Zirconia elements are the only elements that can be used in an air atmosphere at temperatures higher than molybdenum disilicide elements. Zirconia elements have only been used in laboratory size kilns because they are only available in small size and are very expensive. Also, these elements must be preheated to 1000 c before conduction even starts. Zirconia elements can be used at kiln temperatues up to 2000 c and SCR controls are required.