Pouring boiling water over a block of dry ice is one of the oldest ways to make stage mist. A five-pound block will produce a stage full of low-lying fog sure to turn your set into a creepy cemetery, gloomy battlefield, or heavenly landscape. Dry ice foggers are easy to make, or can be purchased complete. When dry ice--frozen carbon dioxide--is exposed to hot water, the carbon dioxide turns instantly back into gas, creating a heavy white mist that hugs the floor.
Chemical fog machines heat special fluid until it becomes vapor. Glycol/glycerine foggers heat a mixture of water and glycol or glycerine, called "fog juice." As the machine heats the liquid, the glycol turns to mist, mixes with air, and creates white clouds. Warm glycol/glycerine fog rises, making it excellent for fog and mist effects. If you want fog that stays on the ground, the warm mist must pass through a chiller in the machine.
Foggers that use mineral oil atomize the oil inside the machine and pump it out into the air. When it leaves the machine and comes in contact with water vapor in the air, mist or fog is formed, resulting in thick white clouds. To keep them lower on the stage, the clouds can be passed through a chilling unit in the fog machine.
The first artificial fog machines were invented by Tom Mafee in the early 70s for an exhibit at the Osaka World's Fair. These machines turn ordinary tap water into large amounts of fog in a variety of ways. Water-based fog creates a lot humidity, which can be a benefit in warm outdoor settings. Artificial fog and mist produced this way, however, may cause moisture problems for indoor stages.