Editing Varnish (section)

===Drying oil===
{{main| Drying oil}}
There are many different types of drying oils, including [[linseed oil]], [[tung oil]], and [[walnut oil]]. These contain high levels of [[polyunsaturated]] [[fatty acids]]. Drying oils cure through an [[exothermic reaction]] between the polyunsaturated portion of the oil and [[oxygen]] from the air. Originally, the term "varnish" referred to finishes that were made entirely of resin dissolved in suitable solvents, either ethanol (alcohol) or [[turpentine]]. The advantage to finishes in previous centuries was that resin varnishes had a very rapid cure rate compared to oils; in most cases they are cured practically as soon as the solvent has fully evaporated. By contrast, untreated or "raw" oils may take weeks or months to cure, depending on ambient temperature and other environmental factors. In modern terms, boiled or partially [[polymerized]] drying oils with added [[siccative]]s or dryers (chemical [[catalyst]]s) have cure times of less than 24 hours. However, certain non-toxic by-products of the curing process are emitted from the oil film even after it is dry to the touch and over a considerable period of time. It has long been a tradition to combine drying oils with resins to obtain favourable features of both substances.