Omar Khalid
Clay plasticity is a property exhibited by soft clay, where the shape can be changed with force, and the clay does not return to its old shape. Plasticity is influenced by several factors, including particle size, mineralogy, organics, salts, and the pH of the water used. Improving clay plasticity can be achieved through various methods, such as adding specific minerals like bentonite and ball clays, using cellulosic compounds, or optimizing the packing efficiency of clay particles. Commercially available organic additives have also been marketed to enhance clay plasticity, but their effectiveness may not always justify the cost. Additionally, folk methods involving organic additives like milk, beer, urine, and vinegar have been used, although there is limited evidence supporting their direct impact on plasticity. Techniques like aging clay by storing it for extended periods and controlling the water content can also positively influence plasticity.
| Characteristics | Values |
|---|---|
| Particle size | Smaller particles have more surface area and improve plasticity |
| Packing efficiency | Efficient packing of clay particles improves plasticity |
| Water content | Clay mixed with a minimum amount of water exhibits increasing plasticity over time |
| pH | Mild acidity (pH of 6.5) improves plasticity |
| Aging | Clay artists store clay for long periods, improving plasticity |
| Additives | Epsom salts, bentonite, and organic agents can improve plasticity |
| Temperature | Increasing temperature improves plasticity |
| Minerals | Montmorillonitic clays and ball clays can improve plasticity |
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What You'll Learn
- Add minerals such as bentonite or ball clay to improve plasticity
- Use cellulosic compounds to capture water and make clay more plastic
- The pH of water matters. Aim for mild acidity with a pH of 6.5
- Efficient packing of clay particles improves plasticity
- Additives like milk, beer, urine, vinegar, or Epsom salts may help
Add minerals such as bentonite or ball clay to improve plasticity
Clay with poor plasticity tends to split at edges during wedging and rolling, generates a lot of slip, and is difficult to centre during throwing. To improve plasticity, you can add minerals such as bentonite or ball clay to your mix. Bentonite is a highly plastic common clay that acts as a plasticizer. It has a high affinity for water, which acts as a glue to hold clay particles together and a lubricant that imparts plasticity. When adding bentonite to your clay, it is important to note that it is hyperplastic and won't mix with water as a pure material. Thus, it is recommended to dry out the scrap clay, crush it, and then mix in the bentonite. You can also make a 50:50 ball clay-bentonite powder mix, shake them together in a plastic bag, slurry it up, and dewater it to plastic form. However, ensure that the bentonite does not contain high levels of soluble salts, as this can leave a scum on your equipment.
Ball clay is another mineral that can be added to improve the plasticity of clay. It is a 2:1 aluminosilicate clay particle that imparts plasticity in clay bodies. When using ball clay, it is important to consider the amount of water used, as too much water can make it difficult to get the clay into a usable shape. Additionally, ball clay shrinks a lot during drying, so it is recommended to use it alongside other types of clay, such as kaolin, to prevent cracks.
Both bentonite and ball clay can be added to reclaim or recycled clay to improve its plasticity. Reclaimed clay tends to lose its fine particles, leading to a decrease in plasticity over time. By adding these minerals, you can restore and improve the workability of your clay.
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Use cellulosic compounds to capture water and make clay more plastic
Clay is the longest-known ceramic material, with prehistoric humans using it for making pottery. The defining mechanical property of clay is its plasticity when wet and its ability to harden when dried or fired. Clays develop plasticity due to the presence of clay minerals, which are hydrous aluminium phyllosilicate minerals composed of aluminium and silicon ions. In kaolinite clay, the clay mineral with the highest content in clay soils, the bonding between plates is provided by a film of water molecules that hydrogen bond the plates together.
To improve the plasticity of clay, cellulosic compounds can be used to capture water and make the clay more plastic. Cellulose is an organic compound with the formula (C6H10O5) n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. It is the most abundant organic polymer on Earth and is mainly obtained from wood pulp and cotton for industrial use. Cellulosic compounds have been used in ceramic bodies for a long time, with ancient cultures using straw to bring porosity to clay, easing water entrance and increasing its workability.
Today, CMC (Carboxy-Methyl-Cellulose) is used in high-moisture environments to capture water and improve the workability of clay. However, due to its high-water retention, the risk of cracking or breaking increases during the drying process. To avoid this, the drying process must be controlled gently to prevent the appearance of cracks or even blasts caused by retained water.
BioKeram T is an innovative alternative to these cellulosic compounds, developed by a company with over 50 years of experience in the ceramics market. It increases plasticity and moisture to reach optimum workability without affecting the drying process or the final product's quality. Only very small additions of 1-2 kg per ton of clay are needed to achieve optimal performance in plasticity.
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The pH of water matters. Aim for mild acidity with a pH of 6.5
The plasticity of clay is what allows it to be shaped, pinched, rolled, and stretched. It is caused by the right mixture of water and particle size. The pH of the water used with clay is important as it affects its plasticity. Aim for a mild acidity with a pH of 6.5, which is the typical pH of drinking water.
