The earliest way to preserve wood involved storage in water and impregnation with salts: mercuric chloride, arsenic compounds or acids. Later, products of vegetable or animal origin were used. In the 19th century, mineral oil, petroleum and related materials came into use. In the 20th century organic solvents were proposed as disinfectants. Other techniques of pest control were fumigation with toxic gases; the use of the vacuum, either heating or freezing; supersonic treatment; and radioactive irradiation. Glues, oils, waxes, and natural resins were used for consolidation until synthetic products were introduced for wood conservation.