One of the characteristics of terra sigillata is the uniform opaque color created when it’s applied in multiple layers. As a result, this uniform color can flatten the volume of a given form or, conversely, the flatness of the terra sigillata color can be used to color block areas, highlighting the form. Other desirable traits of terra sigillata include its thin, skin-like covering and its reactivity to other materials and firing processes. With these inherent qualities, terra sigillata lends itself to being adaptable to layers of additional materials, which creates a multitude of potential results from the combination of a few materials.
Washes and patinas offer ceramic fired surface options that provide variations in texture and colors when applied over a terra sigillata. While a wash, or flux wash, adds a sheen and sometimes texture to the terra sigillata surface, a patina enhances the terra sigillata surface because it’s a thin, colorant-rich wash layer that settles into the subtle markings on a surface.
A wash, in this context, is a flux material mixed with water that’s applied over terra sigillata before or after the bisque firing. Washes are most often used in low-fire applications, but some artists also use them in mid-range and high-temperature firings, where the firing process provides the additional surface reaction, rendering the wash layer less necessary.
Washes are used at varying concentrations: most commonly 1 part material to 3 parts water or 1 part material to 6 parts water. The 1 to 3 ratio is more concentrated and produces more noticeable results, while the 1 to 6 ratio is more diluted and produces softer, subtle changes in surface, color, and texture. Suggested materials used to make washes include: soda ash, salt, borax, and lithium carbonate.
When making washes, the use of boiling water helps to fully dissolve the materials and as a result provides a more consistent application. You can use hot tap water, and less of the material will dissolve into the water, which will then deliver more variability to the terra sigillata surface.
For small-scale work, a 1:3 ratio wash batch of 2 tablespoons of materials plus 6 tablespoons of water is a good amount to get started. If you’re working in large-scale or large-quantity, you will want to increase your wash batch size accordingly.
Soda Ash and Salt
Soda ash and salt provide the most similar results at the 1:3 ratio, but vary more at the 1:6 ratio.
Both soda ash and salt tend to darken the terra sigillata color and also create a low-satin sheen at the 1:3 ratio. In the more diluted ratio of 1:6, soda ash and salt washes make soft, circle-like patterns, however the shape of the patterns varies in subtle ways. In my own practice, I don’t burnish the terra sigillata and instead use the salt or soda ash wash at the 1:3 ratio to create a sealing sheen on the terra sigillata.
Borax can be used in its granular form or powdered form. The powdered form dissolves more easily in warm water, but is really more of a convenience than a necessity. Borax, as a wash at the 1:6 ratio, will create tiny iron-like speckles on a terra-sigillata surface. Granular borax can be sprinkled onto a terra sigillata surface, and if enough of the material lands and settles together, it will create its own glaze-like melting at low-fire temperatures. The result of the melted granular borax is a glassy pool with an amber-color cast. It also tends to gather and pull colorants away from the terra sigillata below, which can result in some beautiful effects. For example, when 1 tablespoon of tin oxide is added to a OM 4 ball clay base terra sigillata and granular borax is sprinkled onto it, the result is a vivid pink color around the edges of the melted borax pool (1, 2). If left overnight, the borax wash mixture will harden. In an effort to save materials, I drain off the excess water and re-introduce boiling water to the hardened borax when I need to use it again. In doing so the proportions of the wash aren’t exact, but they’re close enough that I haven’t noticed a distinguishable difference.
3 After sprinkling on granular borax, gently rub the granules into the terra sigillata (before the bisque firing) to scratch through the surface. Photo: Jessie Ann Wade. 4 The fired bowl, fired in an electric kiln to cone 03. 5 Detail of the fired bowl. The soda-ash wash creates the variable white areas. The lithium-carbonate wash produces the golden ochre, crusty skin over the crocus martis terra sigillata line and texture. The granular borax has melted, creating amber colored glass with iron rust speckling around the area.
