In 1770-80, a cobalt glaze was introduced into creamware giving the body a bluish tint, thus came the introduction of Pearlware. This ware was easily distinguished by the blue ring at the bottom of the ware or if too much cobalt had been added, the cobalt spotting throughout the piece. c1770-1890
Literature: "...It wasn't until 1779 that he (Wedgwood) was finally satisfied with the new 'Pearl White' body, which incorporated added Devon clay and flint with a cobalt-stained glaze. Nevertheless he considered this merely 'a change rather than an improvement'. " Source: English Pottery 1620-1840 by Robin Hildyard"