October 18, 2002, Newsletter Issue #112: Terra Sigillata

Tip of the Week

Terrasigillata
Using a glass or clear container, dissolve 5 gms of calgon sodium hexametaphosphate, which is a deflocculating agent) in 2 pints of rainwater (sodium carbonate or potassium carbonate can also be used). Add 230 gof dry crushed clay, sieve through a 60 mesh (or finer). Shake container, mix and leave for three days minimum. The clay will settle into 3 layers with a layer of clear water on top, siphon off this clear water and discard. The next layer is the terrasigillata(colloidal slip).
Note: White clay needs 0.3% sodium silicate...and water to clay ratio of four to one, to deflocculate.
Red clay needs 0.5% sodium silicate...and water to clay ratio of three to one to deflocculate. Terra sigillata is high in alumina. It may be hardened by adding 3-4% sodium silicate to the recipe after deflocculating the sigillata.

Terra sigillata: Application and Firing
Application and Firing: Terra sigillata can be applied by spraying, dipping or brushing at all stages of construction. Apply several coats and burnish or simply polish with your hands or a soft cloth when damp/dry to the touch. Polish between coats for the glossiest surfaces. Terra sigillata can be fired at any temperature but the polished surface will start to break down when fired over cone 04. Use as a slip/engobe up to cone 10 and above.

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