A particularly sophisticated technique of hydrogen purification makes use of thin dense metal membranes. Many metals are capable of reacting with hydrogen at moderately elevated temperatures forming metal hydrides. Atomic hydrogen moves fairly easily around the metal lattice thus allowing hydrogen to pass between the two surfaces of dense metal layers while other gases are held back completely. Usually palladium alloys are preferred for this gas separation application since thin layers of most other metals cannot withstand aggressive atmospheres long enough. Palladium, however, is very expensive and the industrial installation of such membranes stipulates layers of less than 10 µm thickness.