Editing Varnish (section)

==Components of varnish==
Varnish is traditionally a combination of a [[drying oil]], a [[resin]], and a [[Turpentine substitute|thinner]] or [[solvent]] plus a metal drier to accelerate the drying. However, different types of varnish have different components. 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 (chemistry)|curing]] processes, primarily chemical reaction between oils and oxygen from the air ([[autoxidation]]) and chemical reactions between components of the varnish.

Resin varnishes dry by evaporation of the solvent and harden quickly on drying. [[Acrylic paint|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. 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.

Environmental factors such as heat and humidity play a large role in the drying and curing times of varnishes. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. The drying and curing time of all varnishes may be sped up by exposure to an energy source such as [[sunlight]], [[Ultraviolet#Curing of inks, adhesives, varnishes and coatings|ultraviolet]] light, or heat.

===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.

===Resin===
Many different kinds of resins may be used to create a varnish. Natural resins used for varnish include [[amber]], [[Gum-digging|kauri gum]], [[dammar]], [[copal]], [[rosin]] ([[colophony]] or pine resin), [[sandarac]], [[balsam]], [[elemi]], [[mastic (plant resin)|mastic]], and [[shellac]]. Varnish may also be created from synthetic resins such as [[Acrylic resin|acrylic]], [[alkyd]], or [[polyurethane]]. A varnish formula might not contain any added resins at all since drying oils can produce a varnish effect by themselves.<ref>{{cite web| title=Make Your Own Oil/Varnish Blend| url=https://www.thewoodwhisperer.com/articles/make-your-own-oil-varnish-blend/ |date=8 May 2007}}</ref>

===Solvent===
Originally, turpentine or [[ethanol|alcohol]] was used to dissolve the resin and thin the drying oils. The invention of petroleum distillates has led to [[turpentine substitute]]s such as [[white spirit]], [[paint thinner]], and [[mineral spirit]]. Modern synthetic varnishes may be formulated with water instead of hydrocarbon solvents.