urban ecology download

0.55 Chrome red can exist only in basic glazes and fluxes, assuming here that boric anhydride is not an acid. The basicity should be as great as possible and produced chiefly CERAMIC CHEMISTRY. 63 by means of lead oxide. Firing should be short and cooling rapid. Red colours fired higher than cone 9 are obtained from chromium-aluminium compounds. The stains should be prepared at a higher temperature than the ware will be fired at. A violet-red fired at cone 14 is produced from 1.0 K 2 0, 16.53 A1 2 3 , 1.0 Cr 2 3 , 3.27 B 2 O 3 , and an under- glaze salmon from 0.046 K 2 0, 0.658 MgO, 0.296 ZnO, 0.722 A1 2 3 , 0.046 Cr 2 3 , 0.08 B 2 3 . Lethbridge states that the presence of zinc chromate makes chrome-tin-lime pinks more brilliant. He gives the formula Sn0 2 72, Cr 2 3 1.0, CaO 26, for a pink. The lime lowers the temperature of development. Purdy uses the general formula for chrome-tin pink and red glazes : PbO 0.22 ) 0.16 A1 2 O 8 (2.1 SiO 2 CaO 0.59 x SnO 2 (0.47 B 2 O 3 KNaO 0.19 ) y Cr 2 O 3 Of colours other than red, little need be said. The composition of the glaze is of course important. Chrome greens are spoiled by zinc, probably through formation of chromate. Copper greens are easily obtained. Silica and borates may be present to any extent in the glaze,