An understanding of the methods and mechanics of sterilization and disinfection is necessary to prevent bacterial cross-contamination. Since sterilization of impressions is not possible, because of the high temperature and time needed, disinfection is the method of choice. The most effective means of disinfecting impressions is to immerse them in disinfectant solution for 30 minutes. Most hydrophobic impression materials exhibit minimal distortion when disinfected in this manner. Irreversible hydrocolloid (alginate) and hydrophilic silicones distort when immersed and can be disinfected by spraying and placing in a sealed plastic bag for manufacturer's recommended time. Internal disinfection (replacing water with disinfectant before impression taking) is the method of choice for alginate, since it allows immediate pouring of the impression after removal from the oral cavity.