Lithium carbonate creates a dry, crusty texture that encourages flaking of the terra sigillata with tones that range from ochres, greens, and browns depending on the color of the terra sigillata below. Caution: Always work in a well-ventilated area, and wear disposable protective gloves and a proper fitting mask when working with raw lithium carbonate. Lithium carbonate, shouldn’t be used on a form that will come into contact with food.
Application of Washes
Application of washes can occur before or after the bisque firing. When applying washes before the bisque on bone-dry clay, it’s important to remember that adding additional water to the clay increases the risk of it breaking or falling apart. However, applying washes before the bisque allows them to more deeply interact with the terra sigillata surface as it’s also unfired and moveable. When applying washes after the bisque firing, you have more freedom in how much you apply, without the risk of over saturating the clay and causing it to break down.
While each wash can be used on its own to produce interesting results, they can also be used in combination with one another. You might consider applying a soda-ash wash over the entire surface of a form, then apply an area of lithium carbonate wash and finally sprinkle some granular borax over an area that overlaps the previous layers in select areas (3–5).
Patinas are an oxide, carbonate, or commercial stain combined with a flux material that allows them to bond to the terra sigillata surface. Patinas are sometimes called stains, and slightly thicker versions of patinas are used as overglazes with majolica. The idea of the terra-sigillata patina, like a patina used in metalworking, is to enhance and highlight the color and textural markings on the surface of a ceramic form.
A versatile recipe for patinas is 1 part carbonate/oxide/commercial stain to 2 parts flux material with water added to create your preferred thickness. When firing to mid-range or high-fire temperatures, the amount of flux can be reduced, depending on your desired outcomes. Traditionally, the patina is mixed to a watery consistency, but you can experiment and consider mixing to varying thicknesses to achieve different results. The thinner consistency produces a drier, more metallic surface, while a thicker patina will be a bit glossy.
6 The bisque-fired oval form was first covered with a blue/black terra sigillata, then a second layer of white terra sigillata (with Zircopax added to a Grolleg porcelain base terra sigillata) was applied and surface decorations were carved and added. The piece was bisque fired to cone 03. After the firing, three patinas were added: upper left: Gerstley borate with copper carbonate; upper right and middle left: Gerstley borate with Mason stain #6266: Peacock; lower right: Gerstley borate with chromium oxide. 7 The patinas were gently rubbed away with a dry towel and then fired again in an electric kiln to cone 03. 8 Interior of the oval form with a clear glaze applied over the layered terra sigillatas.
Patina Fluxes and Colorants
Gerstley or Gillespie borate, Ferro frits 3110 or 3124, borax, or any other flux-based material can be used in combination with colorants to create patinas. Colorants for patinas can be any carbonate, oxide, or commercial stain. Commonly used oxide colorants include cobalt oxide, black copper oxide, red iron oxide, and manganese dioxide. If using a carbonate, you’ll likely need to add a clear or transparent glaze over top to reveal the color. Using copper carbonate as a patina colorant material requires this glaze layer.
Application of Patinas
Patinas are best applied after the bisque firing using a dry sponge or paintbrush. After the patina has dried on the surface, use a dry, soft cloth or sponge to gently wipe away the patina from the raised areas. You can repeat the application process and apply multiple colors of patinas. When applying patinas, consider wearing disposable latex gloves to prevent unnecessary contact with hazardous materials and to avoid unintentionally transferring the patina stain to another surface or form (6–8).
Firing Washes and Patinas
Because the wash materials fume, have a properly ventilated kiln to fire them in. Consider setting up an additional fan and having open window(s), in addition to proper kiln venting. Washes can be fired with results anywhere between cone 06–7.
Patinas can be fired in the same temperature range as the washes, however, you might consider firing to a final temperature that’s slightly cooler than your desired end temperature. Hold at this lower temperature for about an hour, then allow the kiln to slowly cool. This creates a soaking cycle and allows the patina to mature more slowly, resulting in a more matte finish.
After layering washes and patinas, you can continue to build up the terra sigillata surface by layering clear and colored transparent glazes. This will further diffuse the terra sigillata colors and create subtleties. In all of these experimentations, be sure to record your steps and processes so that you can avoid or repeat them in the future depending on the final results.
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.