A test with a smoke machine running 2% glycerine in distilled water. It produced a surprising amount of visual haze for such a small amount of glycol.
Modern smoke fluid tends to be based on distilled water and one or more glycols like propylene glycol or triethylene glycol. Some still use glycerol (glycerine) for extra fog density and persistence, but it has a stronger smell than the other glycols and tends to create more residue.
I did these tests with standard glycerine bought in the UK from Boots the Chemist. The point of the videos is to show how the amount of glycol in the water varies the effect.
The use of distilled water in home made fog fluids is recommended as the solids in normal tap water can build up inside the heater assembly and block it over time.