Big Ceramic Store Blog

Sgraffito in Terra Sigillata With BCS Customer Lisa Harnish - Part 1

BCS customer Lisa Harnish shares her amazing techniques and recipes for "Sgraffito in Terra Sigillata" in today's post! Visit Lisa's website for more!

Lisa Harnish first began working with clay in January 2002. Her initial education in ceramics began at Chandler Gilbert Community College, and continues to this day. Lisa’s sgraffito designs take their inspiration from nature, such as grass, leaves, seaweed and vines that wind their way around and across the vessel. 

What is Terra Sigillata -

Terra Sigillata is Latin for “sealed earth”. Its usage dates back to ancient Roman times. It was developed as a method of sealing the surface of pottery forms, and for decoration (red and black slip). In today’s ceramics, Terra Sigillata is a generic term for a thin slip made by separating the tiniest, finest particles of clay from the larger particles. Often called “Terra Sig” or even just “Sig” for short, it can be made from almost any clay body.

Lisa’s Terra Sigillata (adapted from Matt Metz)

  • 4,000 grams dry Grolleg clay
  • 2 gallons distilled water
  • 40 grams Calgon

calgon

  • Mixing Instructions: Empty water into a clean, 5-gallon bucket. Measure Calgon, add to bucket,  mix to dissolve. Add Grolleg clay in 500 gram batches, mixing after each addition. Set mixture aside to settle for at least 2 days. Use a turkey baster to siphon off the middle layer in small batches as needed. Leave the water layer on top, to keep it from drying out prematurely. The sig should be the consistency of whole milk.

Coloring Terra Sigillata

Because I’m trying to keep contamination of the sig to a minimum, I prefer to use metallic oxides (or carbonates), rather than stains, the only exception to this is for the satin black, I use Mason Stain Black 6600.

colorants
Applying Terra Sigillata

I've found it difficult to apply terra sigillata to wet or leather hard clay. It seems to not want to “stick”, let alone build up a solidly colored surface, however; bone dry greenware is very absorbent, and will accept the terra sig quite readily.

Sanding the greenware with a coarse grit sand paper (60 grit or lower) raises a “tooth” to the surface, a faint texture that will act like a sponge to soak up the terra sig, ensuring a good fit. Neglecting this step has often resulted in sintering, where the sig layer tries to peel away from the clay surface during firing.

  • Sanding generates a lot of dust, which can be hazardous to breathe in. Wear a properly fitted respirator when sanding greenware, in a well ventilated area (i.e., outside).

I use soft "fan" style brushes, or even inexpensive house paint brushes. I do make a point of keeping the brushes separated by the colorant used to prevent contamination. For instance, Red Iron Oxide is only used in Red Iron Oxide sig.

Join us next week for Part 2 where we will go a little more in depth on using the Terra Sig,  burnishing, sgraffito, and glazing!

If you'd like to share your ideas for our blog, email us at bcscommunity@bigceramicstore.com!

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