Clay: The Shaped Connection Between the Arts, Sciences, and Life
Illustrated by ASHLYN VO
Clay is much more than mud. It is special, extraordinary, even magical. To a child it is a fun “goo” that tickles tiny fingers or toes as it squishes between them. To a potter it is a medium that can be shaped into works of art and function. To an engineer it is a material that insulates, binds or supports. But surprisingly, clay is much more than a substance that can be formed and transformed. Indeed, it is a substance that forms and transforms. It has transformed our societies and shaped our past and it may be that in the relentless process of evolution, it formed us.
What is clay?
The word ‘clay’ comes from the German word ‘kleben’ which means ‘to stick to’. Clay is a common name for a large family of complex minerals that form fine-grained earthy materials that become soft and malleable when wet. The unique properties of clays come from both their makeup and structure.
Clays are made of tiny molecules that bind together into larger, but still microscopic, sheet-like structures. These sheets come in two main varieties: tetrahedral and octahedral sheets. Tetrahedral sheets are made up of silicon (Si4+) and oxygen (O2-) (Figure 1). Octahedral sheets are made up of hydroxide (OH-) and either aluminium (Al3+) (Figure 1) or magnesium (Mg2+).
There are many types of clay minerals—kaolinite, smectite (montmorillonite), illite (mica), vermiculite and chlorite for example. They result from the weathering of various minerals in rock. The different clay minerals are made up of various
combinations of tetrahedral and octahedral sheets. For example kaolinite is made up of one tetrahedral sheet plus one octahedral sheet (1:1 layer) while smectite and illite are made up of two tetrahedral sheets and one octahedral sheet (2:1 layer). The space between each layer (e.g. 2 tetrahedral sheets plus 1 octahedral sheet) is known as the interlayer (Figure 2).
It is the type of bonds, the arrangement of the layers and the replacement of atoms with another of similar properties (isomorphous substitution) in these layers which determines the type of clay and its physical and chemical characteristics. For instance some clays expand and are extremely sticky and plastic when wet, others are slippery and plastic while others do not expand and are not sticky when wet. Clays are rarely found separately. They are mixed with other clays and can contain microscopic crystals of quartz, mica, feldspar, and carbonates.
Clay – Function, Form and Art
Clay is plastic, which means that it can be shaped, formed, rolled, cast, pressed, twisted, bent and endlessly manipulated and still hold the form or shape it has been given. Baking clay at high temperatures however, turnsclay into a hard, durable and rock-like material. Indeed, baked – or fired clay is often called “stoneware” because it is as hard as rock. Because of these properties, and its abundance around the globe, clay has played a defining role in shaping human societies of the past. As a material for both engineers and artists, it has given us artifacts of form and function and it has served as a medium for artistic expression.
The extent to which clay has played a role in art, engineering and society is difficult to appreciate in our modern era when materials like metals, plastics and glass are predominant and when we take shelter and nourishment for granted. For almost all of human history, clay pots and clay ovens were the only widespread and practical ways to cook and bake. Similarly, clay bricks, cement and lightweight aggregates were crucial in building structures when wood, skins or rock wouldn’t do. And then there are other, sometimes surprising ways in which clay was used. One example that remains somewhat mysterious to historians, is what has become known as the “Baghdad Battery” and what appears to be a working battery! Discovered in what is now Baghdad, Iraq, this artifact dates back to 200B.C. which is two thousand years before western science mastered electricity. Replicas have generated voltages up to 2 volts (which is similar to a typical D-type battery) but with minimal currents. What remains a mystery about the Baghdad Battery is what it was used for. People have speculated that it might have been used medicinally, for electroplating or for electrifying statues so that anyone who touched it gota small shock or a magical sense of tingling, but no one is certain.
A contemporary use for clay is as an absorbing material and in particular as a medium for absorbing toxic materials. Absorption of water and other substances by clay is a result of the structure of the clay sheets. Depending on the composition of the tetrahedral and octahedral sheets, a layer will have no charge or will have a net negative charge. The negative charge on the surface of clay sheets attracts positive ions (cations). The ability of clay minerals to hold cations is called the “Cation Exchange Capacity” (CEC). Water molecules are electrostatically attracted to those ions, so they penetrate between the layers and surround the alkali ions – like sheets of paper that attract and trap water between them. Importantly, the amount of material that can be absorbed depends on the surface area between the sheets. The larger the area, the more material can be absorbed. Incredibly, one gram of montmorillonite has a surface area of about 800 square meters – that’s the size of a small parking lot!
For artists, sculptors or potters who use it, clay remains a medium that straddles both art and science. To work with clay, it is important to know how it transforms with heat and reacts with glazing compounds. In addition, it is often important to know its heat tolerance, potential density after firing, mechanical strength, its absorbency, and even its thermal expansion. Much of this can be known, as it has for centuries, through traditional practice, but the science and engineering of clay has advanced dramatically in the last 150 years and many modern day potters and artists are informed in their work by science and the chemistry of clay.
