What Do You Need To Harden Clay?

Clay hardening is the process of transforming soft, malleable clay into a hardened, durable material through drying and firing. Clay is a versatile medium used for pottery, sculpture, and other decorative or functional objects. In its natural state, clay contains water which makes it soft and pliable. To create finished clay items that maintain their shape and are strong enough for their intended use, the clay must be hardened.

There are several steps involved in properly hardening clay. The initial drying process removes excess moisture from the clay. Firing clay at high temperatures sinters the clay particles together, causing the clay to become permanently hard. Additional finishing processes may be used after firing to further seal and protect the clay or apply decorative touches. When done correctly, these processes transform the raw clay into a hardened material that can withstand everyday use without crumbling or becoming misshapen.

The main benefit of hardening clay is that it allows clay objects to become strong, rigid, and durable while still retaining their modeled shape. Hardened clay maintains its form without deforming under the stresses of regular use. Clay that has not been properly hardened and fired may slowly warp, crack, or even dissolve if subjected to moisture over time. Hardening stabilizes the clay material so finished objects will hold up well for their intended purpose. The process also allows the clay to become water-resistant. Overall, hardening allows the full potential and longevity of clay artwork or functional wares to be realized.

Drying

Air drying is the most common method for drying clay. With air drying, the clay is left out at room temperature to allow the water within it to evaporate. There are a few pros and cons to be aware of with air drying:

Pros:

• It is the easiest and most accessible drying method, requiring no special equipment.
• Air drying is inexpensive and budget-friendly.
• The slow drying time allows moisture to evaporate evenly, reducing cracking and breakage.

Cons:

• Air drying takes a long time, typically 1-2 weeks for pieces 1/4″ thick.
• The slow process also means pieces are vulnerable to dust, bugs, and other environmental contaminants.
• Clay can dry unevenly if thickness varies in the sculpture.

To ensure proper air drying, keep these tips in mind:

• Air dry in a clean, dust-free space with stable room temperature.
• Place clay on wire racks so air can circulate underneath.
• Flip pieces over at least once midway through drying.
• Avoid drying too quickly near heat sources like vents or ovens.
• Monitor and allow thicker areas to dry fully before baking.

Firing

Firing is an essential step in hardening clay and ceramic pieces. The goal of firing is to heat up the clay to high temperatures which cause chemical reactions that permanently harden and strengthen the clay.

The most common method of firing clay is using a ceramic kiln, which allows precise control over temperature. Most clays require firing to temperatures between 1800°F – 2300°F in order to fully vitrify and become hard and durable. The exact temperature depends on the type of clay body and any glazes or colorants added.

Some key advantages of firing in a kiln include:

• Ability to control ramp speeds and target temperatures
• Even heating throughout the kiln chamber
• Minimized risk of defects like cracking
• Firing to higher temperatures for maximum hardness

However, firing without a kiln is also possible. Methods include pit firing, raku firing, and primitive firing. While these can achieve charming effects, they have some limitations:

• Less control over temperatures and atmosphere
• More likelihood of cracking or exploding
• Lower maximum temperatures
• Greater unpredictability in final results

For most pottery and ceramic work where durability is desired, kiln firing produces the best and most consistent results. With proper clay preparation, slow ramp speeds, and optimal target temperatures, kiln firing allows clay artists to permanently harden their creations.

Cold Finishing

Cold finishing refers to techniques applied to hardened clay pieces to refine their surface and appearance. Two common cold finishing methods are sanding and burnishing.

Sanding is the process of smoothing clay surfaces using abrasive materials like sandpaper. It helps remove any small imperfections left after the clay has been fired. Sanding can create a uniform, matte look. The main advantage of sanding is that it allows for precision removal of surface irregularities. However, it can be time-consuming for large pieces. Improper sanding may also leave undesirable scratches.

Burnishing involves rubbing the clay surface with a hard object to compact the clay and create a smooth, shiny finish. Tools like stones, steel, or hard wood are commonly used. Burnishing is quick and achieves an attractive polished effect. However, it requires sufficient moisture in the clay and does not work as well for intricate surfaces. Over-burnishing may also close the clay pores excessively.

In summary, sanding and burnishing allow artisans to refine the look of finished clay wares through cold work. The technique chosen depends on the needs of the piece and the preferences of the artisan. Both methods have their pros and cons when finishing pottery or sculptures.

Sealants

Sealants are used to seal the porous surface of clay and ceramics against moisture, staining, and improve durability. There are several types of sealants that can be used on clay:

Acrylic Sealers

Acrylic sealers are water-based and provide good surface protection. They are easy to apply with a brush or spray and dry quickly. Acrylics are non-toxic and affordable. They provide moderate protection against moisture and stains.

Polyurethane

Polyurethane is a common sealant that provides very good surface protection. It creates a hard, durable coating that seals the clay thoroughly against moisture and stains. Polyurethane can be applied by brushing or spraying. It dries slower than acrylics and has a strong odor during application.

Epoxy Resin

Epoxy resin creates an extremely hard, glass-like coating over the clay surface. It provides excellent protection against moisture, stains, scratches and heat damage. Epoxy resin can be tricky to apply evenly and requires mixing two components. It also has a strong odor during application.

Wax

Wax provides a soft, natural-looking seal over clay surfaces. It protects moderately against moisture and stains. Wax is easy to apply by brushing melted wax on the clay and buffing it to a sheen. Multiple coats of wax can be applied to increase protection. Wax can wear away over time and may need reapplication.

When choosing a sealant, consider the level of protection needed, the look you want, and the application method. Testing different sealants on sample pieces first is recommended to ensure compatibility with the clay body and desired results.

