mix of refractory materials[19] and melting materials which serve to fill the pores of the ware[20] to various degrees depending on the availability of melt-material within the clay. In the field of ceramics, “vitreous” refers to the range of physical states including any level of melt and crystal formation, “vitrification” is the process of melt and crystal formation within the kiln, and “vitrified” is the complete end-state of vitrification[21]. Fully-vitrified ceramics have low-to-no porosity, high shrinkage, and low resistance to thermal shock (rapid heating and cooling); Yixing teapots are sintered and vitreous but not vitrified.

Properties of Ceramics

Plasticity

Plasticity is the ability of a clay to form a new shape without an elastic-response towards the prior shape: a low-plasticity / high-elasticity clay will attempt to contract after stretching; elasticity is thus the opposite of plasticity in ceramics. Plasticity is mainly a function of particle size, with finer particles creating more plasticity and less elasticity.

Particle Shape and Particle Size Distribution

The particle shape and particle size distribution of processed clay is a primary determinant for the plasticity, strength, pore size, and texture of the material; control of the distribution is necessary for balancing the tradeoffs between attributes to achieve the desired qualities of the fired ware. Different ores and variations in processing techniques influence the size and shape of the naturally formed particles[22].

Contemporary ceramics are sieved after milling[23] to achieve a maximum particle size. The distribution of particle sizes after sieving are naturally normally-distributed, with a notable long tail of very fine particles[24]. Ceramic bodies with a wider normal-distribution of particle sizes are generally stronger than bodies containing a uniform distribution, while bimodal distributions of particle sizes (a mix of very fine clay and large-particle sand, for example) are weaker with a tendency to form micro cracks, leading to a dull sound when tapped. The average size of the distribution also has an effect: clay composed of a finer (smaller) average particle size is more plastic and easier to shape[25], while clays of larger particle size exhibit increased strength and reduced shrinkage.

In addition to the particle size distribution of clay, the shape and surface topology of each clay-component affects the attributes of the raw and fired material. Surface roughness varies with ore, processing, and firing: while particle size plays a minor role in the level of roughness, the shape and orientation of the particles can either cohere in an organized plane or orientate randomly leading to a rougher material (even with a finer particle size); for example, kaolinite forms oriented flat plates which yield a very smooth surface texture in comparison to other clays, despite their larger particle size.

Porosity

Porosity is the open space distributed throughout the fired ceramic material. Clay naturally forms various pore structures ranging from solid and nearly zero-porosity to a bread-like internal matrix, depending on the ore, processing, and firing.

Both densification and vitrification reduce porosity: firing ceramics above sintering temperature (usually) reduces the total volume of open pore space as the ware shrinks in the kiln and the pores are filled with vitrified melt (leading to greater strength of the material). Over-firing may ignite organic materials within the clay, forcing expanding gas to boil to the surface – a flaw called bloating. Bloating creates its own pore structure and acts as a limiting factor in the minimum porosity of many clay materials[26].

When pores are present, they may form a connected internal network with openings to the surface. If the pores are open to the surface, water can enter the internal matrix (an open pore structure); if the pores are sealed, either with clay or glaze, the matrix may be impervious (a closed pore structure). A closed pore structure can appear in most types of clay, even those which naturally form a highly connected pore-matrix, when vitrified material flows into and seals some proportion of the network. Thus, the majority of fired ceramic materials exhibit a natural ratio of open to closed pores which determines the overall porosity of the ware.

Water intrusion into the internal matrix of a ware’s pore structure can pose a problem for the structural integrity of the ceramic material, depending on the degree of sintering and vitrification[27]. It should be noted that even closed-pour structures can face water intrusion in very long-duration wet environments, such as a shipwreck ware.

Shrinkage

Clay shrinks twice: first during drying after the clay is molded, and again during firing. When discussing shrinkage (or contraction) rate, it is important to separate the drying shrinkage which occurs due to water loss before firing from firing shrinkage which occurs in the kiln; most contraction rates listed in this book and elsewhere only measure firing shrinkage[28].

Drying shrinkage occurs after the ware has been shaped and is allowed to harden via the loss of moisture to the atmosphere. Different clays dry at different rates, ranging from hours to days; in most clay, the majority of shrinkage occurs in the initial drying phase as water present near the surface is easily lost to evaporation. Many factors play into the rate and amount of dry-shrinkage a clay exhibits, including the average particle size, the particle size distribution, the particle shape, the mineralogical balance, and the surface chemistry; while generalizations lead to intuition, there are multiple exceptions and counter intuitive interactions that affect the level of dry shrinkage exhibited by the range of clay materials.

During the drying process, plasticity declines with water content until the ware has hardened; if the rate of drying occurs unevenly throughout the ware, hardening and shrinkage will progress at different rates causing the buildup of stress which can lead to dry cracking. Wares must be protected from uneven drying to prevent dry-cracking[29]; many ceramic centers dry their wares under a cloth cover, which may range from dry to damp or even saturated with water. Other ceramic centers shape components of the ware separately[30] and “glue” them together with very moist clay or slip[31]. Both cases are in common use for wares of complex shapes differing in size and construction; for example, the handle of a teapot is smaller and attached to the body after shaping, which can cause it to dry faster than the rest of the ware. Cracking is most common along “gradients”, or anywhere there is a change in thickness or a joint between pieces (for example, a handle or knob), as the gradient can cause the thinner pieces to dry faster than the thicker parts of the ware.

