It doesn't matter if you call it soap, solvent, surface preparation, or whatever, chemistry is an important part of nearly every cleaning process. The first purpose of the chemistry in a cleaning application is to either dissolve the contaminant or, using some other mechanism, to break the attachment that the contaminant has to the substrate. The second purpose of chemistry in cleaning is to keep whatever has been removed from becoming re-attached to the surface it was removed from. Granted, chemistry doesn't always do all of this by itself. Mechanical energy supplied by spraying, brushing, agitation, ultrasonics and nearly any other imaginable means plays a large role as well in the overall cleaning process. Solvents - Historically a variety of naturally occurring and easily formulated solvents were used to truly dissolve contaminants. Kerosene, various forms of alcohol, naptha, stoddard solvent, mineral spirits were probably among the earliest solvents used in cleaning applications. These were well known mainly because they were studied and used in the formulation of paints used by artists. They were both relatively inexpensive and readily available. In time, advances in chemistry brought us more sophisticated hydrocarbon solvents including things like trichloroethane, perchloroethylene, acetone, MEK, etc. It was also discovered that solvents could be mixed using using defined rules to customize their effects. Solvents could easily be formulated that would dissolve an unwanted contaminants while leaving another desirable component untouched. This notion of selective solubility is used extensively in applications ranging from the cleaning and restoration of masterpiece paintings to the de-caffeinization of coffee beans. More recently developed solvents include the fluorinated hydrocarbon solvents which, today, have been largely banned because of their deleterious effects on the environment. Along the way, we have learned a lot about solvents. For example, certain solvents when mixed in specific ratios form azeotropic mixtures. An azeotropic mixture of solvents when distilled produces a condensate with the same chemical mixture as the starting mix. In simple terms, the mixture can not be separated by distillation. This property of a solvent can be useful especially when it comes to reclaiming solvent for re-use. In simple terms, a solvent dilutes whatever it dissolves. The dissolved component does not go away but remains mixed with the solvent. The more material is dissolved, the more concentrated the solution becomes until the solvent finally becomes saturated and can no longer dissolve any more. This concept is important in cleaning because once a solvent has anything dissolved in it, it will re-deposit what it has already dissolved back on the surface of the item being cleaned. This is like the quandary of how many babies can you get clean in the bath - - the second and successive babies pick up the dirt from those who preceded them. The risk with solvent, then, is that it has a limited useful life and, theoretically can thoroughly remove contaminants only if it is used only once! The answer to the single time use in the cleaning industry was the vapor degreaser. In a vapor degreaser, parts are first immersed in the cleaning solvent. This immersion starts the process of dissolving the contaminant at least to the point allowed by the pre-existing contaminant level of the liquid solvent. After immersion in the solvent, the part being is suspended in vapors of the boiling solvent. As the vapor distillate collects on the surface of the part being cleaned the freshly distilled solvent (less any contaminants left behind by distillation) serve to provide the supposed ultimate in cleaning - the single use solvent. This process, of course, only works with pure solvents or azeotropic blends as described above.