Paints

Painting hardened clay adds color and protects the surface. There are several types of paints that work well on clay:

Acrylics – Acrylic paint is a popular choice for painting clay. It dries quickly, is easy to clean up with water, and comes in many colors. Acrylics work on both bare and glazed clay. Apply acrylics in thin layers, allowing each coat to dry fully before adding another. Acrylic paints can be diluted with water for a transparent effect.

Oils – Oil paints provide rich, vibrant color on clay. Oils take much longer to dry than acrylics, so are better for detailed brushwork. Allow oil paints to cure fully before sealing or applying a topcoat. Oils require solvents like turpentine for thinning and clean-up.

Specialty Paints – Some paints are formulated specifically for ceramics. These often have better adhesion and durability on clay surfaces. Ceramic paint pens allow hand decoration of bisqueware. Metallic paints can create shimmering accents. Test specialty paints on sample pieces first.

Application Tips – Apply paint in thin, even coats using soft brushes. Allow each layer to dry before adding another. Painting bisqueware or bare clay will produce the best adhesion. Clean brushes promptly in the appropriate solvent. Allow paints to cure fully before topcoating or sealing the surface.

Glazes

Glazes are a type of coating applied to hardened clay surfaces to provide an impervious and decorative finish. They are an essential part of finishing and decorating clay wares. There are several types of glazes used in pottery and ceramics:

Types of Glazes

– Underglaze – Applied to unfired clay and becomes part of the surface after firing. Often used for decorative patterns under a transparent glaze.

– Cone 6 Glaze – Matures and vitrifies at around 2200°F. Durable all-purpose glaze suitable for stoneware.

– Low Fire Glaze – Matures at lower temperatures, usually between 1500°F – 1800°F. Used for earthenware and raku ware.

– Mid Range Glaze – Fires between 1900°F – 2150°F. Used for stoneware and porcelain.

– High Fire Glaze – Fires above 2250°F. Used for porcelain and high fire stoneware or porcelain.

Application Methods

Glazes can be applied to bisque ware by various methods:

– Dipping – Submerging ware into a bucket or vat of glaze.

– Brushing – Using a brush to brush glaze onto the surface.

– Spraying – Spraying glaze through an airbrush or spray gun.

– Pouring – Pouring glaze directly onto a piece.

Required Firing

Glazes must be fired in a kiln at the appropriate temperature to mature and create a glassy vitrified surface. Firing temperature depends on the type of clay body and glaze used. Typical ranges are cone 04-6 for low fire, cone 5-7 for mid range, and cone 9-10 for high fire glazes.

Pros and Cons

Some benefits of using glazes:

– Make pieces impervious to liquids

– Create glossy or matte decorative finishes

– Available in endless colors and effects

– Enhance and protect the clay surface

Some drawbacks:

– Adding an extra step compared to natural clay finishes

– Potential for glaze defects if fired improperly

– Glazes can be toxic and require proper handling

Overall, glazes are an indispensable part of most clay ware production, providing beautiful and functional enhancement to ceramic objects.

Varnishes

Varnishes are clear, durable coatings that provide a hard surface for clay pieces. They can be used to both seal and protect the clay surface. There are several types of varnishes to choose from:

Polyurethane varnishes – These provide a very hard, durable finish. They are resistant to water, chemicals, and abrasion. Polyurethane varnishes come in gloss, satin, and matte finishes.

Acrylic varnishes – Acrylics are water-based varnishes that dry quickly and resist yellowing. They provide good protection and can be used for either the final finish or as a sealer under other varnishes.

Solvent-based varnishes – These oil-based varnishes provide excellent durability and resistance to water. However, they contain volatile organic compounds (VOCs) and require proper ventilation when applying.

Varnishes can be applied with a brush, cloth, or by dipping or spraying the clay object. Multiple coats are usually needed for best results. It’s important to allow each coat to fully dry before adding the next. Lightly sanding between coats can help improve adhesion.

Advantages of varnishes include their durability, high gloss finish, and water resistance. Drawbacks are they can yellow over time and darken the original clay color. Varnishes also require proper application and drying conditions to avoid bubbling or clouding of the surface.

Waxes

Waxes are often used as a final protective coating on dried and fired clay pieces. Waxes provide a smooth, durable, water-resistant finish.

There are several types of waxes that can be used:

– Beeswax – This natural wax has a warm, yellowish color. It can be softened with heat and applied as a paste wax.

– Carnauba wax – Derived from palm leaves, this is a hard wax that produces a glossy finish.

– Paraffin wax – This petroleum-based wax is relatively inexpensive. It melts easily for application.

– Microcrystalline waxes – These waxes contain small crystal particles that make them quite durable.

Waxes can be applied in a few different ways. The melted wax can be brushed or sprayed directly onto the clay surface. Alternatively, the clay piece can be dipped into the melted wax. Wax pastes can also be rubbed vigorously into the surface.

The main benefits of waxes are their water-resistant properties and the smooth, protective coating they provide. Waxes also enhance the color and luster of the clay. The wax fills in small cracks and imperfections for a flawless finish.

However, waxes can sometimes give an uneven, blotchy appearance, especially with porous clay bodies. Wax finishes also require maintenance over time, as the wax wears away with use. Wax coatings are easy to restore by reapplying a new coat of wax when needed.

Conclusion

There are several techniques that can be used to harden clay, and the right method depends on the type of clay, the desired hardness level, and the final look you want to achieve. Air drying is the simplest approach and requires no special equipment, just time and airflow. Firing clay in a kiln is necessary for reaching maximum hardness, but can require significant investment in equipment. Cold finishing techniques like sealing, painting, and applying glazes allow for creative customization of hardened pieces. Waxes and varnishes can provide protection and sheen. Evaluate the level of hardness required, budget, timeline, and visual effects when deciding on the best hardening technique for a clay project.