Here are the basic techniques used by working potters and ceramic artists. Many of these techniques are ones that I use, particularly sgraffito, raku and underglaze painting.
A mound of soft, fresh clay is turned, or thrown, on the wheel to form the basic shape of the pot. The potter uses his hands and a variety of tools to form the shape. The resulting pot, called greenware, is removed from the wheel and left to dry slowly.
(The potter’s wheel is used for forming round shapes such as bowls or vases. Other forms can be hand built using clay slabs and coils.)
Once the greenware is dry and firm enough to handle (leather hard), it is placed back on the wheel and trimmed to its final form using a variety of tools to work the spinning pot. After trimming, it is again set aside to dry further.
Although this method might sound somewhat complicated, the steps and textures described will become clear during the practical process of ceramic-making. You can gain an online education in the theory of ceramics, but it is difficult to fully understand the techniques used in this process without taking a more hands-on approach. When you are working with clay, these theoretical approaches will often become obvious. You can then turn to more advanced techniques in order to further your pottery skills and expertise.
Slip Carving, or Sgraffito
Sgraffito is basically a technique used to incise into a surface of applied slip. The process usually starts by applying a layer of a contrasting colored slip on top of a surface of leather hard clay. The piece is then set aside to stiffen up until it’s leather hard. If you cover the piece in plastic wrap, you can often leave the piece to harden for days.
Once the piece is firm enough and the surface is not tacky, a design or pattern can be carved through the slip and into the clay body beneath. Once the design is pulled away by incising, there is a beautiful contrast between the slip and the clay. This contrast is stronger after firing and glazing.
When the trimmed pots are bone dry, they are placed in a gas or electric kiln for bisque firing. Depending on the type of clay used, this initial firing will be from 1800 to 2300 degrees Fahrenheit. The kiln may take a day or more to cool. The pots, now called bisqueware, are hard and relatively strong after this initial firing. Note that during the firing, the pot may shrink as much as 15%. It’s important for the potter to plan ahead!
To determine the temperature in the kiln, potters use cones made of various types of clay. At a particular temperature, depending on the type of clay, the cone will soften and lean over, telling the potter that the kiln has reached the desired heat. (The cones can be observed through peep-holes in the side of the kiln.)
The bisqueware can be dipped or painted with glazes. Stains and special coloring materials called underglazes can also be used to decorate the pots. Normal paints cannot be used for glazing pottery… the pigments are destroyed by the high temperatures of the glaze firing. Some of the best materials for bright, vivid colors (such as lead) are now known to be poisonous. Potters now use guaranteed lead-free, safe chemicals for glazing containers and tableware.
In the final firing, the chemicals in the glaze form a hard, smooth, and usually non-porous surface. The pottery itself goes through further changes in the final firing, becoming harder, stronger, and less porous. Glaze firing temperatures may range from 1900 to 2400 degrees Fahrenheit, or higher.
A raised pattern is formed on the greeware by trailing a line of slip onto the greenware before bisque firing. Other decorative techniques include painting greenware or bisqueware with underglazes (high-temperature paints), or applying dyes either under or over glazes.
Bisqueware is glazed and fired in a kiln to approzimately 1800 degrees Fahrenheit, then removed and placed in a sealed container of combustible materials to draw the oxygen from the piece. Raku pieces may exhibit intricate crackled surfaces, or metallic lusters and sheens, depending on the glazes used. Unglazed areas absorb carbon from the combustion and become black.
Note that raku pieces are purely decorative; they should not be used to hold liquids or foods.
Smoking, or Pit Firing
Greenware is burnished to a smooth polished surface, often using smooth stones. The greenware may be bisque fired, or it may be placed directly in the smoke pit.
In either case, a fire of charcoal or wood is used to heat the pottery to temperatures of up to 1300 degrees Fahrenheit. The pottery is covered in slow-burning materials, such as sawdust, damp straw, or manure. As the material slowly smolders, the pots absorb carbon and other by products from the combustion, taking on rich grey, brown, red and black colors.
Pit Firing is the technique used to produce the famous blackware of the New Mexico pueblos.
Household salt and other chemicals can be placed in the kiln during firing; the resulting chemical vapors are deposited on the pottery, creating rich natural textures and colors. Salt firing often requires a special dedicated kiln, since the chemicals leave deposits that can contaminate future firings.