FEEL THE CLAY
Terminology of Ceramics
In order to understand clay as a raw material we have to consider its geologic origins, its role in human history, chemical composition and its aesthetic and functional value. Clay has the finest particle size of all soil types. It contains various rock minerals depending on where it originates from. Industrially produced clay bodies are mixed from clay minerals and other materials according to a recipe. Ceramicists either use these ready-made clay bodies or mix the clay themselves. Clay shrinks when it dries and when it is being fired. The shrinkage is between 10 – 15 % depending on the type of clay. The choice of clay is important for all the next steps in the process. Knowing the clay and its characteristics is crucial for all kinds of clay work.
High-fired clay. As according to its name, this clay can take high temperatures up to 1350 degrees. There are clays of different qualities and colors available.
Stoneware clays mature at 1200 – 1350 degrees. Their fired colors ranges from almost whites to buffs and through tans, dark colors and blacks.
Porcelain is a mixture of different clays containing a major amount of feldspar. After firing (up to 1450 c) it is white, dense, and often translucent.
Earthenware clays are low-fired between 900 – 1150 c. After firing the body is slightly porous
Red clay is low-fired and rich in iron. The firing temperature is between 900 – 1080 c. During the firing the color of the clay changes from grey to red brown. Local clays in Finland are mostly red clays.
Grog is fired clay and crossed and ground- up. It is used in clay bodies to give strength and structure.
Paper clay is a mixture of clay and paper fibres. They make the clay stronger and lighter. Paper clay also shrinks less. Paper fibres burn out in the firing.
Casting clay is a fluid mixture of clays which is poured into a plaster mold.
Casting clays can be low-fired or high-fired and of diffent colors like other clay bodies.
Ceramics is fired clay. Raw, dry clay can be recycled into a workable clay by soaking it in water. After firing ceramics is hard and can no more become clay. Ceramics is durable, often almost rocklike. Archaeologists have found ceramic pieces older than 20 000 years.
Drying Clay. It is important, when working with clay, to master the process of drying. If the clay object dries too fast, it may crack. It is often best to work with clay when it is leather-hard. Then the surface of the clay is still moist but stiffened and detailed designs can be made on the surface. It is not recommended to work on dry clay because by doing so harmful clay dust will fly around. A dry clay object is also very fragile.
Forming and modelling clay
For modelling and particularly for throwing clay must be kneaded or wedged to remove all air. Clay must be smooth and homogeneous.
Handbuilding. With this technique clay is formed by hand. Other handbuilding methods can be combined together.
Pinching. Fingers and tumbs are used as tools to make a clay object.
Coiling. An age-old method of constructing hollow forms by rolling and attaching coils of soft clay together.
Slab-building. In this technique a rolling pin or a slab-rolling machine is used to make slabs, flat pieces of clay. Slabs are then cut and joined together to make a desired form.
Throwing. In throwing, pottery is made on a rotating wheel. Only round forms can be thrown. They can then be altered later. Usually, the bottom of pots are cleaned and trimmed with trimming tools. A foot ring can be added on the bottom of a piece.
Using molds. Clay can be pressed into different shapes with molds. Very
often these molds are out of plaster because plaster takes moisture out of clay. A clay slab can be formed into a hallow mold or draped over a hump mold.
Casting. Casting clay is fluid, deflocculated clay which is poured into plaster molds. The clay can stay a while in the form. Plaster sucks moisture out of the fluid clay. In this way stiffened clay wall is built. Extra clay is poured out and the clay object remains in the form.
Tools. Hands are for a ceramicist the most important tool but there are many other helpful tools: wooden boards, rubber and metal kidneys, rolling pins, modelling tools, trimmming tools, cutting wires, sponges, potters’ knives, cloth for rolling slabs...Many things that you find in your own kitchen can be used as tools.
The decoration with colors when working with clay can be done in different stages. Clay and colors change in the firing and can look totally different after firing. Therefore, it is important to make and fire test pieces in order to see what the end result looks like.
Engobes are colored, fluid clays. They are applied to the surface of a leather-hard piece. Even big areas can be decorated this way.
Underglazes. These colors are a mixture of oxides, commercial stains and other ingredients. They can be painted with a brush and used either before or after the firing.
Glazes are a mixture of natural and commercially produced minerals. They melt in the firing and make a thin, glass like layer on the ceramic body. Glazes are used for decoration and durability and hygenic reasons. Glazes have to be chosen according to clay body and firing temperature. The variety of available glazes and glaze recipes is enormous. A glaze behaves differently on different clays. A glaze can be glossy, transparent, semi-matt or matt. When making pottery for nutrition the glaze has to be food-safe.
Guilding, Platinum, Lustre. In guilding real gold , platinum and lustre is used They are applied with a brush on a glaze-fired surface of a piece. A new decoration firing between 550 – 1100 degrees is is needed.
The clay objects must be totally dry before they are fired. Usually everything is fired twice. First comes the bisque firing and then the pieces are glazed, followed by glaze firing. Bisque firing. The firing temperature is between 850 and 1050 centigrade depending on the choice of clay. After firing the ware will be porous and can be glazed.
Bisque firing. The firing temperature is between 850 and 1050 centigrades depending on the choice of clay. After firing the ware will be porous and can be glazed.
Glaze firing is done between 1000 and 1300 degrees depending on the type of clay used. The purpose of the firing is to make the glazes melt at the right temperatures.
There are many types of ceramic kilns. Firing can be oxidising or reducing.
Oxygen or the lack of it can influence the chemical reactions in glazes and the way glazes look like after firing.
Electric kilns are the main way of firing in Finland. Such kilns come in all shapes and sizes. The atmosphere in an electric kiln is oxidising.
Gas kiln. This type of kiln is fired with gas. The way it is contructed is similar to an electric kiln. A big difference is that a gas kiln can be fired both oxidising and reducing.
Wood-fired kilns are tradtional and take time to be fired, as fuel wood is used. There are different types of wood-fired kilns and each of them have their own character. The atmosphere in the kiln is almost always reducing.
The Potter’s Guild here has constructed such a kiln in Koroinen near Turku. The volume is 2 cubic meters. Depending on weather conditions the kiln is fired for 12 – 16 hours until it reaches the temperature of 1300 centigrades. The lack of oxygen, ash and flames in the kiln produce beautiful, nuanced colors, that are not possible in an electric kiln. Getting together for the firing, meeting other members of the Potter’s Guild is an essential part of it all.
Raku-firing. For a raku-firing you need a special kiln and gas for fuel. The clay has to be shock resistant. The kiln is fired to 900 – 1000 centigrades. The pots are lifted glowing hot out of the kiln with tongs and placed in a closed container filled with straw, sawdust or dry leaves.
Aluminium Saggar Foil. In this new method of raku-firing an object is made of light-colored clay and then polished and bisquit fired. After that the piece is treated with ironchloride, copper or coboltsulfate and wrapped in organic material, like banana peels, sugar, leaves, steel wool or copper wire. Then the object is covered with aluminium foil and fired to 700 c. The result is a color-saturated surface with occasional organic designs.