resin, any of a class of amorphous solids or semisolids. Resins are found in nature and are chiefly of vegetable origin. They are typically light yellow to dark brown in color; tasteless; odorless or faintly aromatic; translucent or transparent; brittle, fracturing like glass; and flammable, burning with a smoky flame. Resins are soluble in alcohol, ether, and many hydrocarbons but are insoluble in water. When heated, they soften and finally melt. Their chemical composition varies, but most are mixtures of organic acids and esters. Resins are generally classified according to their source or by such qualities as hardness or solubility. Natural resins are found as exudations, often as globules or tears, on the bark of various trees (mostly pines and firs) or on other living plants; they also occur as fossils or as exudations from the bodies of certain scale insects (see lac). Some natural resins, called oleoresins, contain both a resin and an essential oil; they are often viscid, sticky, gummy, or plastic. Other resins are exceedingly hard and resistant to most solvents, softening only at high temperatures. The primary uses for most resins are in varnish, shellac, and lacquer, in medicine, in molded articles (e.g., pipe mouthpieces), and in electrical insulators. See amber; balsam; benzoin; Canada balsam; copaiba; dragon's blood; mastic; rosin; turpentine.