In modern terms, boiled or partially polymerized drying oils with added siccatives or dryers (chemical catalysts) have cure times of less than 24 hours. By contrast, untreated or "raw" oils may take weeks or months to cure, depending on ambient temperature and other environmental factors. 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. Originally, the term "varnish" referred to finishes that were made entirely of resin dissolved in suitable solvents, either ethanol (alcohol) or turpentine. Drying oils cure through an exothermic reaction between the polyunsaturated portion of the oil and oxygen from the air. These contain high levels of polyunsaturated fatty acids. There are many different types of drying oils, including linseed oil, tung oil, and walnut oil. The drying and curing time of all varnishes may be sped up by exposure to an energy source such as sunlight, ultraviolet light, or heat. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. Oil, polyurethane, and epoxy varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive stages from liquid or syrupy, to tacky or sticky, to dry gummy, to dry to the touch, to hard.Įnvironmental factors such as heat and humidity play a large role in the drying and curing times of varnishes. Acrylic and waterborne varnishes dry by evaporation of the water but will experience an extended curing period for evaporation of organic solvents absorbed on the latex particles, and possibly chemical curing of the particles. Resin varnishes dry by evaporation of the solvent and harden quickly on drying. After being applied, the film-forming substances in varnishes either harden directly, as soon as the solvent has fully evaporated, or harden after evaporation of the solvent through curing processes, primarily chemical reaction between oils and oxygen from the air ( autoxidation) and chemical reactions between components of the varnish. However, different types of varnish have different components. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent plus a metal drier to accelerate the drying. Therefore, many manufacturers list proper disposal practices for rags and other items used to apply the finish, such as disposal in a water filled container. Thus, oil-soaked rags and paper can smolder and ignite into flames, even several hours after use if proper precautions are not taken. All drying oils, certain alkyds (including paints), and many polyurethanes produce heat (an exothermic reaction) during the curing process. These are flammable in their liquid state. polyurethanes) or resins as their binder in combination with organic solvents. Many varnishes contain plant-derived oils (e.g. Safety īecause of flammability concerns, many product containers list safety precautions for storage and disposal for varnishes and drying oils as they are flammable, and materials used to apply the varnishes may spontaneously combust. The Tang Chinese used medieval chemistry experiments to produce a varnish for clothes and weapons, employing complex chemical formulas applied to silk clothes of underwater divers, a cream designated for polishing bronze mirrors, and other formulas. Varnishing is also recorded in the history of East and South Asia in India, China and Japan, where the practice of lacquer work, a species of varnish application, was known at a very early date. Varnishing was a technique well known in ancient Egypt. A false etymology traces the word to the Greek Berenice, the ancient name of modern Benghazi in Libya, where the first varnishes in the Mediterranean area were supposedly used and where resins from the trees of now-vanished forests were sold.Įarly varnishes were developed by mixing resin-pine sap, for example-with a solvent and applying them with a brush to get the golden and hardened effect one sees in today's varnishes. The word "varnish" comes from Mediaeval Latin vernix, meaning odorous resin, itself derived from Middle Greek berōnikón or beroníkē, meaning amber or amber-colored glass.
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