LOW FIRE Red or Dark Earthenware Clays White or Buff Earthenware Clays

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1 About Clay For the Claymobile, we use a low fire whiteware and a low fire terra cotta. However, there are as many different clay bodies as there are cookies. Below are just the few main categories. Information are taken from pottery.about.com LOW FIRE Low-fire bodies are defined by when the temperature at which the clay body matures, generally considered to be between cones 09 and 02 (1700⁰ and 2000⁰ F or 927⁰ and 1093⁰ C). Low-fire clays tend to have good workability and usually will not shrink, warp, or sag excessively. However, they are softer which means that they are less durable and will absorb liquids. Low-fire clays are divided into two types according to their color after firing. Darker-colored bodies (most commonly red), and the white and buff clay bodies. Red or Dark Earthenware Clays Darker earthenware clay bodies can range from a orange-red to a dark brown, with red being the most common. Their color derives from the iron-bearing clays used their clay bodies. The iron already within the clay body acts as a fluxing (melting) agent, which matures the clay at relatively low temperatures. Earthenware clays melt at such low temperatures that they seldom become fully vitrified. Because of this, the fired ware will continue to absorb liquids. For this reason, functional ware is almost always glazed. Appropriate, non-toxic glazes must be chosen, however, as some glazes are also mildly absorbent at this temperature range. White or Buff Earthenware Clays Because of an increased interest in low-temperature firing, new varieties of low-fire clay bodies have been developed. These clay bodies have also been given the label earthenware due to the fact that they mature in the earthenware temperature range. The idea of low-firing white clay bodies actually began further back to Europe, when pottery factories began trying to duplicate the porcelain ware that had become available from the eastern Asia. These clay bodies required large quantities of fluxing agents, in

2 order to lower the melting temperature for the relatively clean mixtures of kaolin and ball clays. The white bodies of today are still composed of about half clay and half added fluxing agent, such as talc. MID RANGE AND HIGH FIRE What are mid-range and high-fire clay bodies? These clay bodies fall into two basic types, stoneware clay bodies and porcelain clay bodies. Differences Between Mid-Range and High-Fire Clays The main distinction between mid-range and high-fire clay bodies is implied in their names: the distinguishing factor is the temperature at which the clay matures. Mid-range clay bodies high to maturity between cone 4 and cone 7. High-fire clay bodies are usually considered to be those that mature between cone 8 and cone 11, although some porcelains go all the way to cone 14. The ingredients used in mid-range and high-fire clay bodies are very similar within their type. The main difference is that, in relation to high-fire bodies, mid-range bodies will have either less refractory elements, more fluxing agents, or a combination of these two. Stoneware Clay Bodies Generally speaking, stoneware clay bodies fire to a gray, buff, tan, or light brown color. Color will vary with the same clay, depending on the kiln's atmosphere. Darker colors are possible with the addition of slips, such as Alberta Slip, or of coloring oxides. Stoneware clays get their name from the dense, rock-like nature of the clay body when it is fired to its maturation temperature. There are some naturally occurring stoneware clays that need little modification. Usually, however, a stoneware clay body adds other ingredients for optimal performance. For example, ball clays may be added for plasticity, or fire clays may be added to raise the maturation temperature of the clay body. Porcelain Clay Bodies Porcelain clay bodies are known for their hardness, their extremely tight density, their whiteness, and their translucence when the pottery's walls are thin. Kaolin clays are the foundation of all porcelain clay bodies. Kaolin is the purest form of clay. It is also so nonplastic as to be nearly unworkable if not mixed with other clays. Another difficulty is that porcelain clay bodies are very prone to warping during drying and in the kiln. The purest porcelain bodies are fired at the highest temperatures used in pottery, usually between cone 11 and cone 14. However, many porcelain clay bodies are modified to make the clay more workable and also to bring the firing temperature down. SPECIALIZED CLAY BODIES Specialized clay bodies are sometimes required for certain activities, whether in the process of creating ware or in the end use of ware. This includes clay bodies specifically formulated for functional ware, ovenware, raku, salt (soda) firings, and slip casting.

3 Functional Ware Functional ware are those pots that are used for food and drink, including plates, bowls, mugs, goblets, pitchers, and teapots. Clay bodies used for functional ware should have a tight, dense clay body after firing. General guidelines are that functional ware made from mid-range or high-fire stoneware should have an absorption rate of 3% or less. Porcelains should have an absorption rate of 1% or less. Too high of an absorption rate can lead to crazing of the glaze, which in turn will allow the clay body to absorb more water, compounding the problem. Crazing in the glaze may lead to the growth of harmful bacteria both in the cracks and under the glaze. Pots with crazed or crackle glazes should not be used for food or drink. Ovenware Any pottery that will be used in the oven or microwave must be very resistant to thermal shock. There are two different approaches to meet this requirement. Ancient traditions use soft pottery. This type of ovenware is generally underfired; the clay body is very open and porous and able to withstand sharp temperature changes. The problems with this approach are that pots tend to become unsanitary quickly, and they are easily broken. The other approach is to use a stoneware body with the following characteristics: as low in silica as possible uses lithium-bearing feldspars such as petalite, spodumene, or amblygonite fired to cone 10-11, but have an absorption rate of at least 3% or more be very low in iron fired in oxidation. Tips for creating ovenware pottery: pieces should be compact and rounded forms to reduce internal stresses glazes must fit and be evenly applied inside and out the piece's walls and floor should be as uniform in thickness as possible. Raku ware Raku firing involves tremendous thermal shock to the clay. Stoneware bodies can work well with the addition of 30% grog or sand. Crushed seashells, expanded vermiculite, and volcanic ash have also been used by some potters. Kyanite, an aluminum silicate, can also replace some of the clay content. (Kyanite promotes strength in the fired clay.) Salt-Fired (Soda-Fired) Ware Most mid-range and high-fire clay bodies will do fairly well in a salt firing. The best saltfire clay bodies are: low in iron low to moderate in silica content if grit is used in the clay body, use silica sand instead of grog to promote the orange peel effect. Slip Casting Clay Bodies Casting slips (and the clays they are made from) have specific needs:

