Any chemical element that vaporizes (that is, boils) at high temperatures or condenses from a gas at high temperatures. Examples are calcium (Ca), aluminum (Al), uranium (U), titanium (Ti), the rare earth elements such as cerium (Ce), europium (Eu), and gadolinium (Gd); tungsten (W), zirconium (Zr), and thorium (Th). The opposite of refractory is volatile.