In the Studio: Terra Sigillata Line Blends Rhonda Willers

Appears in the Sept/Oct 2017 issue of Pottery Making Illustrated.

A multitude of colors can easily be produced by adding colorants to base terra sigillatas. The common practice of creating a glaze line blend applies to terra sigillata as well, but with modifications because terra sigillata recipes are described by volume.

The line blend process produces a monochromatic line of color with a singular colorant and a singular terra sigillata base. The process can be further used by mixing colorants with additional singular terra sigillata bases, or with blended bases.

By-Volume Mixing Method and Limits

Terra sigillata lines blends are based on an amount, X, of colorant per 8 ounces or 1 cup of liquid terra sigillata. A simple way to begin measuring colorants is to use teaspoon (tsp) and tablespoon (tbsp) measurements, thereby adding the colorants by volume, mirroring the by-volume cup measurements of the base liquid terra sigillatas. Begin this process with 9 separate containers, each holding 1 cup of liquid terra sigillata. The following amounts of colorants are each added to one of the containers of 1 cup liquid terra sigillata: ¹⁄₈ tsp, ¼ tsp, ½ tsp, ¾ tsp, 1 tsp, 1½ tsp, 2 tsp, 2½ tsp, and 1 tbsp (which equals 3 tsp).

Between the amounts of 2 tsp and 1 tbsp, the colorants saturate the base terra sigillata, which is why there’s no need to test or add more than 1 tbsp of colorant. You can weigh colorants out on a gram scale and record information in this method, but most often the by-volume measurements are sufficient for this material.

To prepare for a standard terra sigillata colorant line blend, label your nine containers with the following information: A) Terra sigillata base, B) Colorant measurement, and C) Colorant type. Example: Grolleg Base + ¼ tsp CoCO₃. After containers are labeled with each of the combinations from ¹⁄₈ tsp to 1 tbsp, add your liquid base terra sigillata in 1-cup increments to each container.

Colorant Line Blends

To create a blue line blend use cobalt carbonate (CoCO₃) as the colorant. Cobalt carbonate is most often used to produce blue colors in glazes and it’s capable of doing the same in terra sigillatas. Commonly used colorants come in the form of carbonates, oxides, and commercially produced stains. One issue with the commercial stains is their lack of solubility, which means they don’t dissolve very well or at all. One solution to this is to ball mill the terra sigillata with the commercial stain to break down the stain’s larger particle size. If a stain or a colorant are not mixed well, the mixture may produce a streaky color rather than an even color tone.

To produce a full range of monochromatic colors, I recommend using the following terra sigillata bases:

Grolleg kaolin base

OM 4 base

Cedar Heights Redart base

one base blend mixed at a 50/50 ratio such as Grolleg kaolinbase and Cedar Heights Redart base

Next, begin adding the colorant following the amount listed on the container’s label (1). To mix the colorant in, best practice is to use an immersion blender, but you can also vigorously shake the container, then blend with a stiffer brush prior to application (2).

Test Tile Types and Application

Terra sigillata line blends are applied best by brushing on a flat, red earthenware test tile (3) that’s fired to cone 03 in an electric kiln, but you can apply terra sigillata to any clay body and use any firing method. The results, of course, will vary greatly depending on your firing method and final temperature. The shape of your test tiles or test objects should match the way you plan to use the material on your artwork. When brushed on a vertical or curved surface, terra sigillatas will run as you apply them, but once dry, they stay put in the firing.

Finally, label your test tile or test object, matching the information on the container label and the firing method to the tile. Repeat this process for each mixture in the line blend.

When loading the tiles (4) into the kiln, test tiles and test objects can touch or be stacked. The only caution is that some materials, like cobalt oxide, will have a ghosting effect, leaving a halo of color near their placement. After all the tests are fired, you will have a set of results that will likely inspire you to continue testing and creating line blends with colorants and terra sigillatas (5).

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.