Redart + OM 4 Sig Kate Marotz

Instructions

I have been making terra sigillata from clay, deflocculants, and water using the process outlined on digitalfire.com. I use Cedar Heights Redart and OM 4 ball clay to create my terra sigillata and use sodium silicate with soda ash as deflocculants.

The water to clay ratio is 1 quart of cold water for every 2 pounds of dry clay. Add the dry clay to the water, mixing thoroughly with a hand blender or drill mixer attachment. The combined amount of deflocculant should be 0.5% of the weight of the dry clay. Dissolve the deflocculants in warm water before combining with the clay.

The specific gravity of terra sigillata should be carefully measured using a hydrometer or digital scale, especially for those looking for a high-gloss surface. After combining all ingredients, the specific gravity should be 1.15 for ball clays or 1.2 for all other clays.

Set a timer and allow the mixture to settle undisturbed for 20 hours. I use a ½-inch flexible tube as a siphon to decant everything except the heavy sediment at the bottom of the container. I even keep the layer of water at the top because it can hold the finest particles of terra sigillata. With ball clays, where it is more difficult to discern where the heavy sediment begins, siphon out the top two thirds. With clays like Redart, there is a more distinct layer of sludge.

At this point, the terra sigillata is far too thin to use, and it will need to be concentrated by evaporation or by heat. Tip: Put it in a crock pot on medium. Evaporate the water until the terra sigillata reaches a specific gravity of 1.10–1.2, which is ideal for most applications.

My final step is to add clay-body stains or titanium dioxide to adjust the color. I use a large rock tumbler to ball mill stains. I apply terra sigillata to bone dry clay (no burnishing) and then bisque fire my work.

Find additional information on terra sigillata in Rhonda Willers’ book Terra Sigillata: Contemporary Techniques, available at the Ceramic Arts Network shop  https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqnEAA.

This recipe was shared by Kate Marotz in the May 2021 issue of Ceramics Monthly.