4 fine-particled clay with no grog or sand a deflocculant must be added, low in iron and free alkali. Deflocculants such as sodium silicate and soda ash are used to ionize the clay particles, making them repel each other. A seemingly solid piece of damp slip casting clay can become liquid when vigorously shaken. Deflocculants reduce the amount of water needed, which reduces problems such as shrinkage and warping. Generally speaking, the clay body will only need 0.3% to 1.5% (by weight) deflocculant added. Producing a problem-free slip casting clay body can be very difficult. It takes patience, expertise, and a great deal of testing. CHARACTERISTICS OF HAND BUILDING CLAYS Hand building with clay is fun. Good hand building clays make the experience even better. What are the characteristics of a good hand building clay body? The primary requirement of a hand building clay body is strength, with plasticity running a close second. Happily for us, and unlike a throwing clay body, water absorption is not an issue. Strength Because many hand built forms need a high degree of strength in the green stage, hand building clay bodies often have 20% to 30% grog or other non-plastic filler. The grog can either be fine or coarse, depending on the surface qualities the potter wants. Such high addition of grog also results in an open clay body. This means the clay will dry quicker and with a reduction in the possibility of cracking. When extreme levels of strength are needed from the clay as it is being worked, a few handfuls of chopped nylon can be added to each 100 pounds of dry clay as it is being mixed. Warning: Do not use chopped fiberglass as a substitute. Plasticity A hand building clay still needs to be plastic enough to be workable. The more extreme the shapes being built or being used in the building, the more plasticity the clay will need. For example, it takes a high degree of plasticity in the clay body in order to roll out long, thin coils that do not have any cracks. Weight Hand built forms can be vary large. When that is the case, it is a good idea to decrease the weight a finished product will have. This is accomplished by mixing combustible substances into the clay. Some materials that have been used successfully by a number of potters are sawdust, coffee grounds, ground nutshells, and ground fruit pits. Organic additions generally burn out of the clay body without any difficulty if a normal firing schedule is followed. Perlite (pearlite) is also sometimes used to reduce weight. For perlite and the organic fillers, as well as grog, it is best to add them when the clay body is being mixed. CHARACTERISTICS OF THROWING CLAY BODIES

5 Throwing on the potter's wheel makes some heavy demands on clay bodies. There are three key characteristics necessary for a clay body to work well for throwing. Throwing clays must have a high degree of plasticity, they cannot absorb too much water while being thrown, and they must be strong enough to hold their shape while being worked. Plasticity The absolutely crucial characteristic of all clays used for throwing is that of plasticity. Without a very high level of flexibility, the clay simply won't be workable on the wheel. Even moderately plastic clays can take a toll on the potter's hands and wrists, due to the strength and pressure required to make the clay move. Ball clays are highly plastic and are often used in throwing clay bodies. However, this is a trade off; the more plastic a clay is, the more it will shrink and the greater its tendency to warp while drying. Plasticity must be maintained in a balance with other clay body characteristics. Strength Throwing clays must be highly plastic, but also retain enough strength to stand upright when thrown into thin-walled, tall forms. To this purpose, throwing clay bodies contain some coarser particles to give it strength. Throwing clay bodies are likely to contain fire clays, fine sand, grog, or a combination of these. When grog is used in a throwing body, it is best if it is sized to pass through a 30-mesh screen, but not through an 80-mesh screen. This "30-80" grog allows for the strength needed for throwing, without being so coarse as to be painful on the potter's hands. In general, a throwing clay body should contain no more than 8% to 10% of coarsely particled material. Water Absorption Clay absorbs water while it is being formed on the potter's wheel, which is why the longer a clay is worked on the wheel, the softer it becomes. Water absorption, then, can become a problem. Plastic clays that are on the stiff side when being wedged will absorb less water when they are on the wheel. Unfortunately, this also means that the clay is harder to work with and is tiring to the potter. Working with stiff clay can also hurt muscles and joints. Plastic clays do absorb less water than coarse materials such as fire clay or grog, which open a clay body up. Again, a good throwing clay must be balanced between plasticity, strength, and water absorption. Effects of Aging Aging lends better workability to a clay body. To age clay, it is left open to the air for a day or two after mixing and before covering. This allows microscopic organisms to move into and grow within the clay. These organisms produce organic polymers, which add more plasticity to the clay with none of the drawbacks of using too much fine-particled clay to achieve the same level of plasticity. The one drawback aged clay has is that is stinks like swamp mud. Many potters mix their clay, let it develop a colony of organisms, then store it for one to six months before using it. Clay bodies that have nepheline syenite in them, however, should never be stored more than three months, due to possible ionization of the clay particles.