The process by which all types of micro-organisms are destroyed. Autoclave sterilization, utilizing both steam and pressure, is the most effective method for routine clinical dental practice. See autoclave. Dry heat sterilization is achieved using a hot air oven on a standard setting of 160°C (320°F) for 2 hours and is useful for sterilizing powders but inappropriate for dental instruments. Chemical sterilization can be achieved by the immersion of materials or instruments in liquids such as gluteraldehyde and formaldehyde solutions. This process can take 12 hours or more to kill all spores and the liquids are volatile and toxic to tissues, and is therefore inappropriate for dental instruments. Ethylene oxide is a gaseous form of chemical sterilization used commercially in the manufacture of many disposable medical devices. Sterilization by flaming an instrument until it glows red provides effective sterilization but will detrimentally alter the properties of the working tip of a metal instrument. Glass bead sterilization and hot salt sterilization have been used for root canal instruments but are only effective for that part of the instrument which is in contact with the glass beads or salt, and therefore sterilization is incomplete. Radiation sterilization, such as x-rays or gamma rays, may be used in the commercial manufacture of some materials and equipment. Ultraviolet (UV) sterilization is only effective on surfaces and some transparent materials. Boiling in water for 15 minutes will kill most bacteria and viruses but is ineffective against prions and many bacterial spores and is therefore unsuitable for achieving sterilization in clinical dental practice. See also decontamination.