One of the most important decisions a ceramic artist makes concerns their choice of clay body. Commercial clays sold by pottery suppliers are both convenient and reliable. But, there are alternatives for artists who want something different than what’s simply pulled off the shelf. Today, many ceramic artists explore the potential of adding something to their clay, and their reasons for doing this are as varied as the additions themselves.
Adding Materials to Clay
Many artists collect granular materials from their local geography, which bestow works with a specific sense of place. Others travel with collecting bags, looking for the opportunity to gather interesting and unique materials from far-off locations.
Fiona Byrne-Sutton, a UK-based ceramic artist, makes large press-molded vessels (see above) that explore the heritage of central Scotland. She embeds her vessels, made from a deep red clay with a higher percentage of iron oxide (1), with ferns, Scots pine, and small boulders. The boulders (small bits of dug clay) are pressed into the surface of her large vessels and painted with a topsoil slip, then the surface is pressed with weeds collected from roadside crevices.
Hard or Non-Combustible Materials
Additions of hard, non-combustible materials to a clay body open up the body and reduce shrinkage. Hard additions can also be used to enhance the surface of forms, such as when clay is scraped with a metal rib, exposing a rough, pocked surface.
Hard, non-combustible materials include (but are not limited to):
- Grog (broken up fired clay, china, or sanitary ware)
- Glass bits
- Brick (crushed or chunks)
- Metal scraps, flakes of rust scraped from metal objects
- Concrete or cement (crushed or chunks)
- Feldspar (small chunks)
All caution should be taken concerning the addition of found materials in ceramic work. All non-combustible fragments should be washed and pre-fired in order to prevent damage to ceramic work, kiln furniture, and kilns.
Organic or Combustible Materials
All organic matter is combustible and when it’s fired above 572°F (300°C) it burns up and breaks down into carbon and water. This action is used by artists to provide interest and develop textures. You will see smoke issuing from the kiln and what will be left is an ash residue. If the organics are mixed into the clay, all that remains after firing is a cavity where the organics have burned out.
Organic and combustible materials could include (but are not limited to):
- Wood shavings, sticks
- Perlite (sold in garden centers)
- Coffee (beans and grounds)
- Fibers (use organic not synthetic)
Organic matter decays when it is added to damp clay. The decay frequently appears as growing mold. When left undisturbed, spores are produced by the mold, which can be released into the air and are easily inhaled. Here are some suggestions for limiting your exposure to this and other health hazards:
- Prepare just enough clay for your needs and dry leftover clay for storage.
- Mold can grow on clay work that has been wrapped in plastic for a few days or longer. If possible, carry the work outside before unwrapping or unwrap it in a well-ventilated area and wear a dust mask and disposable gloves.
- Before reusing plastic clay bags, wipe them down with hot water and disinfectant and then hang them up to dry.
- Provide good ventilation when firing pieces containing organics, because between about 302–662°F (150– 350°C), smoke will be produced from the burning additions.
- A slow firing is recommended to allow combustibles to burn in a controlled manner.
Don’t let these safety considerations deter you from using organic additions.
Byrne-Sutton’s forms start off in a plaster disk-shaped slump mold lined with black clay. Next, she presses vermiculite (see 1) into the clay before filling the mold (see 3). Vermiculite helps the thick walls of her vessels dry evenly and reduces the overall weight, which is an important consideration in large forms. She advises that care be taken with vermiculite, as it can cause the clay to blister. To prevent blistering, it should be pressed into, rather than wedged into, the clay. Pressing vermiculite into her black clay gives a warm, toasty speckle to the ceramic body (2).
She then presses in small hand-dug boulders (4), which are a gray clay that fires an orangish white. This allows the orange tones of the dug clay to stand out from the black base once fired (see 6).
Seasonal plants are pressed in next and painted over with clay slip dug from the ground (5). The slip will fire white or different shades of orange, depending on which clay strata it was dug from.
Red iron oxide, manganese dioxide, and copper wire are added and all give different blacks when fired onto the black clay body, building up a painterly surface. She works intuitively and very quickly at this stage to create pattern, color, and texture.
After the clay vessel stiffens up, it is removed from the mold and allowed to fully dry. Byrne-Sutton gives the vessel a long soak in the once firing, before taking the temperature up to 2120°F (1160°C) (6).
Excerpted from Additions to Clay Bodies by Kathleen Standen, which is copublished by The American Ceramic Society and available for purchase at https://ceramicartsnetwork.org/shop/additions-to-clay-bodies.