What Do You Do To Clay To Make It Hard?

Clay is a type of fine-grained natural soil that exhibits plasticity when mixed with water. It is made up of hydrated aluminosilicates and other minerals that give it unique properties. When clay is first dug from the ground, it is soft and malleable. However, through various processes, clay can be hardened to create pottery, ceramics, bricks, and other objects.

There are two main methods for hardening clay: drying and firing. Drying involves slowly removing the water from the molded clay object through evaporation. As the water evaporates, the clay particles move closer together and bond with each other, creating a sturdier structure. Firing involves heating the dried clay object to high temperatures in a kiln, which causes chemical and physical changes that permanently harden the clay.

This article will provide an in-depth overview of the different techniques used to turn soft clay into a hardened, durable material suitable for functional and artistic purposes.

Drying

Drying is one of the simplest ways to harden clay. It involves leaving clay pieces out in the open air to allow the moisture in the clay to evaporate. As the water leaves the clay, the clay particles move closer together, causing the clay to become dense and harden.

There are a few pros and cons to air drying clay:

Pros:

• It’s very simple and doesn’t require any special equipment – just access to air.
• It’s inexpensive and accessible.
• It works for most types of clay.

Cons:

• It can take a long time for clay to fully air dry, usually days or weeks depending on factors like the size and thickness of the piece.
• Clay can crack or warp during the drying process if it dries unevenly or too quickly.
• Air dried pieces are quite fragile and porous.

The drying time depends on several factors like the size of the piece, the thickness of the walls, the type of clay used, and the humidity and air circulation in the drying area. Smaller pieces may air dry within a few days whereas larger or thicker pieces can take weeks to fully dry. Turning and rotating pieces periodically allows for even drying.

Firing

Firing clay is the process of exposing hardened unfired clay to high temperatures in a kiln, which vitrifies the clay and makes it permanent. Proper firing is essential for creating strong, finished ceramic pieces. There are several key aspects of firing clay:

Kiln Firing

The most common method for firing clay is in an electric or gas kiln, which allows precise control over temperature. Kilns heat up at a gradual rate to prevent cracking, until reaching a peak temperature anywhere from earthenware (1000-1200°F) to stoneware (2200-2400°F) ranges. The clay must be held at the peak temperature long enough for the materials to vitrify.

Advantages of kiln firing include:

• Consistent results with precise temperature control
• Ability to fire a number of pieces at once
• Programmable controllers for automatic firing
• Cleaner and more environmentally friendly than some other firing methods

Potential disadvantages include:

• High upfront equipment costs
• Learning curve for programming and operating kilns
• Space requirements for the kiln
• Electrical power requirements

Bisque Firing

Bisque firing, also known as biscuit firing, is an essential step in transforming soft clay into a hard, finished ceramic piece. It is done before any glazes are applied. Bisque firing strengthens the physical form of the clay by driving out any remaining water and organic matter through evaporation and chemical decomposition.

The ideal temperature for bisque firing ranges between 1600-2100°F depending on the type of clay. Firing too low can result in a piece that is still fragile, while exceeding the maximum temperature can cause bloating, slumping, or cracking.

When bisque firing unglazed pieces, it is important that the clay is fully dry to prevent cracking from escaping steam. The slower ramp up in temperature allows water molecules to evacuate without damaging the structure. Once the peak temperature is reached, holding time should be sufficient for heat to fully penetrate the clay body.

For pieces that will later be glazed, bisque firing prepares the surface to properly accept and bond with the glaze. The porous bisque surface provides spaces for the glaze to interlock and adhere. Firing to the right cone level hardens the bisque enough to support the glaze application without deforming.

Glazing

Glazes are vitreous coatings applied to ceramic ware to create a glassy surface and decorative effects. The glaze is made up of a mixture of materials that fuse to the clay body during the firing process, forming a hardened glass coating.

Glazes contain three main ingredients: glass formers, fluxes, and stabilizers. The glass formers include materials like silica, boron, and feldspar, which form the glass matrix. Fluxes like alkali metal oxides lower the melting temperature of the glaze so it flows and fuses at the kiln temperatures. Stabilizers like alumina prevent the glaze from becoming too runny and help the glaze adhere to the clay body.

There are many techniques for applying glazes. Dipping directly coats the piece with glaze. Spraying through an airbrush or spray gun creates a fine mist and even coat. Brushing is good for targeted glaze application. Pouring and sponging create effects like running drips. Glazes can also be screen printed for detailed patterns.

Once applied, the glazed wares are loaded into the kiln and fired to high temperatures like 2200°F to melt the glaze ingredients, allowing them to fuse to the clay surface. Specific firing profiles are used to control the heating and cooling rates for an ideal glaze finish. The final result is a durable, decorative coating that adds visual interest and makes pieces functional and waterproof.

Cold Finishing

After firing, there are several cold finishing techniques that can be used to alter the look and feel of finished clay pieces. Three of the most common cold finishing methods are sanding, waxing, and painting.

