It is occasionally desirable to avoid the use of a hot material. At such times bituminous products are used in so-called cold applications, with the bitumen in liquid form as a cutback or an emulsion. After application of the bituminous solution to the solid surface, the solvent evaporates leaving the bituminous film to act as a coating. Numerous solvents are sufficiently volatile and are good solvents for bitumens, but many are either too expensive or too hazardous. To control the drying times of the solution, care must be taken in the selection of the bitumen and solvent.
Emulsions are dispersions of very small drops of bitumen in water.
A satisfactory emulsion is smooth in appearance, usually brown In colour, and can be made from bitumens having a wide range of consistencies. Emulsions usually contain a mineral material and are often called clay emulsions, although the mineral can also be coal, shale, metallic oxides, portland cement or asbestos. The most widely used is a special clay known as bentonite. An obvious advantage of emulsions over other bituminous products is that they are easy to handle, addition of water being all that is necessary to decrease their viscosity. Among the other factors to be considered are water absorption and photo-oxidation.