Flow Blue. A deep cobalt blue which was used for under-glaze printing on pottery. As the name indicates, the colour tended to flow into the glaze, giving a blurred effect; this result was obtained by placing FLOW POWDER (q.v.) in the saggar containing the ware, chlorine evolved from the powder and combined with some of the cobalt, thus rendering it slightly soluble in the glaze.
Flow Powder. A mixture formulated to evolve chlorine at the temperature of the glost firing of pottery and used in the production of FLOW BLUE (q.v.). For ware covered with a lead glaze a suitable composition is (per cent): NaCl, 22; white lead, 40; CaCO3, 30; borax, 8. For use with a leadless glaze a suitable mixture is (per cent): NaCl, 15; MgCl2, 55; KNO3, 15; CaCO3, 15.
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