Functional utility in ceramics requires attention to the constraints of what the function will be. When a potter makes a functional object such as a dinner plate or teapot, it should be stable, durable, mechanically sound and properly glazed to ensure cleanliness and impermeability to water, soap, oils, acids, a full range of food and bacteria, the use of knives and forks, and all the myriad of considerations daily life. If they will be used in an oven, they must exhibit heat tolerance without breakage due to expansion and contraction under heat applied stress. The glaze should not easily chip off or be gouged. All of these demands require fairly specific knowledge and the science of ceramics is helpful to potters in this regard. In recent years, computer programs designed to help potters have been developed for theoretical glaze calculation and analysis and serve as a complement to experiential knowledge. Indeed, potters quantify the firing process by a measure of heat and time called a “cone”. The use of cones became an organized element of clay scientific knowledge through the work of Herman Seger in the late 19th century. The specific cone to which clay is fired, depends on the internal chemical and physical makeup of the clay, that is, the relations of alumino-silicates to fluxes and metallic oxides, diverse impurities and the complex relations of particle size to the application of extreme heat through time.
The notion of functionality also plays an important role in the basic shape and form of a pot, or in the heat range and strength of the clay because of the utility which the pot will serve. Whether clay is porous, heat resistant, fragile, smoothly textured and neatly glazed, etc., all play a role in daily use and the utilitarian function we assign a pot. In this way, scientific knowledge about density, mechanical strength, and the nature of the glassy matrix is important. When it comes to other aspects however such as the overall shape, colour, and comfort to the hand, scientific knowledge gives way to art and experience. In this way art and science come together in the act of sculpting clay.
The role of aesthetics is also a question for the broader realm of art such as sculpture. Ceramic sculpture opens the realm of clay work to ideas of representational meaning, expressive intention, conceptual expression, social relevance and similar issues which are imbedded in the domain of art. The role of practice and the ingenuity of human imagination and the creative hand are essential elements of creating. When artists work with clay, science contributes what it can but it takes a back seat to the creative process which is driven by imaginative and aesthetic energy. The functionality and practicality of clay is based in the process of transforming it from a soft, changeable plastic material, to one that is hard, durable and resistant to change.
The discovery that extreme heat causes a permanent change in the properties of clay was critical for human development. It changed societies as dramatically (though not as quickly) as the computer has changed our own. For science in particular, the manipulation and firing of clay might very well be the first deliberate act of chemical engineering by human beings.
The most common forms of clay used by potters and artisits – the base of which is kaolinite – are soft and malleable when wet because water is trapped between the clay layers (platelets). When clay dries without being fired the trapped water is evaporated and the clay hardens. But if dry kaolinite is put back into water it returns to its soft and malleable state because the water is re-absorbed between the plate-like layers. However, when dry kaolinite is heated to extreme temperatures, above 800°C, there is enough energy for the atoms in the clay to rearrange themselves into different compounds – resulting in a permanent chemical and structural change. For kaolinite, the chemical change is;
The first two substances in the equations above (mullite and silica) are solids and they form interlocking crystals that are permanently hard. Gone is the plate-like layered structure of the clay that absorbed and trapped the water. The mullite-silica crystal structure will not absorb water – the change is irreversible. The last substance in the equation presented above is water – water that comes from the kaolinite itself and not water that was trapped between the layers. This water evaporates in the heat that drives the reaction and can never return.
Clay and Nanotechnology
A quick look around may seem to indicate that clay is a material of the past. Plastics, metals and composites are materials of the present and nanotechnology represents the promise of the future. Nanotechnology is the science and technology of the very very small and it promises fantastic advances, including in the field of materials. The word “nano” is used in science as a prefix that means “one billionth”. So a nanometer is a billionth of a meter – about the scale of atoms. A nano-material is a material that is assembled and engineered at the scale of nanometers – a scale that is far too small to see with the naked eye, and only barely visible by the most powerful optical microscopes.
Nanotechnology offers the promise of the fantastic – strings the width of a human hair that can be used to pick up a small van, materials that can repair themselves, materials that are light and virtually unbreakable to name a few. Yet, as with many things, nature is a step ahead. Nature already has its own nano-material: clay! Indeed, as we have already seen, clays are formed of tiny (in fact, nano-scale) sheet-like structures with dramatic properties. Science is now re-discovering clays as naturally occurring (and abundant) nanomaterials.
The most widespread use of clay as nano-material is as an additive to polymers. Polymers are materials made up of large molecules connected together in repeating structural units. Plastics are polymers. Adding clays to polymers can significantly (and cheaply) improve their properties. In the automotive industry nanoclays are reported to improve the intrinsic barrier properties of resins for use in paints by 5 to 15 times – making them scratch resistant. In fuel tanks, nanoclays reduce the transmission of solvents and gases through polymers because the tiny sheets of clay can be shaped into a maze that the solvent or gas never works its way through. Nanoclays added to polymers improve the strength of polymers and make them more resistant to heat and the wear and tear of time. In 2006, global consumption of nanocomposites was almost $300 million USD and almost 25% was accounted for by clay nanocomposites. By 2011, consumption is expected to rise to 850 million USD and clay nanocomposite’s share will rise to almost 45%. Indeed, the chances are that if you have scratch resistant paint on your car, or if you buy a magazine with a glossy cover, you have nanoclay technology already at your fingertips.
