In the Studio: High-Fire Terra Sigillata Rhonda Willers

Appears in the Sept/Oct 2020 issue of Pottery Making Illustrated.

High-fire terra sigillata? Is this even possible? Yes! Terra sigillatas remain stable from low fire through mid range to high fire. Because I fire in both low-fire electric and high-fire wood kilns, I began experimenting with terra sigillatas at these various ranges and have discovered great results in both.

Expectations for terra sigillatas must change as you use them across temperature ranges. For example, while many colors remain relatively consistent between these temperature ranges, some do change. Terra sigillatas are known for their waxy sheen when fired in the cone 08–04 range. One can spend hours burnishing terra sigillata to bring out a gloss, but the burnished surface typically disappears if the piece is fired to temperatures above cone 1 (2079°F (1137°C)). Also, as you fire terra sigillatas hotter, their capacity to seal the surface diminishes and you must instead rely on the clay body’s vitrification and/or utilize a liner glaze on interior surfaces that will contain liquids after firing.

Preparing Terra Sigillata for High Fire

I find the 1.15–1.20 range of specific gravity to be more effective when using terra sigillata in a high-fire atmospheric kiln because it provides a more opaque coating, whereas the lower specific-gravity terra sigillatas tend to disappear in the firing process. I keep the Grolleg-kaolin base terra sigillatas near the 1.15 range as they are more prone to flaking if they become too thick/dense. Terra sigillata bases made with OM 4 ball clay and Newman Red clay work best at a specific gravity of 1.20 and even up to 1.30.

To prepare a terra sigillata with a denser/higher specific gravity, siphon the base terra sigillata as you normally would after the recommend settling time. Then allow the siphoned terra sigillata to remain uncovered to evaporate the water away until the higher specific gravity is achieved. What doesn’t work well is to siphon the terra sigillata from you base mixture too early in the settling process. If you do this, you’ll end up with heavier clay particles that settle out as your terra sigillata rests between uses. Your goal is to only have the smaller particles with less water.   

Lower specific gravity terra sigillatas (1.10 and lower) burn away from the accumulation of wood ash and the abrasive internal kiln atmosphere. This also happens to thinly applied terra sigillatas, where brushstrokes become very evident after the firing.

High Fire Terra Sigillata Application Tips

If you plan to layer multiple terra sigillatas, the same basic principles apply. Use no more than three layers or you run a higher risk of having the terra sigillata flake after the bisque firing. With a thicker specific gravity (1.20–1.30), no more than two layers are needed. When creating decoratively patterned surfaces, light terra sigillatas layered over dark terra sigillatas often end up fading into the darker terra sigillatas as a result of the firing process.

To preserve decorative patterns, use a dark terra sigillata for the patterning, applied over a layer of a light terra sigillata. You can also consider layering terra sigillatas underneath high-fire glazes, such as shinos, to help shift and develop even richer surfaces.

Exciting Results

In the McKeachie-Johnston anagama kiln where I fire, we often do a heavier reduction and fire to finished temperatures of cone 11 (2361°F (1294°C)). Because of this I use a wood-fire porcelain made by Continental Clay so the terra sigillatas appear brighter. When they are fired on stoneware, which has more iron in the clay body, the terra sigillata colors become muted and darker, and the flashing of color variations isn’t as visible.

Terra sigillatas are highly reactive in atmospheric kilns; those that are lighter in color can flash a wide range of colors. Terra sigillatas created with OM 4 ball clay and Grolleg kaolin produce some of the most varied results. While flashing slips are traditionally formulated to achieve different colors in atmospheric firings, terra sigillatas are equally capable of producing such varied results. Terra sigillatas also cover a surface in a delicate way that preserves subtle touch marks and textures. Conversely, the thicker nature of flashing slips often obscures these gentle marks and moments.

As the temperature increases in a wood firing, melting wood ash begins to move downward and across the forms. During this movement some terra sigillatas, particularly the darker blue colors, will move along with the wood ash, creating natural melting patterns that remain after the firing is completed

Rhonda Willers is the author of Terra Sigillata: Contemporary Techniques, published by The American Ceramic Society, available in the Ceramic Arts Network Shop at https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqnEAA.