How Can You Harden Clay?

Clay hardening is the process of removing moisture from clay to improve its strength and durability. There are several techniques used to harden clay, each with their own benefits and drawbacks.

The most common methods used to harden clay are air drying, oven drying, adding hardening agents, and firing in a kiln. The choice depends on factors like the type of clay, the desired hardness, the resources available, and the final application.

Hardening clay is an important step when creating pottery, sculptures, or other clay crafts and projects. The benefits of properly hardened clay include:

• Increased strength and durability for long-lasting pieces
• Prevents cracking, warping, or damage to the clay work
• Allows painting, glazing, staining, and other surface techniques to be applied
• Enables clay to withstand repeated use and cleaning

Air Drying

One of the simplest ways to harden clay is to leave it out to dry. As the water evaporates from the clay, the clay particles move closer together, forming strong bonds that harden and stiffen the clay. Air drying takes advantage of this natural process.

The time required for air drying depends on factors like the climate/humidity, the thickness of the clay piece, and the type of clay. In a warm, dry environment, small pieces made with earthenware or raku clay may air dry within 24 hours. Larger pieces or clays with higher grog or mineral content may take a few days up to several weeks to fully air dry. Allow even more time if drying in a cool or humid environment.

Tips for optimal air drying include:

• Place clay pieces in a warm, dry, dust-free area with good airflow.
• Elevate pieces off direct contact with any surfaces to allow airflow all around.
• Start with a slowly drying period to avoid cracks – cover pieces with plastic for the first 8-12 hours.
• Turn pieces periodically while drying.
• Avoid drying too fast, which can cause warping or cracks.
• Monitor drying pieces and spray with water if they start to crack from drying too rapidly.

Oven Drying

Using an oven is an effective way to thoroughly dry clay. The key is to use a low temperature over an extended period. Most clays should be dried between 150-200°F. Start by placing the clay on a cookie sheet or baking dish, making sure the pieces are not touching.Heat the oven to 200°F and bake the clay for 8-12 hours. Check on the clay every few hours, rotating the pieces if needed for even drying. The clay is ready when it is dry all the way through and is no longer cool to the touch. To test, break a piece in half and look at the center to ensure no moisture remains.

Adding Hardening Agents

You can mix in various dry materials as hardening agents to help strengthen and solidify clay when air dried or fired at lower temperatures:

Sand

Adding sand is one of the most common ways to harden clay. Use about 1 part sand to 3 parts clay. Too much sand can make the clay brittle and prone to cracking.

Cement

Cement chemically reacts with clay to harden it. Use sparingly, no more than 10% of the total amount. Excess cement may cause cracking.

Grog

Grog is pre-fired and ground up clay that creates texture and absorbs firing shrinkage. Add up to 50% grog for highly durable outdoor sculptures. Too much grog can also make clay prone to cracking.

In general, hardening agents will make the clay less plastic and workable. Only add as much as needed to achieve the desired hardness and durability when dried or fired at lower temps.

Firing

Firing clay in a kiln is the most effective method for hardening clay permanently. Clay undergoes an irreversible chemical reaction when fired at high temperatures, causing the clay particles to fuse together into a hard, durable material.

Most clays require firing temperatures between 1800-2200°F to fully vitrify and become hard. Specific clay bodies have different maturation temperatures based on their composition. For example, earthenware clays mature at lower temperatures around 1800°F, while stoneware clays require higher temperatures near 2200°F to mature.

A proper firing schedule is necessary to bring the kiln through the necessary temperature range at the right pace. The schedule should include a warm-up phase to completely dry any moisture and slowly bring the kiln to temperature. The firing phase heats the kiln to the peak temperature and soaks there to allow the clay to vitrify. Finally, a cooling phase brings the kiln back down in temperature slowly to prevent cracks and breakage.

Firing produces permanent physical and chemical changes in the clay that cannot be undone. The intense heat causes clay particles to partially melt and fuse together, resulting in a dense, non-porous material. Firing also permanently sets any colors or glazes applied to the clay. The end result of firing is a hardened clay object with increased strength and durability.

