A kiln is a type of oven that can achieve higher temperatures than an open fire. As an invention often associated with the creation of stoneware and ceramics, it has transformed since ancient times, evolving into safer and more efficient methods of heating. A thermally insulated chamber, kilns are also used to dry green lumber, create charcoal, dry tobacco leaves or malted barley, and refine metals from their ores. So-called "ovens" heat special crystals that resonate at a precise frequency and serve as standard time references in numerous contemporary instruments, clocks, and radios.