What Is In A Clay Body?

A clay body refers to the composition of a clay mixture used for pottery and ceramics. It is made up of different natural materials that provide specific qualities when fired.

The primary component of any clay body is clay. Clays are sedimentary rocks made up of fine particles that become plastic and malleable when wet, allowing them to be shaped. The type and amount of clay determines the plasticity, shrinkage, porosity, color, and firing temperature of the clay body.

In addition to clay, a clay body contains other ingredients called fluxes, fillers, colorants, and additives that modify the clay’s working properties, appearance, and physical characteristics when fired. The ingredients and their proportions make up the clay body’s recipe. Finding the right recipe allows potters to achieve their desired results.

Understanding clay bodies is fundamental for potters looking to throw functional pottery or create ceramic art. The composition directly impacts the workability, drying, firing, strength, color, and texture of finished ware.

Clay

Clay is the primary ingredient in any ceramic clay body. There are three main types of clay used in pottery:

• Earthenware clays – These clays contain lower levels of fluxes and require lower firing temperatures, typically between 1800-2100°F. They result in absorbent clay bodies.
• Stoneware clays – Stoneware clays have higher flux content, requiring firing between 2200-2400°F. This results in low absorption clay bodies that are durable.
• Porcelain clays – Porcelains have the highest kaolin clay content with very low impurities. They require the highest firing temperatures, around 2300-2600°F, and produce non-porous, highly vitrified ware.

The clay composition determines important properties like plasticity, drying performance, firing temperature, absorption, color, and texture. Plasticity refers to how easily the clay can be worked and shaped. Higher plastic clays have longer molecular chains while shorter chains produce less plastic clays. Very plastic clays can stretch and hold their shape well but may be prone to warping and cracking during drying. Less plastic clays may crumble more easily when worked but can dry and fire with less distortion.

Clay is mined from deposits across the world with varying properties depending on their composition and location. Secondary clays are sedimentary deposits washed into rivers, lakes, and seas from weathering rock while primary clays are residual deposits found closer to their parent material. Some clays are mined overseas and exported while others come from local deposits. Unique regional clays give potters a range of options to create desired results.

Flux

Flux is an essential ingredient in clay bodies that helps the clay vitrify and mature at high temperatures. The main purpose of adding flux to clay is to lower its melting temperature so that it can become glassy and non-porous during firing.

The most common fluxes used in ceramic glazes and clay bodies are feldspar, talc, dolomite, and nepheline syenite. Feldspar is the most popular ceramic flux and melts at around 1100°C to 1200°C depending on the type. It promotes vitrification and improves the strength and workability of clay. Talc and dolomite have lower melting points around 900°C and can create issues like bloating if too much is used. Nepheline syenite melts at around 1050°C and helps control thermal expansion.

Adding an appropriate amount of flux controls the firing behavior of clay. With higher amounts of flux, clay will vitrify at lower temperatures. Too much flux can lead to over-firing effects. The type of flux also impacts the final color and surface quality. Overall, flux is essential for maturing clay bodies and facilitating vitrification during firing.

Fillers

Fillers are non-plastic particles added to clay to improve its working properties, drying behavior, and fired characteristics. There are several common types of fillers used in clay bodies:

Grog

Grog is ground up fired clay or brick. It comes in different particle sizes from coarse to fine. Grog reduces shrinkage and warping, increases porosity, and can help stiffen bodies. The coarser the grog, the more porous and less plastic the clay body becomes.

Sand

Sands are silicas in ground form. They reduce shrinkage and warping, but not as much as grog. Sands increase thermal expansion slightly which can help counteract excessive vitrification in some clay bodies. They also aid in vitrification at lower temperatures.

Vermiculite

Vermiculite is a lightweight mica that expands when heated. It is useful for reducing weight in sculptures or planters. Vermiculite increases porosity and absorbs firing shrinkage in the clay body.

Perlite

Like vermiculite, perlite is a lightweight volcanic glass that expands when fired. Perlite reduces weight and increases insulating properties in clay bodies. However, it breaks down at higher temperatures so is generally used in low-fire applications.

Organic Materials

Some organic materials like sawdust, leaves, or straw can be used as fillers in low-fire clay bodies. When the organic matter burns out during firing, it leaves behind pores and reduces shrinkage. Too much can weaken the body so organic fillers are generally kept below 20%.

Colorants

Colorants are added to clay bodies to achieve different colors and visual effects. Clay artists have two main options for adding color – natural mineral pigments or synthetic stains. Each type of colorant produces different results in the final fired clay.

Natural mineral pigments are made from iron, cobalt, copper, and other metal oxides that are mined from the earth. These pigments are ground into a fine powder and mixed directly into the clay body. Natural pigments produce earthy tones ranging from deep reds and browns to grays. The advantage of mineral pigments is that they are very stable at high temperatures so the color remains vibrant after firing. However, mineral pigments can be messier to work with compared to synthetic stains.

Synthetic stains are made of concentrated ceramic chemicals combined to produce specific colors. They are available in liquid and powder forms. Synthetic stains create bright, intense colors like blues, greens, yellows, oranges, pinks, and purples. The downside is that some synthetic stains can burn out or fade at high temperatures. So careful testing is required to find stains that remain colorfast through firing. In general, synthetic stains are easier to use and mix more evenly into clay compared to natural pigments.

