I have grown to love terra sigillatas for their versatility in color, sheen, and because they’re a relatively low-tech way to make a ceramic surface. I’m drawn to the waxy surface of the terra sigillata and its ability to heighten subtle markings on a clay surface. Terra sigillata’s integration and reactivity with other surface and glaze materials is also of great interest to me.
When burnished, terra sigillatas provide the functional purpose of creating a watertight surface on an earthenware clay body. Burnishing aligns the clay particles in the terra sigillata against the surface of the clay form by rubbing it with an additional material, such as a smooth stone, soft plastic or fabric, or even your fingers. Keep in mind, the watertight surface will still allow water to gradually pass through the walls, through a slow seeping process as it’s not a glass-glazed surface. Note: The sheen created by the burnishing process disappears if works are fired above cone 1. If you aren’t making forms that will contain liquid, then the watertight quality is of lesser concern and burnishing may be skipped unless a glossy sheen is desired.
Terra Sigillata Bases
A common starting place for terra sigillata exploration is working with a singular, industrially produced clay such as Cedar Heights Redart, Grolleg, or OM 4 ball clay to create a base terra sigillata. Each of these base terra sigillatas create a distinctive color and sheen. For example, a Grolleg-base terra sigillata produces a cool white color with a high-gloss sheen when burnished. A Cedar Heights Redart base terra sigillata produces a deeper iron red with medium- to high-gloss sheen when burnished. Taking this idea a step further, you can mix the base terra sigillatas in various proportions with one another to create new colors and sheens.
Quick Mix Summary
1. Mix 1 part clay to 2 parts water, blend well with a drill mixer.
2. Add ¼–½% (of the dry weight of the clay) of a deflocculant such as sodium silicate or Darvan 7 then mix again with a drill.
3. Allow heavier clay (red clay) terra sigillatas to settle for at least 6–8 hours, and lighter clays/ball clays to settle for 24–48 hours.
4. After the mixture has settled into distinct visible layers, siphon out the middle section of the mixture.
Measuring Specific Gravity
Specific gravity is a measurement of the relative heaviness of any kind or portion of matter. This is typically described in comparison to an equal volume of a known substance, such as water. A desired range of specific gravity for terra sigillata is: 1.10–1.20, with the ideal being 1.15.
1. Use a clear plastic cup and digital scale or triple-beam balance scale, to measure 100g water (remember to tare/zero out the scale with the cup on it first).
2. Mark the cup to indicate the 100g water level.
3. Empty the cup and refill it with the mixed terra sigillata base exactly to the 100g mark.
4. Weigh the terra sigillata using the scale.
5. The weight should be between 110g and 120g, with 115g being the ideal weight.
6. Once you’ve measured the terra sigillata, simply divide the number by 100 (or move the decimal over two places from the right) to convert to a measure of specific gravity. It should be between 1.1 and 1.2, with 1.15 as the ideal specific gravity.
Terra Sigillata Bases and Base Blends
Terra sigillata should be applied before the bisque firing. This can be done on firm leather-hard to bone-dry ware. I prefer applying at the bone-dry stage, as the terra sigillata coats more evenly and thickly. You can brush (I recommend a soft bristle brush to avoid brush marks), spray, or dip to apply the terra sigillata. Use caution when dipping terra sigillata as greenware is very fragile, and it could take on too much water and potentially crack the clay. A general rule of thumb is to apply no more than three layers of terra sigillata or you run the risk of it peeling and flaking off after the firing.
Combining Base Terra Sigillatas
Once you have three to four base terra sigillatas prepared, you can mix them in various ratios to create additional colors and sheens. As examples, I have used the following clays to make base terra sigillatas: Grolleg porcelain (A), Newman Red clay (B), Cedar Heights Redart (C), and OM 4 ball clay (D) (1). Each of these clays offers particular results when used alone: Grolleg and Cedar Heights Redart were previously discussed, Newman Red clay produces a brighter red-orange with moderate gloss sheen and OM 4 produces a warmer white with potential for a high-gloss sheen.
Let’s say you want to have the red-orange brightness of Newman’s but desire more burnishing potential. A great solution is to mix the Newman Red clay terra sigillata with the Grolleg porcelain terra sigillata. You can experiment with the proportions of these two to achieve both a pleasing color and a desired sheen. One of the easy parts of combining base terra sigillatas is that they can be mixed by volume after they’ve settled and have been siphoned.
Note: When mixing colorants into terra sigillatas, recipes are provided as an amount, X, of colorant per 8 ounces or 1 cup of liquid sigillata. To follow theses practice recipes, combine the terra sigillata bases using cup-based amounts. I use a standard red earthenware clay body for my test tiles, but you can apply terra sigillata to any clay body and use any firing method. The results, of course, will vary greatly depending on which clay and firing method you use.
Rhonda Willers is an artist, educator, wife, and mother who lives and works in rural Wisconsin. She has taught at the University of Wisconsin-River Falls since 2007. To see more of her work visit: Instagram @r_willers and www.rhondawillers.com.
Subscriber Extra: Archive Article
Click here to read the archive article Cleaner Mixing by Jeremy Randall, which originally appeared in the June/July/August 2012 issue of Ceramics Monthly.