I have been making hanging planters lately, decorated with colored terra sigillatas. I love the surface of these pots—matte, but not chalky after a good burnishing. But to get terra sigillata just right, you need to be sure the specific gravity is correct. If the term specific gravity is new and sounds unsettlingly scientific to you, today’s post will dispel any fears. In this excerpt from her book Terra Sigillata, Rhonda Willers defines the term and explains how to easily calculate it for your terra sigillata. –Jennifer Poellot Harnetty, editor.
After you have mixed and siphoned terra sigillata, you need to measure its specific gravity. Specific gravity is a measurement describing the relative heaviness of the terra sigillata in relation to same volume of water. In practice, it describes the terra sigillata’s thickness and relates to application. You can measure the specific gravity easily by: weighing 100 grams of water in a clear plastic cup on a gram scale (remember to tare the scale with the cup on it) (1), marking a line on the cup at the meniscus of the water (2), emptying the cup, refilling it with terra sigillata to the meniscus marking, and re-weighing the cup filled with the terra sigillata (3). The resulting weight should be between 110–120 grams. Divide this weight by 100 (the weight of the volume of water) to determine the specific gravity. You can also measure the specific gravity with a hydrometer (4).
The workable specific gravity range for terra sigillata is 1.10 to 1.20, with 1.15 being ideal for application and colored terra sigillata mixing. At the low end of specific gravity (1.10) you will need to apply multiple layers to create an opaque coating. This could be advantageous if you want to layer a variety of colored terra sigillatas and have colors peek through the layers. The ideal specific gravity range of 1.15 means there is enough body or thickness to the terra sigillata to aid in the suspension of the colorants. At this range, approximately three layers of terra sigillata may be applied to a piece. If too many layers are applied, or if the terra sigillata is too thick, it will peel away, which may or may not be desirable.
Excerpted and adapted from Terra Sigillata: Contemporary Techniques by Rhonda Willers. This book is available in the Ceramic Arts Network Shop.