James L. Murphy, "New Roseville Progressive Pottery Find," Roseville Legend, Winter 2004, pp. 1, 4.
A recently discovered earthenware bowl impressed "Twentieth Century German/Acid Proof," can be attributed to the Progressive Pottery Co., Roseville, Ohio, which appears to have operated only ca. 1901-1906. Matching sherds from the pottery site corroborate the identification of this bowl, which provides an example of a previously unknown mark for this cookingware manufacture.