What Is The Formulation Of Clay Body?

A clay body refers to the formulation of different clay materials and additives used to create a clay for pottery or ceramics. Properly formulating a clay body is essential for achieving the desired characteristics in the final fired ceramic piece.

There are three main types of clays used in most clay bodies – earthenware clays, stoneware clays, and porcelain clays. The different clay types have their own properties that lend themselves to certain applications. For example, porcelain clays are highly refined and create smooth, bright white ceramics that are strong and somewhat translucent. Stoneware clays are less refined than porcelain but create natural, earthy tones and have good strength. Earthenware clays are softest when fired and suitable for hand-building and modeling.

Some key properties that determine the characteristics of a clay body include plasticity, shrinkage, porosity, color, and firing temperature. The goal of clay body formulation is to achieve the optimal combination of plasticity, workability, color, strength, and other desired qualities based on the clay application. Even small variations in clay body composition can significantly impact the final ceramic piece.

Due to the complexity of balancing these different factors, clay body formulation is part science and part art. Testing and adjusting is typically required to perfect a clay body recipe for the particular needs of the ceramic artist or production pottery. An immense variety of clay bodies can be created through careful formulation.

Clay Types

There are several main types of clay used in ceramic formulation:

• Kaolin – Also known as china clay, this is a white, pure clay that is plastic and has high fired strength. It is low in impurities like iron and melts at very high temperatures, making it ideal for porcelain bodies.
• Ball clay – A secondary clay that is very plastic and gives good workability and green strength to clay bodies. Ball clays are more impure than kaolins but they melt at relatively low temperatures.
• Fire clay – Refractory clays that are heat resistant and lend high fired strength to bodies. They are less plastic than ball clays.
• Stoneware clay – These clays are highly vitreous at lower cone temperatures like cone 6. They contain various impurities that aid vitrification.
• Porcelain – Made mainly from kaolin, this clay fires to a white, translucent body at cone 10-14 temperatures.
• Earthenware – Lower firing clays, including terra cotta, that mature at lower cone temperatures like cone 04-6. They contain high amounts of fluxes.
• Terra cotta – A red colored earthenware due to iron impurities. Often used for flower pots and garden ornaments.

Clay Properties

There are several key properties of clay that impact the formulation of clay bodies:

Plasticity

Plasticity refers to the ability of clay to be molded and shaped. Clays with high plasticity, such as ball clays and bentonites, are essential for making clay bodies that can hold their shape during forming. Plasticity is affected by factors like particle size and water content.

Shrinkage

As clay dries and is fired, it shrinks in size. The amount of shrinkage depends on the clay composition. High shrinkage can lead to cracking, so understanding the shrinkage rate of different clays allows control of the overall body shrinkage.

Porosity

The porosity of the fired clay determines properties like strength and absorbency. Adding porous non-clay materials like grog can increase porosity. Dense clays like kaolin have low porosity.

Color

The natural color of clay after firing plays a large role in the look of finished ceramic pieces. The color is influenced by impurities like iron, calcium, and magnesium. Some clays can also influence colors of stains and glazes.

Strength

Strength indicates a clay’s durability and resistance to breakage. Strength comes from the structure and particle bonding after firing. Porosity, shrinkage, and composition affect strength. Strong clay bodies are less likely to crack or warp.

Goals of Formulation

The formulation of a clay body refers to the specific ratios of clays, fluxes, and other additives used to achieve desired properties and behaviors. Some key goals to consider when formulating a clay body include:

Workability

The clay body should be plastic and smooth enough to wedge, throw, handbuild, or extrude without excessive effort. Factors like clay plasticity, sand or grog additions, and water content affect workability.

Drying Performance

The formulated clay body should dry at an even, gradual pace without cracking or warping. Plasticity, density, water content, and additive particles impact drying behavior.

Firing Behavior

The clay body should fire to the appropriate hardness and color at the desired temperature. Clays, fluxes, and atmospheric conditions in the kiln affect firing outcomes.

Final Strength

The fired clay object should have suitable durability and resistance to breakage for its intended use. Strength is influenced by clay types, firing temperature, and additives like grog.

Texture

The clay formulation impacts the smoothness, grittiness, or visual quality of the fired surface. The sizes and proportions of clay, flux, and non-clay particles contribute to texture.

Color

The fired color depends on the clays used, fluxes, kiln atmosphere, and firing temperature. Formulation can be adjusted to achieve a specific color result.

Common Clay Ratios

When formulating clay, there are some common clay ratio “recipes” that are often used as starting points for different types of clay bodies. Here are some of the most common:

Triaxial Blend

A triaxial blend consists of three types of clay – ball clay, kaolin, and fire clay. A typical triaxial stoneware blend would be:

• 25% ball clay – for plasticity and bonding
• 25% kaolin – for whiteness and workability
• 50% fire clay – for strength and thermal properties

Adjusting the ratios allows you to fine tune the properties. More kaolin makes the body whiter, while more ball clay increases plasticity.

Porcelain

Porcelain clay bodies have high kaolin content, usually 50% or more. A typical porcelain recipe is:

• 50% kaolin
• 25% feldspar or nepheline syenite – for fusibility
• 25% silica or flint – for strength

The high kaolin content gives porcelain its white color and translucency.

Stoneware

Stoneware clay has less kaolin and increased amounts of other clays and fluxes. A common starting recipe is:

• 20% ball clay
• 20% kaolin
• 20% fire clay
• 40% feldspar

This produces a strong stoneware body that fires to a tan or brown color.

