(Ideal for FX workshops where the characteristics can be adjusted to suit the application.)
The most common type of smoke machine takes a glycol based fluid that is pumped into a heated chamber. The normal components used are a solenoid pump to push the liquid in, and a fibreglass lagged heater block based on a sandwich of aluminium plates, a heating element and a long piece of copper capillery tubing snaked around between the heater plates. In some units the heater is tubular with the capillery tubing wound round it, but the effect is the same.
At switch on the unit will not pump liquid until the heating block has come up to the correct temperature, whereupon the pump can run and squirt the fluid into the block. When it does, the fluid evaporates very quickly and the resultant increase in pressure not only causes it to form a dense superheated vapour, but forces it out of the front of the machine via the exit port, which can be as simple as the end of the capillery tubing being poked out, or in some cases a small pinhole orifice to make sure that the internal pressure is kept high.
The resultant dense vapour exits the front of the machine and upon contact with the cool air it forms a dense cloud that is a very close relation to real fog.
In relative terms the ingredients used in smoke fluid are pretty safe, but there is always an ongoing debate in the entertainment industry about whether the output of a smoke machine is safe or not. When exposed to strong concentrations of the fog many people tend to get watery eyes and dry throats and noses. This is because the glycols are hygroscopic (moisture loving) and tend to remove the natural moisture from whatever they come into contact with. The eyes and throat are the most sensitive to this effect and the result is a dryness and slight discomfort. As soon as the person moves back into an unfogged area, the natural moisture returns and the symptoms disappear.
It's a good policy to ensure that where smoke is being used in the vicinity of performers it is kept to the minimum required to achieve the desired effect.
In the early 1940's some experimentation was done which involved hazing the air in children's wards with triethylene glycol to reduce the risk of cross infection. The idea was that the glycol atmosphere destroyed airborne viruses by sticking to them or removing their moisture (or something like that). During the years in which the wards were hazed to the point that a slight haze could be seen, the only complaints were that a few nurses got headaches because of the visual haziness. During the tests, no other significant ill effects were detected.
Only a few different glycols tend to be used in modern smoke fluids these days, and the ones that are used are also found in food too which must say something about their relative safety. The anti-fog brigade point out that the fog is being generated by heated pressurisation these days instead of the older technique of screw-pump atomisation. It is claimed that the use of heat can change the nature of the glycols and make them carcenogenic, but the manufacturers of the machines point out that the change of state can only occur if the glycols are burned, and modern machines have accurate temperature sensing circuitry to keep the heater block at the safe temperature. In reality the risk of burning occurs at about 400 degrees centigrade and most machines operate down in the 200 - 300 degree region.
Most smoke machine manufacturers present their fluids as top secret formulas that are specific to their particular machine. They warn that if any other manufacturers fluid is used then your machine will be damaged and your warranty will be instantly invalidated. In reality most commercial fluids use triethylene glycol mixed with water, and are generally interchangeable.
To elaborate, smoke fluid is a mixture of between 10 to 75% glycol to water depending on how dense the fog is desired. The water tends to be distilled/deionised/demineralised to remove contaminants that could cause furring of the machines heater block, and it is also pretty sterile to reduce the risk of bacteria growing in the solution. Propylene glycol is sometimes used in combination with the triethylene glycol to create different smoke characteristics, but the older glycerol (glycerine) recipe is rarely used since it's high density fog tends to leave quite a sticky residue after prolonged use.
For effects use the older glycerine recipe can be very useful since it allows the easy creation of a fluid that offers anything from a gentle haze to a thick high persistence white out.
For the sake of this article I'll be referring to glycerine as glycerol which is it's chemical name, or glycol which is the generic chemical family that glycerine belongs to.
For club use it's better to buy commercial fluid since it avoids the complications of purifying and sterilising the water, and the triethylene versions are much more venue friendly. However for effect use it is relatively simple to make up glycerine based fluid and adjust it's characteristics to suit your requirements. The glycerine can be bought in bulk from most good bakery suppliers and it is important to use pure glycerine as opposed to the sorbitol based substitute which has the same effect as burning sugar!
I bought a gallon of glycerine from Ingram Brothers in Glasgow (Scotland) for about £12. To make the fog fluid you should get a spotlessly clean plastic container and pour in the required percentage of glycerine before topping up with demineralised water or in the case of Scotland where our tap water is both clean and soft, I just top the bottle up from the tap. The next step is to shake the bottle vigorously, and that's basically it. You now have a customised haze/smoke/fog fluid. To keep track of different densities it might help to put a tiny amount of food colouring in the fluid to colour code it. I use a touch of yellow for a 75% glycol mix, red for a 50% mix, green for a 25% mix and blue for a 10% mix. If you really must, then you can give your fluids snappy names too, like "midnight mist" or "haunted forest".
This bits down to experimentation. With a gallon of glycerine you can have many hours of fun trying out endless ratios of dilution (I certainly did). You will be surprised at how little glycol is needed to create a light haze in the air, while the higher concentrations of glycol (up to about 75%) produce a dense plume of long lasting fog. For typical use a mix of 30% glycol is about right and also seems to be the concentration that inhibits growth of mould in the water. Depending on the type of smoke machine, you might be restricted in the ratio that you can use without causing spitting, where little bursts of glycol are spat out of the machine while it is producing fog. The best way to detect this is to sit the machine in front of a sheet of newspaper laid flat to see if any spots of liquid appear on it. You can add aromas to the fog if you can find good water based aroma fluids, but be aware that any impurities run the risk of leaving deposits in the smoke machines heater block.
Experimentation has been done in the past with adding coloured dyes to smoke fluid to produce coloured smoke, however, the amount of dye required is high and the risk of staining is also too high to make the technique worthwhile. In the pyrotechnic coloured smoke devices dye is also used to add the colour, and these smoke devices also pose a high stain risk. The best advice for producing clean coloured smoke is to colour the smoke with light using beam control to selectively colour smoke in a scene.
Don't attempt to make glycerine based fog fluid for commercial applications. The high density of the fog has a price in that the glycerine precipitates out of the air into a slippery film that doesn't smell as clean as the triethylene fluid. Old timers will recognise the glycerine smell as the typical stale smell of the earliest fog fluids (guess why!). The risk of storage also has to be considered with custom fluids. While fluid borne bacteria is most likely to be pastuerised as it goes through the fog machine, anything organic runs the risk of leaving deposits in the machines heater section which can ultimately block the machine if they are allowed to build up. The same can be said of hard mineralised water, and even some of the commercial fluids are guilty of causing heater block failure over time. The risk of heater blockage can be minimised if you carry out the simple flushing procedure detailed below. In general, for very low glycol concentration fluids, the fluid should be prepared as required and not left in the machine when it is stored.
Whichever glycol fluid you use, it's advantageous to occasionally flush out the heater block in smoke/fog machines with distilled water, especially when super high concentration glycol has been used. This can help remove deposits from the heater block assembly, and will prolong the life of your machine. To do this, simply run a cupfull of distilled water through the machine in the normal manner. You'll just get steam out of the smoke nozzle as the liquid goes through. This is a maintenance routine suggested by many machine manufacturers, and is very quick and easy to do. Some manufacturers go further and suggest adding 20% white vinegar to the water. Since white vinegar is generally water with 5% acetic acid, this equates to running a very dilute 1% acid solution through the unit. If you use the vinegar method then do it outside, since the acidic steam is quite unpleasant to breathe.
After flushing the machine run some standard fog fluid through it, particularly if it is going into storage. The pump is best kept lubricated with the fog fluid to keep it from sticking.