How Can You Increase The Plasticity Of Clay?

What is Clay Plasticity?

Clay plasticity refers to the ability of clay to be reshaped and molded without cracking or breaking apart. Plasticity is one of the most important properties of clay and is vital for most clay applications.

When clay is plastic, it can be formed into shapes by applying pressure. Highly plastic clay is very flexible, easily deforms into new forms, and retains fine detail. Less plastic clay is stiffer and tends to crack when molded.

Plasticity allows clay to be crafted into pottery, sculpture, bricks, and other structures. Without sufficient plasticity, clay objects would be too fragile and brittle. The plastic quality of clay enables reshaping and adding creative detail throughout the design process.

Factors that Affect Clay Plasticity

Clay plasticity refers to the ability of clay to be shaped or molded without cracking or becoming permanently deformed. There are several key factors that impact the plasticity of clay:

Water Content – The amount of water mixed into the clay greatly affects its plasticity. More water makes the clay more flexible and easier to mold without cracking. However, too much water can make the clay sticky or cause excess shrinkage when drying.

Clay Composition – The mineral makeup of the clay determines its natural plasticity. Clays high in kaolinite tend to be more plastic and workable than those dominated by illite or smectite. The particle size also matters, with extremely fine clays often being more plastic.

Additives – Materials blended into the clay body can alter its plasticity. Certain organic additives called plasticizers make the clay more flexible and easier to work with. Inorganic materials like grog or chamotte can reduce plasticity and make clay stiffer.

Increase Plasticity by Adding Water

Water is crucial for activating the clay particles and allowing them to slide over one another. When clay dries out, the water between the clay platelets evaporates, causing the particles to bind tightly together. Adding water helps separate the platelets again so they can move freely.

The ideal water content can vary based on the type of clay, but generally 25-35% water by weight is a good target. Start with less water and add gradually until the clay reaches the desired consistency. Too much water can lead to floppiness, while too little water will leave the clay stiff and unworkable.

There are a few techniques for adding water to clay:

• Spritz clay with a spray bottle, mixing thoroughly as you spritz.
• Knead in water by hand, a little at a time.
• Place unwrapped clay in an airtight plastic bag with a damp paper towel. The clay will absorb moisture from the towel.
• Allow clay to soak in a bucket of water for 10-30 minutes, then knead by hand to distribute moisture evenly.

Always knead the clay after adding water to blend it throughout the clay. Check the consistency and feel of the clay as you knead, adding more water if needed. The clay is ready when it’s smooth, flexible, and molded easily without sticking excessively to your hands.

Use Plasticizers

Plasticizers are additives that can be mixed into clay to increase its plasticity and workability. Plasticizers work by bonding to the clay particles and lubricating them, allowing them to slide over each other more easily. This gives the clay more flexibility and stretch.

Some common plasticizers used in clay include:

• Glycerin – A clear, odorless liquid that is hygroscopic, meaning it attracts moisture. Glycerin bonds with the clay and keeps it from drying out.
• Vegetable oils – Oils like linseed oil provide lubrication and prevent clay from cracking as it dries. They allow clay to be worked without becoming too sticky.
• Liquin – A popular oil-based liquid plasticizer made for oil paints. It helps disperse pigments smoothly.

The amount of plasticizer to add depends on the specific clay body and desired plasticity. Start with a small amount, such as 1-2 teaspoons per pound of clay. Knead it in thoroughly and test the plasticity. Add more as needed, up to around 2 tablespoons per pound. Too much can make the clay sticky or prevent it from hardening properly.

Add Flexible Materials

Adding certain flexible materials to clay can help increase its plasticity. Some common materials used for this purpose include:

Bentonite

Bentonite is a type of clay formed from volcanic ash. It has excellent water absorption properties and high plasticity. Adding a small amount of bentonite, usually 5-10%, will increase the plasticity and workability of clay. Bentonite helps clay particles slide over each other, improving its flexibility.

Paper Pulp

Mixing recycled and dried paper pulp into clay body adds fiber content. The cellulose fibers from paper help bind clay particles together while allowing flexibility. Paper pulp increases plasticity and also reduces cracking and shrinkage during drying and firing.

Vermiculite

Vermiculite is a mineral that expands greatly when heated. Adding a small quantity of vermiculite, around 1-5%, can enhance the plasticity of clay. The platy structure of vermiculite improves clay’s workability. Vermiculite also benefits clay by reducing shrinkage and acting as a filler material.

