What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
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, it 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 dead micro-rocks).
Clays have plasticity. This property is a product of the fact that the surface chemistry attracts 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 can be viewed at '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 on hydration). 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 (where most particles are clay itself). There are thousands of places on the planet where you can easily see mountains being worn away and eroding down on the the valley or flat lands around them. The planet is sand, gravel and clay making machine, mountains constantly pushing upward and erosion wearing them down.
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 minerals particles. Each particle type has its own mineralogy, chemistry and physics. And its own solubility and thermal stability properties. 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 overall chemistry of an new and unknown clay material to firing behavior in a body, which does not melt, (e.g. temperature of vitrification, color, strength, efflorescence) can thus be quite misleading. It is much better to think in terms of physics: properties that you can measure and rationalize. Often the data sheets 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?
SHAB test bars, an LDW tester for water content and a DFAC test disk about to be put into a drier. The SHAB (shrinkage-absorption) bars shrink during drying and firing, the length is measured at each stage. The LDW sample is weighed wet, dry and fired. The can prevents the inner portion of the DFAC disk from drying and this sets up stresses that cause it to crack. The nature of the cracking pattern and its magnitude are recorded as a Drying Factor. The numbers from all of these measurements are recorded in my account at Insight-live. It can present a complete physical properties report that calculates things like drying shrinkage, firing shrinkage, water content and LOI from these measured values.
Electron micrograph showing Dragonite Halloysite needle structure. For use in making porcelains, Halloysite has physical properties similar to a kaolin. However it tends to be less plastic, so bodies employing it need more bentonite or other plasticizer added. Compared to a typical kaolin it also has a higher fired shrinkage due to the nature of the way its particles densify during firing. However, Dragonite and New Zealand Halloysites have proven to be the whitest firing materials available, they make excellent porcelains.
These are the results of a detailed elemental composition analysis of a sedimentary clay. The first column of numbers is ppm (parts per million), divide them by 10,000 to get percent. The Fe here, for example, is 50,868 or 5.1%. The second column is +/- error. Notice that this test does not detect boron or lithium, they require a different method. By contrast, the chemical analysis shown on the data sheet of a typical ceramic material shows only the principle ceramic oxides (less than a dozen), but all of these trace elements will still be present.
When companies ship materials they often include these with the shipment. The information reported is often very basic and properties important to ceramics are often not found.
The ppm items are not oxides, they are elements. Ba for example, is shown as 4276 ppm. We do not know the form. It could be barium sulphate, barium carbonate, barium nitrate, barium chloride. But altogether they supply this amount of Ba. The same is true of chrome, strontium, nickel and vanadium.
A term used by potters and in the ceramic industry. It refers to the earthenware, stoneware or porcelain that forms the piece (as opposed to the engobe and covering glaze).
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.