What is fog, and what's it good for?

• Provide creepy look or spooky atmosphere.
• Serve as a projection surface for light and images.
• Provide a minor obstacle, such as confusing things in a maze.
• Hide details of the enviroment.
• Distract.
• Provide additional detail or sensory input, making something more realistic - It's easier to believe a pot is boiling if "steam" is coming out of it.

We tend to use the terms "fog" and "mist" interchangeably, but there is a difference. Here's a particularly clear explanation from a mailing list posting (sorry, I don't have the full original posting):

> MISTS are droplets of liquid suspended in air. They are spherical in 
> shape with a size ranging from a few micrometers to over one hundred 
> micrometers in size.  Mists are generated by the splashing, bubbling, or 
> spraying of liquids.  The fine droplets comprising the mist remain the 
> same as the original liquid from which they were generated. So a toxic, 
> irritating, or infectious liquid will produce a mist with similar 
> characteristics when it is aerosolized. 
> 
> FOGS are similar to mists. They differ in the way they are formed. 
> Mists are formed when some type of vigorous actions cause droplets of 
> liquid to be suspended in the air.  Fog is formed when vapors of a solid 
> or liquid condense in the air to form suspended droplets.  The droplets 
> in fog are usually one to ten micrometers in diameter, which is slightly 
> smaller than the average mist droplet. 

I would like to add "smoke" to this list, and would be tempted to define it as particles or vapor in the air as a byproduct of combustion.

What should I look for?

In order to decide what is best for you, you must first think about your requirements in various areas:

• safety - Is this technology safe for my actors, my patrons, and myself? Are there hazards associated with the equipment and/or supplies? Does this otherwise safe technology cause problems for people with allergies or chronic health conditions, like asthema?
• thickness - Do you want a pea-soup fog that you can hardly see through, or a light haze?
• volume - How much fog do you need? Just enough to fill a cauldron, or enough to fill your front yard?
• enclosure - Is the target area enclosed, or is it in the open, where wind will blow the fog away?
• timing/duration - Do you need continuous fog, or just a little burst now and then?
• cost - How much does the equipment cost? Is it expensive to repair? How much does it cost for the consumable supplies necessary to operate it?
• reloading - How much fog do you get each time you load the fog machine? Will you have to reload in the course of a Halloween evening? How hard is that?
• power - Does the fog machine require a power supply? Do you have the necessary power at the machine location?
• size - How big is the fog machine? Can you fit that in your set, or duct the fog where you need it?
• controls - Does the machine run by itself, or via remote control, or timer? Does the machine require supervision or safety checks?
• low-lying - Do you need the fog to lie low on the ground, or do you want it billowing up?
• regulatory compliance - Will the Fire Marshal let you use it? Does your insurance company object?
• Will it set off the smoke detectors?

Will it set off the smoke alarms?

Well, in truth, "maybe". But you don't want false alarms, so you should assume that your fog will cause false fire alarms, and then look for ways to prevent it.

The first step in guarding against false fire alarms is to find out exactly what kind of alarm system you are dealing with. There are numerous kinds, and they are often used in combination.

Here are some of the things you might run into:

• ionization - This is the most common smoke alarm that you can find in home improvement and drug stores is the ionization detector. These are the $10 models operated by a 9V battery.

  They are designed to sniff the air for the byproducts of combustion, and are unlikely to react to modern fogging technologies. They will react to pyrotechnic smokes and incense.

• optical - This type of detector shoots a small beam of light across a small caged gap. A light detector at the other end of the gap looks for variations in the beam, as might be caused by smoke wisping through.

  Since this detector is triggered by the fact that smoke is visible and capable of casting a shadow, it will trigger on all visible fogs, smokes, mists, etc.

• IR detectors - These detectors attempt to detect fire by looking for IR light. They are similar to heat detectors, but sensitive on a slightly different wavelength, and take less energy to trigger.

• heat detectors - I'll use this category to discuss detectors that require a substantial amount of heat energy to trigger.

  This would include temperature-driven devices like thermostats. It also includes fusible-link systems, like the sprinklers that are triggered by melting a piece of plastic or metal.

• UV detectors - These detectors are usually found in industrial applications, watching for the UV-rich flame of certain burning chemicals. They are sometimes used to monitor the flame in gas-burning furnaces.

  These detectors won't be triggered by most fogging technology, with the possible exception of pyrotechnic mixtures. But I'll bet that a black light sets them off!

• CO detectors - These detectors sniff the air for carbon monoxide, a byproduct of incomplete combustion. It is currently fashionable to install them in houses, especially near furnaces and water heaters.

  These detectors might be triggered by pyrotechnic smoke or incense.

Dry Ice

Trade-offs:

• Consumable supplies are expensive. May be hard to get.
• To make a lot of fog, you need a lot of material.
• Fog lies low.
• Can be done without power, but doesn't last log without it.

Water-based (glycol) fogger

Pressurized Water Mist

Industrial mist systems are used to surpress dust and odors, neutralize static electricity, and cool people and things. They are particularly handy in places like chicken farms, because it isn't economical to air-condition a barnyard.

Systems like this tend to be expensive, because they require:

• good filters to keep the water clean
• precision-machined nozzles
• high-pressure pumps

Ultrasonic Mister

Steam

Humidifiers and Vaporizers

• thermal - The cheapest kind of vaporizer you can buy is the thermal kind that tells you to "add a pinch of salt to the water." This gadget uses the salt as an electrolyte to run electricity through the water, heating it up, producing steam.

• mechanical - I have seen vaporizers with a relatively heavy top that comes off, with some kind of wheel attached to it. These may be sold as "cool mist vaporizers". The wheel spins around, scooping up the water and flinging it around until it hits the inside walls of the device and splatters into tiny droplets.

  If you use "hard water" in such a device, prolonged use will produce a coating of white dust over everything in the room.

• ultrasonic misters

In general, humidifiers put less water into the air, and might not even produce a visible effect. My grandmother liked to put a pan of water over the heating duct in the winter, to add humidity.

Pyrotechnic smoke

Now, let's think about this - Haunts have a reputation {deserved or not} as fire-traps. Haunters often have strained relationships with the local Fire Marshal. And you are thinking for even a minute that it's a good idea to add burning stuff to your haunt, when other technologies exist?

Incense

Incense is available in sticks, cones, bricks - all of which are capable of continued smouldering once you have lit them and gotten them going. You can also get powdered and resin forms of incense that won't continue burning on their own, and must be placed on a burning piece of charcoal.

I don't really like the idea of any real fire in a haunt, even a smouldering stick of incense.

Oil Vaporizer

Oil Cracker Hazer

Water-based hazer

Liquid Gasses

The materials most often used for this are liquid nitrogen and liquid carbon dioxide. These gasses have the advantage of being relatively cheap and unreactive.

Other materials could theoretically be used, such as liquid oxygen (extremely hazardous), and noble gasses (expensive). But should be avoided.

Liquid gasses are expensive, difficult to use, and dangerous for numerous reasons. Fogging through compressed gasses should be considered an exotic technique, best avoided by all but experts.

Notable hazards are:

• The released gas may be non-toxic, but will displace oxygen in the air, causing suffocation.
• Extreme cold can cause tissue damage if it comes into contact with living creatures.
• The pressure necessary to maintain liquid gas is extreme. This demands highly rated valves and equipment (expensive), and presents hazards if pressure is accidently released.