Avoid Overworking the Clay

Overworking or over-kneading clay can actually decrease plasticity and flexibility. Kneading aligns the clay particles and breaks down the long-chain molecules, making it stiffer.

Signs your clay is overworked include it cracking easily when bent, brittleness, crumbling, and resistance to taking shape. The clay may also feel quite warm or hot to the touch from friction if it’s been kneaded extensively.

A good rule of thumb is to knead just until the clay is smooth, pliable, and thoroughly mixed – no more than a few minutes usually. Additionally, avoid re-wedging scraps of clay excessively to preserve plasticity.

Store Clay Properly

Proper storage is crucial for maintaining the plasticity of clay over time. Here are some tips for storing clay:

Seal clay in airtight packaging or containers. Exposing clay to air causes it to dry out. Store clay in sealed plastic bags or airtight plastic containers. Press out any excess air before sealing to prevent air pockets from forming inside.

Control humidity. Keep clay in a space with consistent humidity between 40-60%. Drastic changes in humidity can alter the moisture content in clay and affect its workability. Avoid storing clay in places with large humidity fluctuations like a damp basement or dry attic.

With proper sealing and humidity control, clay can retain its plasticity for months or even years when stored. Always check clay for optimal moisture content before using if it has been stored for extended periods.

Recondition Old Clay

If your clay has been sitting unused and exposed to air for an extended period, it can become dried out and lose its plasticity. Luckily, there are ways to recondition old clay and restore its workability.

The first step is to remoisten the clay by misting it with water and allowing it to soak in. Start with small amounts of water and mix thoroughly before adding more. Be patient during this process, as it may take 15-30 minutes for the clay to absorb enough moisture. Test the clay’s plasticity periodically by pinching and bending it.

Once the clay is pliable enough, you’ll need to re-wedge it thoroughly. Wedging is the process of cutting and slamming clay against a hard surface to mix in the moisture and eliminate air bubbles. Knead, fold, and press the clay continuously for 5-10 minutes until it has a smooth, uniform consistency. The clay should now be soft and workable again.

With some TLC and moisture, even very dry and crumbly clay can be revived. Just be careful not to make the clay too wet during reconditioning. Allow time for thorough absorption. With the right technique, old clay can become fresh and plastic again.

Test and Monitor Plasticity

There are a few simple tests you can do to check the plasticity of your clay:

Ribbon Test

Take a small piece of clay and roll it into a ball. Press it between your thumb and forefinger to flatten it into a ribbon shape. Measure the length of the ribbon before it breaks. Higher plasticity clays can be rolled into longer ribbons without breaking.

Rod Test

Roll a piece of clay into a cylinder rod shape approximately 1/4 inch thick. Hold the rod horizontally and observe how much it bends before breaking. The more it bends without breaking, the higher the plasticity.

Assessing as You Work

As you are working with the clay, notice how easily it bends and flexes without cracking. Clay that is cracking or breaking as you sculpt with it likely needs more moisture to increase plasticity.

When to Use Less Plastic Clay

While very plastic clay is ideal for most pottery and sculpture, sometimes a less plastic clay body is preferable. Here are some of the benefits of using a stiffer, lower plasticity clay:

• Holds shape better – Less plastic clay will hold its shape when sculpting or throwing on the wheel. The sculpture or pot won’t deform as easily before drying and firing.
• Less shrinkage – A stiffer clay body will shrink less during the drying and firing process, reducing cracking and warping.
• Crisp details – A clay body with low plasticity can create sharper edges and crisper detail in sculptures, carvings, and decorations.
• Sturdy forms – Lower plasticity clay is great for making large, hollow pieces like pots and vases, as it provides more structural integrity.

There are a few techniques to improve workability with less plastic clay:

• Knead thoroughly – Spend more time wedging and kneading the clay to increase pliability.
• Use stiff rib tools – Stiff metal ribs can shape less plastic clay effectively.
• Add water sparingly – Use water to lubricate and smooth clay as needed, but avoid over-wetting.
• Work slowly – Take your time; don’t rush shaping and smoothing stiff clay.
• Let pieces firm up – Allow dried clay pieces to stiffen adequately before detail work.

With care and practice, less plastic clay can become highly workable while providing advantages over extremely malleable clay.