Man has been using soap and water as cleaning agents for thousands of years. The first soap was made in the middle east about 5000 years ago. The discovery of soapless detergents is not very old. The first synthetic detergent was not invented until 1916, but since then, the manufacture of non-soap detergents became a major development of the petro-chemical industry. New methods of fabric cleaning came into use, such as dry cleaning. Dry cleaning is a method of cleaning fabrics with chemical solvents instead of soap and water. Many of these solvents are derivatives of crude oil. Petrol is the most important of them. Benzene is also used in dry cleaning. Their fumes can be dangerous if inhaled and they catch fire easily. Some after synthetic chemicals such as polychloroalkanes and alkanes have also been developed. The most common dry cleaning chemicals are carbon tetrachloride and trichloroethylene. In a dry cleaning establishment, clothes are usually treated first for stains. Then they are placed in the dry cleaning machine with the cleaning fluid or solvent and tumbled slowly for up to half an hour. After a rinse in clean fluid, the clothes are spun around rapidly to extract the liquid, and are finally fluffed in hot air. Any stains remaining are removed by hand and clothes are then steam pressed. Dry cleaning has several advantages over ordinary soap cleaning. Cleaning fluids can dissolve stains (especially oil and grease) which soap and detergents cannot remove. The process is most useful for delicate or expensive silken and woollen fabrics because it does not have any undesirable effect on them. For instance, the colours do not fade, as they might in water.