Clay with a minimum amount of water, known as dry mixed clay, exhibits increasing plasticity over the first two to four weeks of storage as it becomes fully wetted. On the other hand, clay mixed with an excess of water, or slurry mixed clay, achieves full plasticity within three days. This is because the excess water used in the process wets the clay more efficiently. The amount of water necessary for plasticity is related to the packing efficiency of a given body of clay. The higher the packing efficiency, the less water is needed for plasticity.
The pH of the water in a clay body can be lowered by adding organic materials such as milk, beer, urine, and vinegar. However, these additives may have negative side effects, such as developing unpleasant odors and promoting the growth of unhealthy bacteria or molds.
There are also certain minerals that can be used to improve plasticity, such as Bentonite and ball clays. These options can provide the desired plasticity without significantly increasing costs. Additionally, cellulosic compounds can be used to capture water and improve the plasticity of clay bodies, but the drying process must be controlled gently to avoid cracks or blasts caused by retained water.
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Efficient packing of clay particles improves plasticity
Clay is a highly plastic material, which means it can be deformed continuously under a finite force and will maintain its new shape. The plasticity of clay is influenced by several factors, including particle size, mineralogy, organics, and salts. Efficient packing of clay particles improves plasticity by increasing the electrostatic attraction between particles.
Clay particles are very small, typically around 1 micron in size, and have a large surface area relative to their weight. The efficient packing of these particles can dramatically improve the plasticity of the clay body. Research has shown that a packing efficiency of around 68% is ideal, as it minimizes the space between particles while allowing them to move easily against each other. This higher packing efficiency also reduces the amount of water needed for plasticity.
The pH of the water used in a clay body also affects its plasticity. Ideally, the water should have a very mild acidity with a pH of around 6.5, similar to drinking water. However, it is important to note that potable water may contain additives that can impact the natural aging process of clays.
Additionally, certain minerals can be added to improve plasticity. For example, Bentonite and ball clays are highly plastic and can be used to increase the plasticity of a clay body without significantly increasing costs. However, the availability and cost of these minerals may be influenced by geopolitical factors.
Another alternative is to use cellulosic compounds, which capture water and help clay bodies behave as if they were more plastic, making extrusion easier. However, the drying process must be carefully controlled to avoid cracks or blasts due to retained water. Overall, by understanding the factors that influence clay plasticity and employing efficient packing techniques, it is possible to enhance the plasticity of clay for various applications.
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Additives like milk, beer, urine, vinegar, or Epsom salts may help
Clay artists believe that aging clay improves its plasticity. This can be achieved by storing it for a long time after mixing it with a minimal amount of water. This process can take up to two to four weeks. However, some artists use commercially manufactured organic materials to improve plasticity. There is also potter folklore about using organic materials like milk, beer, urine, vinegar, or Epsom salts to improve plasticity.
Milk
There is limited information on how milk can be used to improve the plasticity of clay. However, it is worth noting that milk contains lactic acid, which can act as a mild acid to dissolve calcium and other minerals in the clay, which could potentially change its plasticity.
Beer
Beer can be added to the water before it is mixed with clay. This will start mold growth in the clay/water mixture, which increases the binding action or attraction of the clay platelets, improving plasticity.
Urine
Like beer, urine can be added to the water before mixing with clay. Urine contains urea, which can act as a plasticizer, improving the workability and flexibility of the clay.
Vinegar
Some clay artists add vinegar to their clay to dissolve calcium and other minerals. This process can take several years, with the mixture being left in buckets to grow bacteria and being mixed every few months.
Epsom Salts
Epsom salts can increase the attraction of clay platelets in moist clay, causing the clay to become flocculated. Clay platelets are drawn together, creating a tight, plastic clay body with good throwing properties. The recommended amount is about 5 oz of Epsom salts per 100 lbs of dry clay formula (approximately 0.3% of the formula). However, it is important to note that too much Epsom salt can cause salt migration to the drying clay surface, resulting in blistering and carbon being trapped in the clay.
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Frequently asked questions
Plasticity is a property exhibited by soft clay. Clay can be moulded into a new shape, and it will retain this new shape without attempting to return to its old one.
The water content of clay is crucial to its plasticity. Clay with a minimum amount of water (dry mixed) will exhibit increasing plasticity for the first two to four weeks of storage as it becomes fully wetted. Clay mixed with an excess of water (slurry mixed) achieves full plasticity faster, within three days of being mixed.
The pH of the water in a clay body also affects its plasticity. Very mild acidity (a pH of 6.5) is ideal. The organic matter in the clay can also affect the pH, and therefore the plasticity.
There are several minerals that can be used to improve plasticity, including bentonite and ball clays. Epsom salts can also be added to increase the attraction of clay platelets, resulting in a tight, plastic clay body.
Particle size is another important factor influencing plasticity. Efficient packing of clay particles improves plasticity due to increased electrostatic attraction between closely packed particles.
