Dry Ice

• Making fog with dry ice is the granddaddy of all special effects. Dry ice is the solid form of carbon dioxide and very cold. When immersed in hot water, the temperature difference causes a cloud of water vapor -- fog -- as well as a furious bubbling that makes it ideal for Halloween displays. The fog doesn't last very long unless you continuously heat the water and keep adding more dry ice, which can get expensive with a large scale display. This method is great for a buffet table centerpiece. Use only food grade dry ice if you put it in punch or other beverages.
  
  

Mist

• A fog-like vapor can be made by spraying purified water from a pinhole nozzle under high pressure. Boiling water also creates a misty sort of fog in the form of steam. For Halloween displays, a more viable choice is an ultrasonic mister. Small, fairly inexpensive units are available and can be placed in water to create tabletop effects such as fog-wrapped skulls or steaming fountains. Some have light-emitting diodes to add spooky color to the display.
  
  

Fog Machines

• Glycol-based fog machines are the most efficient, cost-effective way to produce fog for a larger home display. They work by spraying a special mixture of water and glycol into a heated chamber, resulting in plumes of dense fog. Plans for cobbled together homemade devices abound, as do formulas for "fog juice." Considering that a fog display of this type will include a scalding hot machine producing vapor you might inhale, a safer bet would be to buy a reasonably priced, commercially available fog machine and solution. The hot fog emitted by these machines tends to rise into the air, so if you want it to swirl eerily along the ground you will need to make your own homemade fog chiller.
  
  

Fun With Fog

• You can use fog to enhance props in your display, such as a glowing cauldron of bubbling, steaming witches' brew. It can also be an effect in its own right, the sort of fog that turns your front walk into a deserted London alley. For smaller, short duration displays, consider dry ice and ultrasonic misters. With a good fog machine and some plastic pipe, you can make a larger volume of fog appear to rise from unhallowed ground, hanging wraith-like in the air or pouring ominously from almost any opening, including your own front door.