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.

Details

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 strictest sense, the term 'clay' refers to flat microscopic particles from which that lump is composed. 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 are just micro-rocks).

Clays have plasticity. This property is a product of the fact that the particles attract water electrolytically. The water thus becomes a glue and a lubricant that gives billions of particles the opportunity to express that collective property of plasticity. Simplistically clay particles thus can be viewed as 'water magnets'.

Clays are born when parent rocks, referred to as 'clay-making minerals', break down physically (by weathering) and hydrate to form new mineral particles with new properties. This hydration involves insertion of complete water molecules into the crystal structure (whereas with most minerals oxides are converted to hydroxides). From a mineral point of view, clays are hydrous-layer silicates of aluminum. Many clay minerals exist. "Kaolin" is the purest clay mineral, its theoretical chemistry is Al₂O₃.2SiO₂ (one part alumina oxide, two parts silicon dioxide). Other clay minerals include ball clay (the most common), illite, montmorillonite, smectite, 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, found within eye-shot of the site of alteration, 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 reveal very pure clays. 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. It is also a water-making machine, volcanoes release it (because a significant percentage of lava is water).

Sedimentary clays, by contrast, have been transported, often hundreds of miles. During the move, mother nature grinds them to finer and finer particle sizes. She also mixes them with many other mineral particles. 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) does not really have much significance unless the clay is being melted in a kiln and thus decomposing and yielding its oxides to the melt. So, in ceramics, connecting the overall chemistry of a new and unknown clay material to firing behavior in a body, which does not melt, (e.g. temperature of vitrification, color, strength, efflorescence) is thus misleading. It is much better to think in terms of the physics: properties that can be measured and rationalized. 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 pure 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. And 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.