Abstract: A solid substance is cleaned by a vapor cleaning method. In the method, a cleaning vessel which is opened to the atmosphere at its top is used. A hazardous solvent is vaporized at the bottom of the vessel and cooling coils within the vessel condense the vapors as they travel upwardly to the open top. The solid substance to be cleaned is placed in a zone where the vapors are condensing. The invention resides in blanketing the top of the condensing vapors with an inert gas which is less hazardous than the volatile solvent thereby forming a buffer layer between the atmosphere and the solvent. The gas has a density at the conditions existent within the buffer layer which is greater than the density of air at ambient conditions, and has a boiling point below the boiling point of the solvent.