Clays with finer (smaller) average particle size[32] exhibit greater plasticity and higher dry-shrinkage; balancing the mineral components of the clay can lead to better plasticity and less shrinkage, reducing the risk of dry cracking. The addition of sand or re-milled fired-ceramic (“grog”) of the right particle size[33] will slightly improve drying performance at the expense of dry-strength, in addition to coarsening the texture of the fired ware.

Firing shrinkage is not a linear function of temperature, but a step function of firing schedule: any temperature above the sintering temperature will cause densification of the material; firing shrinkage is thus correlated to strength, as higher levels of shrinkage in the kiln are indicative of greater levels of densification yielding a stronger ware[34].

In addition to densification, wares with a higher proportion of vitreous material shrink more during firing[35]: the total level of shrinkage is largely determined by the amount of feldspar and other flux which form vitreous glass and mullite crystals of high density – resulting in a stronger, denser, material of melt-glass filled pore structures.

[1]Specifically crystal structure; the same compounds in different ratios form different crystal or non-crystal structures with different interactions and physical properties. For example, pure carbon can make diamonds, graphite, carbon nanotubes, Buckminsterfullerene (a buckyball), and many other structures; a single element arranged differently can exhibit wildly different physical and chemical attributes.

[2]The total percentage of silica in a Yixing teapot, for example, isn’t meaningless, yet it isn’t quite meaningful.

[3]Composing less than 1% of the volume of the earth.

[4]Clay is (confusingly) multi-defined depending on the field of study; within geology or sedimentology it is usually defined as both an amalgamate material within a range of particle size, and a material composed of clay-minerals (which are naturally within the expected clay particle size range).

[5]A mineral integral to the formation and properties of clay. Silica is very common across Earth’s terrestrial rock.

[6]Where it may undergo additional weathering different from the weathering which broke down the original material.

[7]Any non-silicate compound within clay is considered an impurity, in geological terms, even if desirable for the manufacture of ceramics.

[8]Any predominantly clay-mineral soil with an average particle size less than 2μm (0.002mm) is “clay” in geological terms; processed clay is alternately defined as an average particle size less than 4μm (0.004mm) in ceramic terms.

[9]A form of grinding; the adjustable particle size distribution from milling has a large impact on the attributes of the clay.

[10]Many processing techniques call for a cleaning step, where either some or all of the water is changed; or where impurities that float to the surface of the ore-water slurry are skimmed from the top.

[11]Usually a manual process of beating the clay flat, folding it over itself, and repeating the beating; the goal is to re-mix the clay into both a homogeneous material and texture.

[12]Milled-ore or fired-and-re-milled clay; grog may be added to the clay either before or after aging for various effects.

[13]With few exceptions.

[14]The field of ceramics uses terms-of-art with specific definitions that differ from other fields (such as glass); all terms here are used with their ceramic definition.

[15]A Firing-schedule is the time-temperature curve within the kiln.

[16]Sintered ceramics are not necessarily vitreous.

[17]This is one of the novel wonders of the material: clay naturally forms structural sheets that adhere to each other yet remain shapable; the sheets harden with the removal of water (becoming brittle before firing) and fuse through densification in the kiln.

[18]In practice, all vitreous wares are also sintered, with the degree of vitrification controlled by the temperature of the kiln and the properties of the clay.

[19]In ceramics, a “refractory” is any material which doesn’t melt at normal kiln temperatures. The bricks used to construct the kiln are, for example, refractory.

[20]This explanation does not consider the melting properties of glaze which is considered separately within the field.

[21]These terms cause great levels of confusion when discussed in terms of Yixing teapots both because of the closeness of the related terms (vitreous versus vitrified) and because of the specific definitions used in the field of ceramics.

[22]Clay is preprocessed by nature – the particle sizes of the clay-minerals are already small (definitionally) from weathering and deposition, with inclusions of various sizes that must be ground into a usable state (milling).

[23]Material left over in the sieve (too large to pass through the filter) are either re-milled and re-sieved or discarded; clay is usually ground to ~200 mesh (a very fine powder).

[24]The reasoning for a normal distribution post-sieving is not obvious. Many individuals harbor the incorrect assumption that the sieve achieves a hard limit on maximum particle size, resulting in a half-normal distribution; in actuality, the square mesh of the sieve can only filter on the smallest-axis of the particles, which are generally spheroid or ovoid. Particle size is measured on the longest axis, and thus oblong particles form the symmetric larger-than-mesh side of the distribution.

[25]Because smaller particle size clay has a greater surface area, leading to more plasticity.

[26]While porcelain, the densest common ceramic, can be fired to near zero porosity, stoneware (including zisha) have a larger average particle size and cannot (generally) achieve less than 3% porosity.

[27]Yixing teapots, porcelain, and other high fired wares are completely sintered and do not slack in water (under most conditions). Usable wares of unchanged material-strength have been recovered after a few centuries in seawater.

[28]From the molded ware’s pre-firing dry state to its post-firing sintered state.

[29]or, with highly plastic clays, deformation.

[30]With an attempt to match moisture content of differently sized components.

[31]A clay and water slurry of variable viscosity made from either the same or different clay as the ware body; multiple uses, including as a coating layer, a filler, a method of coloring details, or a clay-glue.

[32]And thus, with greater surface area, allowing for more intra-particle water absorption.

[33]Smaller particle size grog reduces overall shrinkage, larger particle size grog may help prevent the development of cracks.

[34]With the exception and limitation of over-fired wares, which may bloat, negating some of the effects of shrinkage and strengthening from densification.

[35]vitreous melt particles are “more shrunk”, on average, than sintered materials.