Alexei Volkov
Plastic clay is an extremely rare mineral found in only a few places worldwide. It is a base material in the manufacture of ceramics due to its plasticity, which is a clay's ability to be shaped and its ability to assume a new shape without reverting to its old form. Plasticity is caused by the right mixture of water and particle size, allowing clay to be shaped, pinched, rolled, and stretched while staying in one piece. The addition of grog or sand can make plastic clay more workable, and potters have also experimented with additives like milk, beer, and vinegar to improve plasticity. However, the role of water in plasticity is not yet fully understood, and the drying and hardening properties of plastic clays are reversible.
| Characteristics | Values |
|---|---|
| Definition | Plastic clay is an extremely rare mineral with plastic properties that can be shaped without cracking or tearing. |
| Composition | Clay particles, water, feldspar, silica, mica |
| Particle Size | 1 micron (one millionth of a meter or one-thousandth of a millimeter) |
| Water Content | Typically around 20% by weight |
| Workability | Can be made more workable by adding grog or sand, improving its ability to stand without slumping |
| Colour | Plastic clays have a wide range of colours, but certain clays can result in pure white ceramics when fired |
| Extraction | Extracted using hydraulic 'back-hoe' excavators from localized deposits around the world |
| Processing | Blended, shredded, dried, ground, and treated by calcination to produce powdered clay and chamotte |
| Applications | Ceramics, glaze recipes, ceramic slurries, artware casting, and as a base material in the sanitaryware sector |
| Additives | Organic materials like milk, beer, urine, and vinegar may be added to improve plasticity, but there is limited evidence of their effectiveness |
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What You'll Learn
Plastic clay is a rare mineral
Plastic clay is an extremely rare mineral with a unique set of characteristics that make it highly valued in the world of ceramics and other industries. It is found in localized deposits in only a few places worldwide, making it a sought-after resource for artists and manufacturers alike. The rarity of plastic clay can be attributed to its sedimentary origin, which requires a unique combination of geological processes and historical factors.
The plasticity of this mineral is what sets it apart from other types of clay. Plasticity is the property that allows clay to be shaped, moulded, and transformed into various forms without cracking or tearing. It is caused by the right mixture of water and particle size, with clay particles being attracted to each other by weak electrostatic forces. Water acts as both a glue and a lubricant, allowing clay particles to slip past each other without breaking apart. This gives plastic clay exceptional workability and makes it ideal for creating ceramics, as it can be easily shaped on a potter's wheel or by hand.
The process of making ceramics with plastic clay begins with extracting the raw material from the earth. Plastic clay is typically found in seams within the quarry, and hydraulic 'back-hoe' excavators are used to carefully extract it. The extracted clay is then blended according to specific recipes to ensure consistent characteristics. The blended clay is shredded into smaller, more regular lumps before being dried and ground into a powder. This powdered form of plastic clay can then be purchased by ceramic manufacturers and artists for their creations.
The rare mineral's unique properties also extend to its drying and hardening characteristics. Unlike ceramic powders that use organic binders, plastic clay can be reused multiple times. A raw glaze made from plastic clay can be scraped off, rewetted, and applied again without losing its effectiveness. This reusability is a significant advantage for ceramic artists, as it allows for experimentation and the correction of mistakes.
The colour range of plastic clays is quite diverse, but when fired, certain types of clay can produce a pure white result. This characteristic has made plastic clay a sought-after material in the creation of ceramics with a white finish. The rare mineral's plasticity, workability, and colour range have contributed to its importance in the ceramic industry, particularly in the sanitaryware sector, where it has helped reduce manufacturing process costs.
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Plasticity and workability
Plastic clay is an extremely rare mineral found in localised deposits in a few places around the world. It is also known as 'ball clay', a name that comes from the early methods of mining when specialised hand tools were used to extract the clay in rough cube shapes of about 30cm. As the corners were knocked off during handling and storage, the cubes became rounded and ball-shaped.
Plastic clay gets its name from its plasticity, which is the property of clay that allows it to be shaped, pinched, rolled, and stretched while staying in one piece. Plasticity is caused by the right mixture of water and particle size. Clay particles are attracted to each other by weak electrostatic forces, and water conducts these forces. Thus, dry clay mixed with about 20% water by weight becomes the plastic material clay artists use. Water acts as a lubricant that allows clay particles to slip past each other without breaking away from each other. Clay particle size has the greatest influence on plasticity, and clay particles are ordinarily about 1 micron in size. Such tiny particles have a large surface area for their weight.
The plasticity of clay is sometimes mentioned in the context of its use in glaze recipes or ceramic slurries. The plasticity of a clay body is a combination of its plasticity and wet strength. The addition of grog or sand may make a plastic clay more workable, allowing it to stand taller without slumping. Clay bodies lacking plasticity will show signs of tearing and cracking when bent sharply or tied in a knot.
Organic materials like milk, beer, urine, and vinegar have been added to clay bodies to improve their plasticity. However, there is no evidence that these additives improve clay plasticity. It is likely that bacteria in the clay feed on these additives and give off wastes that lower the pH of the water, improving plasticity but causing unpleasant odours and promoting the growth of unhealthy bacteria or moulds.
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Organic additives
Plastic clay is a rare mineral with a unique plasticity that makes it ideal for use in ceramics. This plasticity is a result of the right mixture of water and particle size, allowing clay to be shaped, pinched, rolled, and stretched while staying in one piece.
Another organic additive is paper pulp, which can be mixed with clay to create paper clay. Paper clay has increased strength and reduced weight after firing. However, it is prone to mould and produces unpleasant odours during firing due to the paper smouldering in the kiln.
When using organic additives, it is important to consider potential drawbacks, such as the growth of unhealthy bacteria or mould, unpleasant odours, and changes in the clay's properties that may affect the final product. It is also crucial to source pure clay and test each batch when using organic additives to ensure consistent and desired results.
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Extraction and processing
Plastic clay, also known as ball clay, is an extremely rare mineral found in localised deposits in very few places worldwide. Its name is derived from the early methods of extraction, where specialised hand tools were used to dig out the clay in rough cube shapes of about 30cm. As the corners were knocked off during handling and storage, these cubes became rounded and ball-shaped.
Today, ball clay is extracted using hydraulic excavators working at benches cut into the quarry to access the clay seams. Individual raw clay selections are carefully blended according to predetermined recipes to yield a product with consistent characteristics and behaviour. The first stage in processing is to shred the blended clay into small pieces. Further processing through drying and grinding yields powdered ball clay. Treatment by calcination produces "chamotte" or "grog", a calcined clay containing a high proportion of silica and alumina.
Ball clay is formed from ancient rivers and streams washing kaolinite (formed from decomposed granite) from its parent rock. As the streams flowed from upland areas, the kaolinite mixed with other clay minerals, sands, gravels, and vegetation before settling in low-lying basins to form overlaying seams of plastic clay. Ball clays are fine-grained, highly plastic clays that are principally used in the manufacture of ceramic whiteware and sanitaryware, where they are valued for their plasticity, unfired strength, and light colour when fired.
Ball clay can be made from dry clay using a dry clay mix and water to create a plastic stage consistency. Slip can also be turned into the plastic stage by removing some of its moisture. This plastic clay is used by many potters as it is dry enough to retain its shape but still wet enough to be easily moulded and shaped. Once moulded, it will keep its shape, making it perfect for ceramics.
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PV Clay
One of the key characteristics of PV Clay is its plasticity, which is a property of soft clay that allows it to be shaped and moulded without returning to its original form. While PV Clay is not as plastic as typical pottery clay, it still exhibits formability and can promote plasticity in ceramic casting slip or plastic clay bodies. This plasticity is a result of the fine particle size of the clay and the presence of water, which acts as a lubricant, allowing the clay particles to move past each other without breaking.
The mineral composition of PV Clay also contributes to its versatility. The feldspar component promotes vitrification, the silica content imparts resistance to glaze crazing and dunting, and the low iron content contributes to the whiteness of the final product. PV Clay is known for its high silica content, good casting properties, low iron content, and long firing range. When fired, it begins to vitrify and progresses to a very glassy white colour, making it ideal for creating white-burning ceramic bodies.
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Frequently asked questions
Plastic clay is a rare mineral that is used as a base material in the manufacture of ceramics. It is formed from ancient rivers and streams that washed kaolinite (from decomposed granite) from its parent rock. The kaolinite then mixed with other clay minerals, sands, gravels and vegetation to form plastic clay.
Plastic clay is extracted using hydraulic 'back-hoe' excavators in quarries. Individual raw clay selections are then blended and processed into smaller, more regular lumps about the size of a golf ball. Further processing through drying and grinding yields powdered plastic clays.
Water is essential to turn clay into a plastic material. Clay needs the right amount of water, usually around 20% water by weight, to become plastic. Water acts as a lubricant that allows clay particles to move past each other without breaking.
