This method of sterilization is usually carried out in ovens such as hot air oven, which are normally heated by electricity or gas and are thermostatically controlled. The basic mechanism of sterilization by this process is dehydration of microbial cell wall followed by oxidation. Higher temperature and longer periods of exposure are needed in this process of sterilization, which indicates its less effectivity in killing microorganisms when compared to moist heat sterilization. Dry heat sterilization is conducted at 150°C to 170°C for not less than two hours. The individual units of articles and substances that have to be sterilized should be small so that the heated air could be freely circulated throughout the chamber. This method of sterilization is employed for substances such as fixed oils, glycerin, petrolatum, mineral oils such as paraffin and various heat stable powders such as zinc oxide and talc, which are not effectively sterilized by moist heat sterilization.