Sofia Martinez
Plastic clay is a term used in ceramics to describe clay that has a specific level of moisture content, allowing it to be shaped without cracking or breaking. This quality, known as plasticity, is caused by the right mixture of water and particle size, transforming dry, crackly clay into a workable material. Plastic clay is an extremely rare mineral found in localized deposits worldwide and is composed of kaolinite, mica, quartz, and other accessory minerals. The plasticity of plastic clay makes it ideal for use as a base material in the manufacture of ceramics, providing artists with the freedom to create a wide range of forms.
| Characteristics | Values |
|---|---|
| Definition | Plastic clay is a rare sedimentary mineral made from kaolinite, or decomposed granite mixed with river action, other clays, sands, gravel, and vegetation. |
| Composition | Plastic clays usually contain three dominant minerals: kaolinite, mica, and quartz. There are also other accessory minerals and carbonaceous material present. |
| Colour | Plastic clays have a wide range of colours. |
| Firability | When fired, selected clays give results that are pure white. |
| Water content | Clay needs the right amount of water to become plastic, usually 20% water by weight. |
| pH of water | Very mild acidity (a pH of 6.5) is ideal for clay plasticity. |
| Particle size | Clay particle size has the greatest influence on plasticity. Clay particles are ordinarily about 1 micron in size. |
| Particle distribution | Optimum distribution of particle sizes allows clay to be better kept together when formed. |
| Particle packing | A percent, always less than 100, representing the volume fraction of solids in a given volume of material. |
| Flocculation | The process of very small particles forming loose clumps due to weak electrostatic attraction between the particles. |
| Workability | The character of a clay that is a combination of plasticity and wet strength. |
| Short clay | Clay lacking plasticity. A coil rolled or extruded from a short clay will show signs of tearing and cracking. |
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What You'll Learn
Plastic clay is a rare mineral with a unique composition
Plastic clay is an extremely rare mineral with a unique composition. It is a sedimentary material, formed from decomposed granite, or kaolinite, that has been mixed with other clays, sands, gravel, and vegetation by river action. The clay particles are attracted to each other by weak electrostatic forces, and when water is added, it acts as a conductor, allowing the particles to move and slide while staying stuck together. This gives the clay its plasticity, or workability, allowing it to be shaped, pinched, and stretched without rupturing.
The composition and particle size of plastic clay can vary, resulting in different characteristics within the same deposit. The dominant minerals found in plastic clay are typically kaolinite, mica, and quartz, along with accessory minerals and carbonaceous material from ancient plants. The pH of the water used can also affect the plasticity of the clay, with mildly acidic water being ideal. Clay artists may age their clay by storing it for several weeks to improve its plasticity over time.
The plasticity of plastic clay makes it highly sought-after for use in ceramics and pottery. It is extracted from the earth and processed by shredding, drying, and grinding to create a consistent and predictable product. The refined clay is then used as a base material in the manufacture of ceramics, where it can be shaped and decorated.
While plastic clay is rare, commercially viable deposits have been found in select locations worldwide, including South West England, Germany, France, Ukraine, the United States, Thailand, Indonesia, and China. The process of extracting clay from the earth has evolved from traditional methods of digging and hand tools to modern techniques using hydraulic 'back-hoe' excavators.
The unique composition and properties of plastic clay make it a valuable and versatile material in the art world, particularly in ceramics and pottery, where its plasticity allows artists to create a wide range of shapes and forms.
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The role of water in achieving the right clay consistency
Plasticity is a defining mechanical property of clay. Clay is plastic when it is wet, and it hardens when dried or fired. The right quantity of water allows clay to become plastic. Water acts as a mediator, transforming clay from a compact mass to a malleable and receptive form.
Water plays a crucial role in achieving the right clay consistency. Clay with too much water loses its strength and shape, while clay with too little water is hard and resistant. Potters must find a delicate balance where the clay achieves perfect plasticity, allowing them to work with ease without the clay collapsing. This balance is achieved through experience and experimentation.
The role of water in clay consistency is not limited to the initial mixing process. Uniform drying is essential to prevent internal tensions in the clay, which can cause cracks and deformations. Potters may cover their pottery with plastic to allow water to evaporate slowly and evenly. Before firing, it is crucial to ensure that all residual water has evaporated to prevent the piece from exploding in the kiln.
Additionally, water plays a role in recycling clay. Potters may use water mixed with fine clay (clay slop) as a basis for reclaimed or recycled clay. They may also use techniques like spreading the clay on a plaster batt to absorb moisture and achieve the desired consistency. Overall, water is integral to achieving the right clay consistency at every stage of the ceramic process.
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How to recycle clay and maintain its plasticity
Plasticity is the property of clay that allows it to change shape without rupturing when force is applied to it. It is caused by the right mixture of water and particle size, which transforms dry, cracky clay into a workable clay body.
Clay scraps can be recycled and do not need to go to waste. However, clay can only be recycled when it is greenware. Once clay has been fired and turned into a ceramic material, it cannot be turned back into workable clay.
To recycle clay scraps, follow these steps:
- Collect clay scraps in a bucket. Include slop from the splash pan and scraps from hand-building.
- When the bucket is halfway full, break any large pieces into smaller bits. Smaller pieces will slake down faster and more thoroughly.
- Add water to the bucket, covering the clay by several inches. If the clay absorbs too much water and re-emerges, add more water to keep it completely covered.
