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Published Apr 1, 2019

Wood Firing Terra Sigillata

Terra sigillata is a material that is primarily used at low temperatures. But when Alan Willoughby was switching from low fire to wood firing, he decided to play around with terra sigillata in the wood kiln. He had plenty of left over sig from his low fire days so why not try wood firing terra sigillata.

He loved the results! In today's post, an excerpt from the April 2019 issue of Ceramics Monthly, Alan tells us about his process for wood firing terra sigillata. He also shares some terra sig recipes and explains how he makes and glazes them. - Jennifer Poellot Harnetty, editor.

Why Not Try Wood Firing Terra Sigillata?

Terra sigillatas are made from the finest particles of deflocculated clays. When burnished and fired at low temperatures, terra sigillatas develop an incredible sheen and richness, with a soft buttery feel to the touch. At high temperatures, terra sigillatas begin to melt. In wood firings, they develop a glossy to silky-satin surface that shares characteristics with low-fired surfaces, while also demonstrating unique qualities. I work with light-burning clays, including a white stoneware clay and a porcelain. Light-colored clays create an excellent ground and foundation for the color in terra sigillatas.

I apply terra sigillatas to greenware. Using graphite, I begin by mapping out the areas to be covered with different colored terra sigillatas or, if I am using a detailed pattern, I draw it directly onto the greenware. I apply terra sigillatas with sponge brushes or by spraying. I use a bristle brush on smaller, hard-to-reach areas but avoid using them on larger areas, as the bristle marks often reappear after firing and can be distracting. I am careful to build up multiple layers, with a brief drying time between coats. It is important to apply the terra sigillata in a thin to moderate thickness to ensure adequate adhesion to the clay body and to avoid excessive shrinkage.

Tumbler set, 20 in. (51 cm) in length, wheel-thrown and handbuilt white stoneware, off-white terra sigillata, black slip trailing, variation of Bruce Dehnert’s Celadon, fired to cone 11–12, 2018.Pair of tumblers, 9 in. (23 cm) in width, wheel-thrown white stoneware, off-white terra sigillata, black slip trailing, glaze, fired to cone 11–12 in moderate reduction, recycled African pallet wood with inlaid tile, 2018.

Making Terra Sigillatas

Mix deflocculent (I use sodium hexametaphosphate, a fabric water softener from the Dharma Trading Company) in warm water, preferably in a blender, pour into a container with correct quantity of water and add dry materials. After sieving, ball mill the terra sigillata 6–8 hours to increase the amount produced in a measured batch and reduce waste. After ball milling, let the terra sigillata settle for a week or more in a clear container, ladle off the clear water layer on top, then pour off the middle layer of terra sigillata. A layer of larger clay particles will have settled on the bottom of the container.

Glazing Over Terra Sigillatas

After bisque firing, the sigillata surface is less absorbent than the clay. When dipping and pouring glazes I adjust the glaze for the correct thickness on the bisque clay and then brush a light coating of additional glaze over the areas with the sigillatas. This includes when I am using the sigillata as an underglaze and where a glaze and sigillata overlap. Additionally, where there is an overlap, a light burning terra sigillata (Base, Off White, or Warm Earth Tone ) will disappear and be absorbed into the glaze, with very little noticeable effect, while the oxide-rich sigillatas can be used as underglazes.

I mix these terra sigillata recipes in 1000 gram batches, which require 8 cups of water. To make the mixing easier, I use wide mouth gallon jugs, and prefer glass so it is easier to see whether the layers have separated.