Whether natural or synthetic, colorants allow clay artists to introduce all the colors of the rainbow into their ceramic work. The percentages and combinations of colorants can dramatically affect the final fired appearance. So endless experimentation with colorants is part of the joy in clay artistry.

Textures

Adding textures to clay can create interesting visual and tactile effects in finished ceramic pieces. There are several tools and techniques potters can use to alter the smooth, flat surface of clay and introduce patterns and visual interest.

One way to add texture is by rolling or pressing various materials into the clay surface. Things like lace, burlap, netting, seeds, nuts, bolts, keys, doilies, and other found objects can be pressed into soft clay to leave impressions. Using texture mats, stamps, and rollers specifically designed for clay can quickly add geometric designs, simulated wood grain, or other patterns.

Potters can also use clay cutting tools to incise lines, hatch marks, and other designs into leatherhard clay. Combing, scratching, and carving clay with various tools creates grooves and etched textures. Using a slip trailer, painters use liquid clay slip to draw freehand textures directly onto bisqueware or greenware clay.

Some potters also add texture by altering the clay surface with brushes and sponges. Pouncing, stippling, and brushing clay in different directions creates visual interest. Sponging and brushing slip onto bisqueware can also produce intriguing effects. With some creativity and experimentation, textures can add incredible visual richness to finished ceramic pieces.

Slip

Slip is a vital ingredient in most clay bodies. It is a suspension of clay particles in water, giving it a thick, creamy consistency. The main purposes of slip in clay are:

• Binding – Slip acts as a natural glue to hold clay particles together and increase plasticity and strength.
• Lubrication – The water in slip lubricates clay particles, improving workability.
• Coating – Slip is brushed on bisqueware to create an even, smooth surface before glazing.

There are several standard slip recipes used in pottery and ceramics:

• 50/50 slip – Equal parts clay and water by weight or volume.
• 65/35 slip – 65% clay to 35% water, creates a thicker consistency.
• Porcelain slip – Made from porcelain clay, provides smoothness and whiteness.
• Casting slip – High in clay content with deflocculants, ideal for slipcasting.

Specific clays like kaolin or ball clay are often used to produce slips with desired properties. The ratio of clay to water can be adjusted to create thinner or thicker slips as needed for each application.

Additives

Clay artists can add other materials besides clay to a clay body recipe to achieve desired properties in the finished piece. Some common additives include:

Grog

Grog refers to pre-fired clay that has been ground into a granular form. Adding grog to a clay body can help reduce shrinkage and cracking as the piece dries and fires. It also makes the clay body stronger and more porous. A coarse grog is good for handbuilding while a fine grog works better for throwing on the wheel.

Vermiculite

Vermiculite is a lightweight spongy material made from mica. Adding vermiculite to clay makes the body lighter and more insulating while still maintaining an open clay-like structure. Vermiculite improves thermal shock resistance and reduces shrinkage.

Perlite

Like vermiculite, perlite is a lightweight volcanic glass that expands when heated. It is useful for reducing the density and shrinkage of clay bodies. Perlite also improves insulation and thermal shock resistance.

Fiber

Natural fibers like straw, sawdust, wood shavings, and paper can be added to clay to increase strength and durability while reducing shrinkage, cracking, and weight. Synthetic fibers like nylon, polyester, and fiberglass serve the same purposes.

The type and amount of additives like these can significantly alter the properties and workability of a clay body. Testing is required to achieve the desired characteristics in the finished ceramic piece.

Recipes

There are many standard ceramic clay body recipes that potters use as a starting point when formulating their own clay bodies. These recipes combine different proportions of clay, flux, fillers, and other additives to achieve desired characteristics. Some common basic clay body recipes include:

Stoneware: 25% ball clay, 25% kaolin, 35% feldspar, 15% silica

Porcelain: 25% kaolin, 25% pottery stone, 50% feldspar

Terracotta: 50% red art clay, 30% ball clay, 20% grog (fired clay particles)

The recipes above can be modified by changing the ratios of ingredients or substituting different specific clays/feldspars. Potters will test variations by doing small test firings to see the effects on color, firing temperature, strength, absorbency etc. Factors like the clay’s plasticity, shrinkage, color, and fired strength can all be adjusted through testing. Having a good knowledge of how different ingredients interact allows potters to custom design clay bodies for their specific needs.

Testing and Adjusting

Once you have a clay recipe, the next step is to test it to see how it performs. This allows you to identify any issues and make adjustments to optimize the clay body before committing to a large batch.

Test Balls

Make a few small balls of clay from your recipe and allow them to dry completely. Check for cracks, warping, or other drying issues. Test balls let you quickly see drying performance without having to throw a large piece.

Test Tiles

Form some test tiles and fire them to your desired temperature. Check for excessive shrinkage, warping, cracks, etc. Test tiles give you visual feedback on how the clay responds to firing.

Firing

Fire test pieces using the same firing schedule you plan to use for finished pieces. Compare results across various temperatures to choose your ideal range.

Tweaking Recipes

Based on drying and firing tests, you can refine the recipe. If it warps, add flux. If it cracks, reduce flux or add grog. The goal is to achieve your desired working properties, firing results, and aesthetic.

With careful testing and adjustments, you can dial in a custom clay body tailored to your needs.