Sealing

Sealing clay products with a protective coating is an important final step to hardening and preserving the item. Sealants like polyurethane or spray varnishes are commonly used. When applying a sealant:

• Make sure the clay is fully dry before sealing
• Apply in thin, even coats using a brush or spray bottle
• Allow each coat to fully dry before adding another
• 2-3 coats are usually sufficient for good protection
• Avoid over-saturation as this can create drips or tackiness

Sealing will protect the clay from moisture, oils, and other elements that can stain or degrade the finish. However, sealants can wear over time with regular handling and use. Periodically check items and reapply a fresh coat of sealant as needed to maintain the protective barrier.

Proper Storage

Properly storing unused clay is crucial for maintaining its workability and preventing it from drying out and becoming unusable. Here are some tips for keeping clay moist and pliable in storage:

Keep unused clay in an airtight plastic bag or container. Exposure to air causes clay to lose moisture quickly. Placing leftover clay in a sealable plastic bag, Tupperware, or airtight bucket with a tight-fitting lid will prevent air exposure and keep clay from drying out.

Use a damp paper towel to line the storage container. Lining the plasticware with a moist (not sopping wet) paper towel provides extra moisture and keeps the clay from sticking to the container.

Mist clay occasionally with water. Every so often, take the clay out and mist it lightly with water if it seems to be losing moisture. The light mist will rehydrate the clay just enough to maintain its pliability.

Avoid temperature extremes. Store clay away from direct sunlight and heating sources which can cause moisture loss. Cool, dark places like cupboards work best.

Following proper clay storage allows you to keep unused clay workable for months, so none goes to waste before your next project.

Troubleshooting

Clay can crack or break apart during the drying and firing process. Here are some tips for troubleshooting common clay hardening issues:

Cracking and Breakage

If your clay is cracking or breaking apart, the most likely culprit is improper drying. Clay needs to dry slowly and evenly to avoid stresses that lead to cracks. Make sure to wrap clay in plastic wrap or store it in an airtight container if you are air drying. For oven drying, use lower temperatures and dry for longer periods.

Also check that your clay doesn’t have air pockets, which can expand during firing and cause cracks. Wedging your clay thoroughly before shaping can help eliminate trapped air.

Checking Moisture Levels

Use a moisture meter to check the moisture level throughout the drying process. Clay should dry to about 10-25% moisture before firing. If there are large variations in moisture levels within your piece, continue drying.

Reconditioning Dried Clay

If your clay has dried out too much, you may be able to recondition it by wedging in some water. Start with a small amount of water and mix thoroughly. Do not over-wet your clay or it may become sticky. Test the consistency and add more water gradually if needed.

Safety Tips

When working with clay, it’s important to keep safety in mind. Here are some key tips:

Proper ventilation – Make sure your clay workspace is well-ventilated. Opening windows or using a fan can help circulate fresh air and prevent buildup of clay dust.

Avoid dust inhalation – Clay dust can irritate lungs and airways. Wear a protective mask or respirator when sanding dry clay, mixing powdered clay, or cleaning up dried clay dust.

Kiln safety – Firing clay requires very high temperatures. Follow all manufacturer guidelines for safe kiln operation. Wear kiln gloves when accessing the hot kiln chamber and make sure the kiln is on a stable, nonflammable surface.

Conclusion

Working with clay and hardening your creations can be an exciting and fun project. In this article, we’ve explored some of the main techniques for hardening clay, including air drying, oven drying, adding hardening agents, firing, sealing, and proper storage.

Each technique has its own benefits and considerations. Air drying is simple but can take a very long time for larger pieces. Oven drying is faster but requires more equipment. Adding hardening agents like PVA glue or Mod Podge speeds up drying time. Firing at high temperatures in a kiln is necessary for stoneware clays but requires specialized equipment.

Sealing your clay creations, properly storing them away from moisture once hardened, and correcting any issues through troubleshooting will help ensure you end up with durable, long-lasting clay pieces. Taking proper safety precautions and following instructions when using materials like lacquer or firing a kiln are also critical.

With the right technique for your needs and clay type, you can successfully harden your handcrafted clay arts and crafts into lasting treasures. Hardened clay pieces make wonderful decorations, jewelry, dishes, vases, sculptures, and more. We hope this article provided helpful guidance on transforming your soft clay into a hardened, durable work of art.