Varnish is a transparent, hard, protective finish or film used primarily in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss sheens by the addition of "flatting" agents. Varnish has little or no color, is transparent, and has no added pigment, as opposed to paints or wood stains, which contain pigment and generally range from opaque to translucent. Varnishes are also applied over wood stains as a final step to achieve a film for gloss and protection. Some products are marketed as a combined stain and varnish.
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 certain 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 almost immediately upon drying. Acrylic and waterborne varnishes "dry" upon evaporation of the water but experience an extended curing period. 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 gummy, to tacky, to "dry to the touch," to hard. Environmental factors such as heat and humidity play a very 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 or heat. Other than acrylic and waterborne types, all varnishes are highly flammable in their liquid state due to the presence of flammable solvents and oils.
Components of Classic Varnish
There are many types of drying oils, including linseed oil, tung oil, and walnut oil. These contain high levels of polyunsaturated fatty acids.
Resins that are used in varnishes include amber, dammar, copal, rosin (pine resin), sandarac, balsam, and others. The word varnish probably derives ultimately via ancient Greek from a placename of a city in ancient Libya where resins from the trees of once existing forests were sold. In modern English the ancient city is referred to as Berenice; otherwise the existing city in modern Libya is called Benghazi.
Turpentine or solvent
Traditionally, natural (organic) turpentine was used as the thinner or solvent, but has been replaced by several mineral based turpentine substitutes such as white spirit or "paint thinner."
For violin varnish, walnut oil and linseed oil are most often used in combination with amber, copal, rosin or other resins. The oil is prepared by cooking or exposing to air and sunlight. The refined resin is typically available as a translucent solid and is then "run" by cooking or literally melting it in a pot over heat without solvents. The thickened oil and prepared resin are then cooked together and thinned with turpentine (away from open flame) into a brushable solution.
Most resin or "gum" varnishes consist of a natural, plant- or insect-derived substance dissolved in a solvent. The two main types of natural varnishes are spirit varnish (which uses alcohol as a solvent) and turpentine or petroleum-based varnish. Some resins are soluble in both alcohol and turpentine. Generally, petroleum solvents, i.e. mineral spirits or paint thinner, can substitute for turpentine. The resins include amber, dammar, copal, rosin (pine resin), sandarac, balsam, shellac, and a multitude of lacquers. Synthetic resins such as phenolic resin are typically employed as a secondary component in certain varnishes and paints. Over centuries, many recipes were developed which involved the combination of resins, oils, and other ingredients such as certain waxes. These were believed to impart special tonal qualities to musical instruments and thus were sometimes carefully guarded secrets. The interaction of different ingredients is difficult to predict or reproduce, so expert finishers were often prized professionals.
Shellac is a very widely used single component resin varnish that is alcohol soluble. It is not used outdoors or where it will come into repeated contact with water such as around a sink or bathtub. The source of shellac resin is a brittle or flaky secretion of the female lac insect, Coccus lacca, found in the forests of Assam and Thailand and harvested from the bark of the trees where she deposits it to provide a sticky hold on the trunk. Shellac is the basis of French polish, a difficult technique that produces an inimitable sheen, and which for centuries has been the preferred finish for fine furniture. Specified "dewaxed" shellac has been processed to remove the waxy substances from original shellac and can be used as a primer and sanding-sealer substrate for other finishes such as polyurethanes, alkyds, oils, and acrylics.
Shellac varnish is typically available in "clear" and "amber" (or "orange") varieties. Other natural color shades such as ruby and yellow are available from specialty pigment or woodworker's supply outlets. "White pigmented" shellac primer paint is widely available in retail outlets, billed as a fast-drying interior primer "problem solver," in that it adheres to a variety of surfaces and seals off odors and smoke stains. Shellac clean-up may be done with alcohol or ammonia cleansers.