Sanding is done to smooth rough edges and surfaces of bisque fired clay. Start with a coarse grit sandpaper and work your way to finer and finer grits for a smooth sanded surface. Be sure to sand gently and watch for softened edges. Sanding bisque ware should be done before any other cold finishing.

Applying wax is another technique to change the look and feel of fired clay. Wax fills in porous clay surfaces creating a smooth texture and slight sheen. Beeswax, paraffin wax, and carnauba wax are commonly used. Apply a thin coat of warmed wax with a brush or cloth and buff to a shine. Multiple coats can be added for a deeper luster.

Acrylic paints specifically formulated for ceramics can be used to hand paint designs onto bisque fired pieces. These acrylics will adhere to the porous clay surface without requiring a glaze coating. Painting can be done in any style from simple accents to complex artwork. Seal painted pieces with a clear acrylic sealer when complete.

Additives

One way to alter the properties and workability of clay is by adding other materials, known as additives or inclusions. Some common additives used in ceramics include:

Grogs

Grogs are previously fired and ground up clay particles that are added back into wet clay. Adding grog makes the clay body stronger, less prone to cracking and shrinking during drying and firing. Grogs increase porosity, allowing gases to escape during firing. They come in different particle sizes and can be premixed into commercial clay bodies. Up to 30% grog can be added to stoneware clays. An excessive amount of grog can decrease plasticity and make the clay difficult to work with.

Grog is often made from fired bricks or clay liner waste. It can also be purchased as ready-made additions for clay. Grogs with larger particles are suited for handbuilding and sculpture. Finer grog particles work better for throwing on the wheel.

Grogged clay requires slightly higher firing temperatures and longer soak times. Pieces may need to be fired on stilts or kiln posts to prevent sagging. Using grog results in a rougher, more textured surface that can add interesting visual effects.

Clay Chemistry

The chemical composition of clay is an important factor in how it behaves and how hard it becomes when fired. The main component of most pottery clays is silica (SiO2). Silica is a refractory material that can withstand very high temperatures which makes it ideal for pottery that will be fired in a kiln. However, silica alone does not form a workable clay. Clays also contain fluxes which lower the melting temperature. Common fluxes include feldspar, dolomite, and calcium carbonate. These fluxes promote vitrification, which is the fusing of the clay particles into a non-porous material.

The amount of fluxes in the clay body affects the final hardness and durability. Clays with higher flux content will vitrify at lower temperatures compared to high silica clays. However, too much flux can cause overfiring which results in warping, bloating, and weakening of the ceramic. The ideal balance is a clay with enough fluxes to vitrify properly at the intended firing temperature, but not so much that the body loses strength.

The mineralogical composition along with the firing process give clay its hardness and other physical properties. Understanding the chemistry behind clay allows potters to select the right clays and firing methods to achieve the characteristics they want in the finished ceramic piece.

Troubleshooting

Clay pieces can develop issues during the drying and firing process that affect their structural integrity. Here are some common problems and ways to fix or prevent them:

Cracks

Cracks often form because the clay dried too quickly. To prevent cracking:

• Allow clay pieces to dry slowly at room temperature.
• Avoid placing wet clay pieces directly under fans or heaters.
• Wrap clay in plastic if it begins drying too fast.

For cracked bisque ware, fill smaller cracks with a clay slip. Larger cracks may require adhesive and support pieces.

Weak Spots

Thin or uneven clay walls can cause weak spots and breaks. To prevent weak spots:

• Maintain an even thickness when sculpting clay.
• Avoid making walls and appendages too thin.
• Reinforce weak areas with additional clay or a permanent armature.

Bisque pieces with minor weak spots can be reinforced with adhesives before firing again. Pieces with major structural issues may not be fixable.

Fixing Issues

Some common solutions for flawed pieces include:

• Filling cracks and holes with clay slip
• Reinforcing weak areas with additional clay
• Drilling holes to prevent cracking during firing
• Using adhesive and support pieces for cracked bisque
• Sanding rough areas smooth

While some flaws can be fixed, severely damaged pieces may need to be discarded and remade for the best results.

Conclusion

There are several key processes that can be used to harden clay and transform it into a finished ceramic product. Drying, firing, glazing, and cold finishing techniques all play an important role.

To summarize, clay first needs to be air dried or dried in a kiln to remove moisture content. Next, bisque firing at lower temperatures prepares the clay for glazing by making it slightly porous. Glazes can then be applied to create a glassy coating. Finally, higher temperature glaze firing hardens and sets the glaze. Cold finishing methods like sanding and polishing can also be used to refine the final surface.

With the right combination of drying, firing, glazing, and finishing, clay can be hardened into a long-lasting ceramic material. The key is patience and allowing enough time for each step. Avoid rushing the process or skipping steps. Test samples first to optimize techniques for a particular clay body and desired results.

Following proper procedures for transforming clay ensures a strong, durable end product that will last for many years to come.