Technically the term 'smoke' is used to describe the products of combustion whereas 'fog' describes droplets of water vapour suspended in the air; usually at or near ground level. Most 'smoke' or 'fog' effects used in theatre and concert work are neither of these however the terms are still widely used. Theatrical 'fog' effects fall generally into two categories:
This information was updated on 29th December 2010.
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Dry ice as used to create the original low-lying mist effect is solid carbon dioxide which has a temperature of around -78.5 deg.C. It can be purchased from various depots around the country, usually as pellets in 10Kg. bags or as 4Kg. blocks. When the ice is immersed in boiling water using a specially designed 'kettle' large quantities of white mist are produced. Although at least one manufacturer (City Theatrical) has invested time and money in researching improvements to the older 'kettle' design, safer systems producing similar effects now exist. See Low Smoke Conversion Machines below.
The mist produced is 1.5 times heavier than air and so will always settle at the lowest point. It is an ASPHYSIATING gas, which means IT DOES NOT SUPPORT LIFE, so ensure that at no time do performer's (including animals) heads go below the mist level, and be careful not to fill up the orchestra pit, basement or other under-stage areas.
WARNING: The transportation, storage and use of 'dry ice' is complicated and closely regulated by safety law. We strongly suggest that you avoid using this effect if at all possible. If you feel that your show must use it please ask for safety advice and guidance before making a final decision.
Machines to produce these effects range in size from tiny ones built into the set or props to large ones flown in the lighting rig and controlled from the lighting desk, however, most of them work in a similar way. A fluid, usually water based, is pumped into a heat exchanger where it rapidly vapourises causing an increase in pressure. This forces the vapour out of a nozzle at a high rate. As the vapour exits the heat exchanger it cools quickly forming the white fog we call smoke. Special machines called haze generators and their associated fluids are also available. These produce a fine, transparent haze rather than a dense cloud of visible smoke and are ideally suited to use in concerts, clubs and discos where the beams of the lights need to be seen as part of the overall effect but without the obscuring caused by normal smoke.
With thought and careful planning these kinds of effects work well and they are comparatively easy to use, but, the smoke produced is lighter than air so its exact position on stage will be affected by draughts and the movement of performers. Also it will eventually rise and be dispersed by the heat of the lights.
If used correctly smoke fluids are non-toxic and in general should not make you cough. Some manufacturers produce scented fluids or a range of scent concentrates which can be added to normal fluids. The fluid used with a machine will have been specially developed for it. For this reason we always advise that you should use only the fluid recommended for your machine by its manufacturer.
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The input to these machines is usually the smoke produced by a standard smoke machine. This is directed into a chamber which has been cooled using either carbon dioxide or nitrogen gas from a cylinder. The result is a low-lying mist similar to the original 'dry ice' effect.
An alternative machine is available which cools the smoke using a refrigeration process to produce a similar effect. These machines, however, tend to be larger and more noisy, and they usually require the use of a special fluid in the smoke generating part of the set up. Having said that they can produce very large quantities of low-lying smoke.
The output of most smoke machines is not real smoke i.e. it is not a product of combustion and so it should not set off a smoke detector but 'better safe than sorry' - always do a check first.
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