Another interesting use of nano-clay is as a catalyst and support structure for the growth of carbon nanotubes. Carbon nanotubes exhibit fantastic properties – worthy of superheroes and supervillans in the best Hollywood has to offer - but they are difficult to make. Now, scientists are learning how to use nanoclay structures to both facilitate and support their growth. Importantly, it is the same property that makes clay absorb water that makes them good as carbon nanotube growth farms. Scientists just have to manipulate the chemical properties of the clay platelets so that they absorb the elements for carbon nanotubes and not water.
Some kinds of clay may be useful as nano-materials in and of themselves and not as catalysts or additives. Halloysite nanotubes are tiny nano-tubes formed naturally by surface weathering of aluminosilicate minerals. These tiny tubes have diameters several dozen atoms to several hundred wide and lengths a thousand times longer but still barely visible in a microscope. They can be coated to give them desirable electrical, chemical or physical properties. They can also be used as tiny containers that can be filled with pharmaceuticals, bug repellants, cosmetics and much more that release their contents in controlled conditions.
Clay and the Origin of Life
At some point in the distant past – probably about 350 million years ago when the violent shower of asteroids stopped hammering the Earth’s surface on a regular basis – life emerged from the non-living. Somehow, self-replicating complex molecules were formed, protected and given the boost they needed to begin down the path shaped by evolution into the world we see around us. How it all began is still the source of great debate within science but a theory introduced in 1966 by A.G. Cairns-Smith along with several discoveries since then have brought clay into the picture.
Clay is the product of the weathering of rocks and minerals in a repeating dance that sees crystals formed, disolved and broken as water is absorbed then evaporated. The plate like structure that results in some clay – including montmorillonite that would have fromed from volcanic ash - may be the womb life needed to get started. Indeed, the closely spaced plates offer protection and their large surface area provides an ideal chemical factory floor for the development of complex molecules. What’s more, the energy that is released when plates break may have provided the pattern for replication and the spark that got it going on its own.
Modern day life is based on complex cells that are programmed and controlled by DNA, proteins and RNA. DNA is, of course, nature’s most efficient way of storing and replicating information, and proteins are the building blocks that DNA requires for its replication. Each are specialized to do their respective tasks. However, there is a problem. DNA needs proteins but proteins need DNA - it’s a proverbial chicken and the egg dilema. Neither could really come before the other. RNA is another story and for that reason, biologists have long supposed that RNA was a key in the beginning. RNA is more stable than DNA but it is less versatile and efficient than DNA and proteins. What makes it special is that it can do the job of both. The problem with RNA is that it can’t replicate itself. It can replicate small pieces of itself but not more. So how might have RNA formed? That’s where clay comes in to the picture. Montmorillonite clay has the interesting property that it can catayze the formation of RNA.
The next question is then, how might RNA have been trained to replicate information? Again clay presents us with an interesting possibility. Clay plates are crystals that form until the mechanical stress inside them, cause them to break into pieces. Within a single crystal the structure is identical so when a crystal breaks, it forms two pieces with the same structure. In a sense a crystal replicates itself when it breaks. If RNA molecules are formed by the chemical reactions at the surface of a montomorillionite clay plate and that plate then divides, the same chemical process will be driven along both crystals. The RNA formed on both surfaces will therefore be the same. So as the crystals divide and replicate themselves so too does the RNA that forms on the surfaces.
In crystal growth there are always slight modifications in structure due to impurities. This means that there is a gradual mutation process in crystal growth. Similar mutations will then occur in the RNA molecules but given the right conditions the RNA molecules may form in such a way that they will begin to minimize these mutations. In this way the RNA molecules might have evolved to the point where they could replicate and minimize mutations. In this way they might continue to grow but not at the mercy of mutations in the “parent” crystals. They would grow independently.
Another interesting property of clay crystals is that they release sparks of energy when broken. Like lifesaver candies that spark in the dark when crunched, clay crystals will also release sparks of energy (that are too dim to see with the eye but detectable with sensitive equipment) when broken – either by an external force or as they dry out and crack under internal stresses. Indeed, scientists have been able to use the energy flashes produced as clays are put through cycles of wetting and drying to form bonds (peptide bonds) and the polymerization of amino acids.
Science is still a long way from a definitive answer on the development of life on Earth. However, it is possible that clay may have played an important role in our origins. It is certainly true that clay has played a formative role in human societies and it is likely to continue to do so in the future in surprising ways. Indeed, it is a fantastic material that not only can be formed and used, it forms and transforms.