Clay

What is clay? How is it different than dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.

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The term 'clay' is used in different ways. Potters often refer to their 'clays' - these are typically recipes or mixtures of clay minerals, feldspar and quartz (more correctly, these are clay bodies, or just bodies). A lump of the material that has been mined from a deposit is also referred to as 'a clay'. However, in its most common use, the term 'clay' refers to flat microscopic particles from which that lump is composed (assuming it is a typical clay mineral). Actually, even more precisely, it refers to 'some of the particles' (since the lump will invariably have particles of many other minerals also). Clay particles have a surface chemistry that imparts an affinity for water (the other particles interact with water far less).

Clays have plasticity. This property is a product of the fact that the clay mineral particles attract water. The particles have surface charges that can adsorb (hold) a thin layer of water molecules onto the surface. These charges also create particle interactions. The water thus becomes a kind of glue and a lubricant that gives billions of particles the opportunity to express that collective property of plasticity. While we can view clay particles as 'water magnets', this is the main, but not complete, mechanism of plasticity.

Clays are born when parent rocks, referred to as 'clay-making minerals'. They do not simply break down physically (by weathering) and insert whole water molecules (grinding feldspar finer does not itself turn it into kaolinite). Chemical weathering alters the parent minerals through processes including hydrolysis, dissolution, and leaching. Physical breakdown exposes surfaces and transports material. Clay minerals can also form through hydrothermal alteration and changes after sediment burial. While water can be adsorbed on particle surfaces, molecular water can also exist between layers in minerals such as smectites. However, kaolinite contains structural hydroxyl groups (OH) within clay mineral crystals.

From a mineral point of view, clays are hydrous sheet silicates, commonly containing aluminum. Many clay minerals exist. The kaolin powder we buy is the purest clay mineral (having a very high percentage of kaolinite); its theoretical chemistry is Al₂O₃·2SiO₂·2H₂O. The ball clay sold is kaolin-rich but with significant other minerals mixed in. Other clay minerals include illite, montmorillonite, smectite, and halloysite. These vary by particle size, shape, surface electrolytics and mechanism of plasticity. Clays used in ceramics have a wide range of purities; they are usually mixes of the above (unless highly purified).

The wide range of particle sizes, shapes and mineralogies are related to the identity of parent rocks, mode of conversion and whether they are primary (on site of alteration) or secondary (moved by water or wind). These factors produce different tactile properties, plasticities, drying rates and shrinkages, dry hardness and strength, behavior in slurries, etc. (physical properties).

Primary clays, are formed at the site of alteration. They are often hiding in what appears to be gravel or sand (or a mix). Screening out the sand and gravel (e.g. at 200 mesh) can remove coarse impurities and concentrate the clay (assuming the quartz, mica, feldspar present are coarser than 200 mesh). There are thousands of places on the planet where one can see mountains being worn away and sedimented on flatlands around them. The planet is actually a sand, gravel and clay-making machine. Mountains constantly push upward and erosion wears them down.

Sedimentary clays, by contrast, have been transported, often hundreds of miles. During the move, usually by water, Mother Nature often grinds them finer but also deposits particles selectively. There can also be further weathering, mixing with other mineral particles, aggregation and later mineral alteration.

Each particle type has its own mineralogy, chemistry and physics. And its own solubility and thermal stability. It follows that an overall chemistry of a secondary clay (that, as noted, is a mix) is insufficient to describe it (unless the clay is being melted in a kiln and thus decomposing in the melt). An oxide analysis is useful, but it cannot by itself predict how an unfamiliar clay will work, dry or fire. Similar analyses can conceal important differences in mineralogy, particle size and soluble salts. For practical body development, chemistry should be interpreted alongside mineralogical information and physical properties that can be measured. Often the datasheets of clay suppliers can even miss the point of what their material really is; they do this by providing only the chemistry. But this does not answer questions like: Is it plastic? Does it have a fine particle size? Does it have a high soluble salts content? What is the porosity and fired shrinkage at various temperatures? What is the particle size distribution? What are the drying properties? What does it look like when fired to various temperatures?

Potters sometimes express the sentiment: “What the clay is used for is entirely up to the clay; I'm just here to help it along.” That is partly true if one attempts to use a single natural clay and a specific process to make ware. But clay bodies are mixtures that combine the strengths and dilute the weaknesses of the pure clays. They also add non-clay materials to further tune properties to an intended use. A specific pure clay can thus find itself used in many product types.