Earthenware

Earthenware clay contains large amounts of ball clay and kaolin with smaller amounts of fluxes. A typical earthenware recipe:

• 35% ball clay
• 35% kaolin
• 10% feldspar
• 10% silica
• 10% bentonite

Bentonite improves plasticity and green strength. The low flux content keeps the firing temperature low for earthenware.

Terra Cotta

Terra cotta clay is high in iron bearing clays that give it its distinctive red-orange color. A basic recipe is:

• 30% red art clay
• 30% fire clay
• 20% ball clay
• 10% concrete grog
• 10% silica

The grog provides texture and absorbency. The high iron content of the red art clay enables the terra cotta color.

Non-clay Additives

While clay is the main component of a clay body, non-clay materials called “non-plastics” are often added to provide certain properties. Some common non-clay additives include:

• Grogs – Pre-fired clay particles added to increase texture and withstand thermal shock, typically 20-30% of the body.
• Vermiculite – Used to reduce weight and improve insulation, typically 5-10% of the body.
• Sand – Provides texture and increases strength, typically 10-30% of the body.
• Calcium carbonate – Reduces shrinkage and increases whiteness, typically 5-20% of the body.
• Feldspar – Provides glassy bond at high temperatures and increases fusibility/vitrification.
• Talc – Increases thermal shock resistance and gives a fine texture.

The percentages of these non-clay ingredients must be balanced with the clay to achieve the desired forming properties, shrinkage, porosity, color, and fired strength.

Glazes vs Slips

Glazes and slips are both liquid coatings applied to ceramic surfaces, but they serve different purposes.

Glazes are glass-like coatings that are fused to the clay body during the firing process. They are formulated from a mixture of minerals, coloring oxides, and fluxes. When fired, the glaze melts and forms a waterproof coating over the surface of the clay. Glazes can add color, texture, and effects to finished ceramic pieces.

Slips are made from clay, water, and sometimes pigments or oxides. They are applied to greenware or bisqueware before firing. Unlike glazes, slips do not fuse to the clay body. Instead, slips soak into the porous clay surface. Slips are used to alter the color and texture of the clay surface without creating an impervious glaze coating.

Glazes and slips serve complementary purposes in finishing ceramic wares. Glazes create glassy surfaces and effects, while slips provide color and texture without sealing the surface. Knowing when to use a glaze versus a slip depends on the needs of the particular piece and the desires of the ceramic artist.

Formulation Considerations

When formulating a clay body, there are several key considerations to keep in mind:

Shrinkage

The amount a clay body shrinks during drying and firing needs to be taken into account. Higher clay and flux contents will result in more shrinkage. Adding non-plastic materials like grog can help limit shrinkage.

Absorbency

The clay body’s porosity and absorbency characteristics should match the intended purpose. Less porous bodies are better for functional ware that will hold liquids. More porous bodies are better if you want unglazed terra cotta.

Vitrification

The clay formulation needs to achieve full vitrification at the desired firing temperature. Using clays and fluxes that melt and form a glassy phase during firing helps ensure vitrification.

Intended Firing Temperature

The types of clays and fluxes used depend on the maximum kiln temperature. Using too many high-fire materials in a low-fire body can prevent full vitrification.

Cost

Local and inexpensive clays and materials can help reduce materials costs without sacrificing quality.

Testing and Adjusting

Once a clay body formulation has been created, it must be thoroughly tested before being used in production. This testing process allows potters to understand the clay’s firing behavior and make adjustments to achieve the desired results.

The most common methods of testing are creating test bars and test tiles. Test bars are small rectangular samples that can be fired to different temperatures. These simple forms allow potters to easily compare clay bodies and get a sense of their shrinkage, absorbency, color, and strength at different firings. Test tiles take this a step further by applying test glazes to bisque-fired tiles. The glaze-clay interactions can then be observed after firing. Pinholing, crawling, fit issues, and glaze melt fluidity are assessed.

During testing, notes should be taken on the clay body’s drying performance, bisque firing color/appearance, glaze surface qualities, and any other observations. Firing behavior is most critical – does the clay fire to maturity at the desired temperature? Is the thermal expansion compatible with the glazes intended to be used?

If results are unsatisfactory, adjustments can be made to the clay formulation. Common tweaks involve changing the clay-to-non-clay ratios, sources of clay, fluxes, or grog additions. The goal is to refine the recipe to achieve the firing behavior, fired color, porosity, plasticity, and working properties desired for the clay body’s intended uses.

Formulation, testing, and adjustment is an iterative process until all characteristics and performance criteria are dialed in. Thorough testing ultimately ensures predictable, high quality results in final fired ware.

Conclusion

Formulating clay bodies involves understanding the properties of different clays and additives, as well as the goals for the final clay body. While there are common clay ratios that can serve as starting points, testing and adjusting is key to achieving your desired results.

In summary, key steps for formulating clay bodies include:

• Deciding on your goals for the clay body such as smoothness, plasticity, firing temperature, etc.
• Selecting the right clays for those goals, such as kaolin for white color or bentonite for plasticity.
• Choosing appropriate non-clay additives like grog or silica for thermal properties.
• Starting with a common ratio, like 50% clay body, 25% flux, 25% filler.
• Rigorously testing your clay body and adjusting the formulation based on the results.
• Repeating testing until you achieve your desired properties.

While formulating clay bodies takes significant testing and patience, having control over your formulation allows you to produce exactly the clay properties you desire. The flexibility and customization of clay formulation is key for potters and ceramic artists looking to create unique pieces.