AbstractThe Tennessee Valley Authority has carried out small‐scale and large‐scale studies during the past five years, directed toward the development of refractories more resistant to corrosion by molten rock phosphate than the present commercial types.Small‐scale tests were made on refractories containing varying proportions of zirconium silicate and oxides of aluminum, beryllium, calcium, cerium, chromium, magnesium, thorium, and zirconium. Promising compositions were then tested by fabrication into standard 13l½‐in. and 9‐in. brick and by determining their resistance to corrosion by molten rock phosphate, basic open‐hearth slag, and electric phosphate‐smelting furnace slag. The most promising of the compositions tested were (1) Cr2O367%, CaO 30%, and Al2O33% and (2) ZrO233.5%, Cr2O333.5%, CaO 30%, and Al2O33%. A 25‐kw. Detroit rocking furnace was lined successively with brick of these two compositions, as well as a commercial superduty firebrick and a commercial unburned magnesite brick, and each lining was tested against molten rock phosphate until failure occurred.The tests show that the two new types of refractories are superior to the commercial refractories and that they may have promising possibilities for use where basic refractories are now