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Using a glass or clear container, dissolve 5 gms of calgon sodium hexametaphosphate, which is a deflocculating agent) in 2 pints of rainwater(sodium crbonate 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.