- Stir the clay and water together to create an even consistency. This can be done with a stick, an old metal ladle, or your hand.
- Let the clay slake, which may take several hours or a couple of days. If a layer of water forms on top, gently pour out as much liquid as possible.
- Transfer the slurry to a wide container to increase the surface area for evaporation. Place the container in a safe spot and let it rest for several days, stirring regularly.
- Allow the clay to dry to a sludge-like consistency, thick but not solid. It should not be too dry.
- Once the clay has dried to the right consistency, it can be compressed or wedged and is ready to use.
To maintain the plasticity of recycled clay, consider the following:
- The amount of water: Clay needs approximately 20% water by weight to become plastic. Water acts as a lubricant, allowing clay particles to move past each other without breaking away.
- Particle size: Clay particles are typically about 1 micron in size. The particle size influences the plasticity of clay bodies, with smaller particles exhibiting greater plasticity.
- Packing efficiency: Efficient packing of clay particles improves plasticity by increasing the electrostatic attraction between particles. This can be controlled by blending clay body ingredients by particle size.
- Bacteria: A small amount of bacteria can boost the plasticity of recycled clay. However, too much bacteria over time can ruin the clay, so manage the amount of water in the bucket to control bacteria growth.
- Additives: To improve the plasticity of poor-quality clay, add a plasticizer such as ball clay or bentonite. Alternatively, add a few drops of white vinegar to the water to improve clay that has become strange in consistency.
By following these steps and considerations, you can recycle clay scraps and maintain their plasticity for future use.
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Clay's plasticity allows it to change shape without rupturing
Plastic clay is an extremely rare mineral found in only a few places around the world. It is a sedimentary material composed of kaolinite, mica, quartz, and other minerals and organic material. The plasticity of clay is what allows artists to shape it without it rupturing. Clay particles are attracted to each other by weak electrostatic forces, and when clay is mixed with water, these forces are conducted, allowing the clay particles to slip past each other without breaking away from each other. This is why clay with water added is described as "plastic".
The plasticity of clay is influenced by the size of clay particles and the amount and pH of the water added. Clay particles are usually about 1 micron in size. The optimum distribution of particle sizes allows the clay to be better kept together when formed. When clay only has one size of particle, there are more gaps between them, resulting in fewer points of contact. This means that when clay with only one particle size is bent or stretched, many of the bonds between particles will break, causing the clay to crack. This is what happens when clay is "short".
The ideal pH for clay plasticity is a very mild acidity of around 6.5, the pH of typical drinking water. Clay artists may improve the plasticity of their clay by ageing it, as clay mixed with a minimal amount of water exhibits increasing plasticity over the first two to four weeks of storage. Clay mixed with a significant excess of water, on the other hand, ages much faster and achieves full plasticity within three days.
The plasticity of clay allows artists to perform a range of forming processes, such as shaping it with their fingers, rolling coils, throwing pots on a wheel, and extruding endless shapes. Clay with good plasticity can also be used to create large slabs that can be attached to the surface of a piece without cracking and falling off.
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Clay decoration: how to avoid cracking and improve adhesion
Plastic clay, in art terms, refers to wet clay that is ready to be used in pottery or sculpting. The clay is described as "plastic" because its particles attract each other and can be shaped when mixed with water. This plasticity allows clay to be moulded and shaped into various forms.
Now, let's delve into some tips for decorating your clay creations while minimising the chances of cracking and improving adhesion:
Preventing Cracking:
- Start with a smooth, consistent, and well-conditioned clay body. Ensure even moisture distribution and density within the clay to facilitate smooth drying.
- Avoid rushing or forcing the air-drying process. Gradual drying is key—set your pieces to dry in a climate-controlled room at around 70°F.
- Avoid direct sunlight, heaters, or drafts that can cause the surface to dry too quickly and lead to cracking.
- Reinforce areas with thickness changes, such as spouts and handles, to ease the transition in rigidity and distribute stresses more evenly.
- Always bisque fire clay pieces to completion before applying any decorations. Firing wet glazed pieces can lead to issues due to the added moisture.
- If cracks appear during the clay-building stage, reinforce the problem area with additional armature materials or epoxy clay.
Improving Adhesion:
- Before attaching clay pieces together, ensure that the surfaces are clean and properly prepared. Remove any dust or debris, and smooth out rough edges using fine-grit sandpaper.
- Slightly moisten both surfaces with water to improve adhesion. This softening of the clay makes it more receptive to attaching other pieces.
- Use techniques like slip trailing and layering with slip to create decorative designs and improve the attachment of clay pieces.
- Press the surfaces together firmly using your hands or a tool to create a strong bond. This ensures that the clay pieces are securely attached, contributing to the overall durability of your artwork.
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Frequently asked questions
Plastic clay is a type of clay that has the property of plasticity, which allows it to change shape without rupturing when force is applied to it. 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 acts as a conductor, allowing clay particles to slip past each other without breaking away from each other.
Plastic clay is used as a base material in the manufacture of ceramics. It is also referred to as 'ball' clay, which dates back to the early methods of mining when specialised hand tools were used to extract the clay in rough cube shapes of about 30 cm.
Plastic clay is an extremely rare mineral, found in localized deposits at a handful of locations around the world. It is a sedimentary material made from kaolinite, or decomposed granite that has been mixed through river action with other clays, sands, gravel, and vegetation.
