Terra Sigillata

Latin for "sealed/ stamped earth," terra sigillata is an extremely refined clay slip which, when applied to bone-dry greenware, results in a glossy polish. Terra sigillata cannot be purchased, but must be made by combining a suitable ball clay with a deflocculant. Deflocculants change the electrical charge of clay particles from positive / negative to uniformly negative, thereby inhibiting their ability to form electrostatic bonds with each another, preventing amalgamation. The specific gravity of the slip is required to ensure that there is just enough water in the solution to envelop each clay molecule, and no more.

Equipment, Tools, & Materials

• Ball clay—any ball clay will do.

• Deflocculants

• Darvan 7 is used with high iron clays and porcelains.

• Darvan 811 is used for stoneware, high fire slips, and red low fire slips.

• Digital scale. They are inexpensive and quite useful.

• Hydrometer. Inexpensive, and takes the guesswork out of determining specific gravity.

• Mask. Clay particles are quite small, and represent a breathing hazard. Masks are cheap—use one.

Sumi Von Dassow's Terra Sig Recipe

In a tall glass receptacle, mix

• Water (2100 grams/ 2.1 liters) [best practice: distilled water]

• OM4 Ball clay (1000 grams/ 2.2 lbs)

• Deflocculant Darvan 7 (25 grams)

Process

Mix by hand, in a liquid ball mill, with a hand blender, or a drill with a stirring attachment.

Let settle for an hour, then siphon water off the top, leaving the bottom layer of sludge. Toss out the sludge.

Evaporate in normal indoor conditions for two weeks. Stir. Using the hydrometer, determine the specific gravity of the mixture. Some insist that the specific gravity needs to be between 1.13 to 1.17; others say that anywhere between 1.1 and 1.2 will do. This number refers to the density of the terra sigillata mixture relative to water (which is 1).

Some artists boil the mixture to shorten the process, but this tends to increase the amalgamation of clay particles, and will influence the behavior of the mixture, and outcomes.

The end volume should be a little more than a third of the original volume.

Application

When the greenware is bone dry, lightly sand with steel wool. Remove all the dust.

Apply two to three coats, taking care not to create any drips, as they will show on the final piece.

Burnish with a polishing kit, a plastic grocery bag, or your fingers (taking care that your hands are clean and dry).