Shalom

Uncoated solid Molybdenum ("Moly") mirrors are simply the toughest, most durable, longest lasting laser mirrors available. Molybdenum has the lowest thermal expansion of any commercially used metal, and will withstand extreme temperatures without significantly expanding or softening. It has a Melting Point of 2,623 °C (4,753 °F). For infra red uses, polished uncoated Molybdenum has a high natural broad band reflectivity, approx 98%.  Molybdenum mirrors need no coating; the extremely hard surface resists weld spatter, fumes, smoke and process debris. Molybdenum mirrors can be scrubbed with brushes, soak cleaned in organic solvents or detergents repeatedly. 
Finally, the phase shift on reflection is very low and reliable (1 degree or so for CO₂ laser) eliminating polarisation issues that interfere with sheet metal cutting. Molybdenum is useful where a mirror has to be used facing upwards, and prone to collecting dirt and dust.