Fog. Smoke. Haze. All words for the same thing, right? Well, no.
Smoke is rarely used to create any sort of visual effect in the theater: the particles in smoke are just not healthy for anyone to breathe in over an extended period. Smoke is defined as solid particulates in the air, created by the process of combustion (i.e. burning something). Combustion is a messy process, potentially releasing all sorts of toxic gases, and carries with it the risk of a flame near an audience.
Haze is small liquid particles in the air which linger for long periods of time. Haze is used to create a general “smoky” effect: the smoke that fills the kitchen when an inexperienced cook makes bacon, or the water particles in the air that show off beams of sunlight. Because theatrical haze is engineered to linger, it’s hard to clear from the air when a scene changes.
Fog is larger liquid particles in the air, which dissipate at varying rates of time. Fog is the liquid/gas coming off a witch’s cauldron, the creeping mist in all movie cemeteries, and the swirling cloud of smoke when sugar drips into the bottom of a hot oven. There are multiple types of fog- some that hug the ground, others that rise into the air- but the defining features of fog is that it’s (generally) safe to be around for long periods of time, and that it must be continually renewed, or it will dissipate.
Fog is used more commonly, in places from theaters to front yards on Halloween, and is made in two ways: either by cooling a liquid, or by heating it up.
For cryogenic fog, like fog made with dry ice, water is first heated and then rapidly cooled when a block of dry ice is dropped into the water. The dry ice almost immediately expands and turns into a gas: this expansion agitates the water and breaks part of it into tiny water droplets, throwing them into the air. This fog tends to stay close the ground as it starts off cold and becomes less visible as it heats up and rises.
Cryogenic fog tends to be used for either small projects, science fairs or the occasional Halloween party, or for very large projects with 55-gallon drums of hot water. When using low-lying fogs, one must remember that it will settle at the lowest point available: a dip in the ground, or perhaps the orchestra pit. As any particulates in the air are not exactly great for humans over long exposure periods, care should be taken to restrain the fog with a simple well-decorated barrier.
Smaller fog machines available to consumers from party stores work on a different principle: instead of cooling a liquid, ‘fog juice’ is essentially dissolved into warm air.
An easy way to think of this is with a simple experiment: dissolving sugar into water. At room temperature, a certain amount of sugar can dissolve into a set amount of water before the water becomes “saturated” and will not dissolve any more sugar: sugar that is added to a saturated solution will fall to the bottom of the container and remain there. If the water is heated more sugar can be dissolved in it, creating a “supersaturated” solution. As the supersaturated sugar/water mixture cools, some sugar will slowly re-form on the bottom of the glass (or on a string, if one is inserted into the mixture- forming rock candy!).
The terminology is slightly different for air (although air and water are both fluids), but the general process is the same. Fog fluid is heated and dissolved into air, then the warm mixture is exposed to ‘cool’ (room temperature) air, and so the fog fluid changes from a high-energy gas to lower-energy small water particles which the eye perceives as fog.
This sort of fog immediately starts rising upon exit from the confined machine (heat rises) and will typically stick around a bit longer than cryogenic fogs. Exactly how this fog behaves is highly dependent on the environment: temperature, relative humidity levels, and (of course) air currents all play an enormous role.
The specific chemical makeup of the fog fluid used also impacts the behavior of the fog: fluid with only water will not last as long as a mixture of fluids. Generally, fog is made using a ratio of water to either glycol or glycerin, which come from the alcohol family (although unlike ethyl glycol, it is not recommended for humans to ingest these) and readily dissolve in water. The glycerin/glycol raises the boiling point of water, so heaters in fog machines have to heat the fog fluid to a higher temperature, but it also takes longer for the room-temperature air on stage to heat the fluid particles up enough to re-dissolve it.
Since it’s known that long-term exposure to particulates is not particularly good for humans, Actor’s Equity Association (the union for professional actors) has developed guidelines for how dense any particular cloud of smoke that a performer must stand in can be. More information as well as a study commissioned by AEA to determine the affects of fog, smoke, and haze on its members can be found here.
ESTA’s Technical Standard’s Protocol: “Introduction to Modern Atmospheric Effects” (scroll to bottom)
Actor’s Equity Association’s “Theatrical Smoke and Haze Regulations”
Lighting Lounge’s “Hazed and Confused- the Differences Between Fog, Haze & Dry Ice”
Theatre Effect’s “Fog FAQs”
Limelight Production, Inc’s “Fog Effects for Stage and Studio”
New York Time’s “Where There’s Smoke, There’s Stagecraft”
American Theatre’s “A Hazy Shade of Theatre: The Case for Clearer Design”
Encyclopedia Britannica’s “Glycol”