Typically, modern commercially produced varnishes employ some form of alkyd for producing a protective film. Alkyds are chemically modified vegetable oils which operate well in a wide range of conditions and can be engineered to speed up the cure rate and thus harden faster. Better (and more expensive) exterior varnishes employ alkyds made from high performance oils and contain UV-absorbers; this improves gloss-retention and extends the lifetime of the finish. Various resins may also be combined with alkyds as part of the formula for typical "oil" varnishes that are commercially available.
Spar varnish (also called marine varnish) is high quality, waterproof, and sunlight-resistant varnish named for its use on ship or boat spars and other woodwork where a glossy finish is desired. Alkyd-modified tung oil and phenolic resins are often used. Better grades of spar varnish have substantially higher amounts of uv-absorbers added.
By definition, drying oils, such as linseed and tung oil, are not true varnishes though often in modern terms they accomplish the same thing. 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 finishers 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 siccatives or dryers (chemical catalysts) have cure times of less than 24 hours. However, certain non-toxic byproducts 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 favorable features of both substances.
Polyurethane varnishes are typically hard, abrasion-resistant, and durable coatings. They are popular for hardwood floors but are considered by some to be difficult or unsuitable for finishing furniture or other detailed pieces. Polyurethanes are comparable in hardness to certain alkyds but generally form a tougher film. Compared to simple oil or shellac varnishes, polyurethane varnish forms a harder, decidedly tougher and more waterproof film. However, a thick film of ordinary polyurethane may de-laminate if subjected to heat or shock, fracturing the film and leaving white patches. This tendency increases with long exposure to sunlight or when it is applied over soft woods like pine. This is also in part due to polyurethane's lesser penetration into the wood. Various priming techniques are employed to overcome this problem, including the use of certain oil varnishes, specified "dewaxed" shellac, clear penetrating epoxy sealer, or "oil-modified" polyurethane designed for the purpose. Polyurethane varnish may also lack the "hand-rubbed" lustre of drying oils such as linseed or tung oil; in contrast, however, it is capable of a much faster and higher "build" of film, accomplishing in two coats what may require multiple applications of oil. Polyurethane may also be applied over a straight oil finish, but because of the relatively slow curing time of oils and the emission of certain chemical byproducts, care must be taken that the oils are sufficiently cured to accept the polyurethane.
Unlike drying oils and alkyds which cure, after evaporation of the solvent, upon reaction with oxygen from the air, polyurethane coatings cure after evaporation of the solvent by a variety of reactions of chemicals within the original mix, or by reaction with moisture from the air. Certain polyurethane products are "hybrids" and combine different aspects of their parent components. "Oil-modified" polyurethanes, whether water-borne or solvent-borne, are currently the most widely used wood floor finishes.
Exterior use of polyurethane varnish may be problematic due to its heightened susceptibility to deterioration through ultra-violet light exposure. It must be noted, however, that all clear or translucent varnishes, and indeed all film-polymer coatings (e.g. paint, stain, epoxy, synthetic plastic, etc.) are susceptible to this damage in varying degrees. Pigments in paints and stains protect against UV damage. UV-absorbers are added to polyurethane and other varnishes (e.g. spar varnish) to work against UV damage but are decreasingly effective over the course of one to four years, depending on the quantity and quality of UV-absorbers added as well as the severity and duration of sun exposure. Water exposure, humidity, temperature extremes, and other environmental factors affect all finishes. By contrast, wooden items retrieved from the Egyptian pyramids have a remarkably new and fresh appearance after 4000 years of storage. Even there, however, fungal colonies were present, and mildew and fungus are another category of entities which attack varnish. In other words, the only coat of varnish with near perfect durability is the one stored in a vacuum, in darkness, at a low and unvarying temperature. Otherwise, care and upkeep are required.
Many modern polyurethanes have been formulated to overcome a variety of problems that plagued earlier polys.
The word lacquer refers to quick-drying, solvent-based varnishes or paints. Although their names may be similarly derived, Lacquer is not the same as Shellac and is not dissolved in alcohol. Lacquer is dissolved in Lacquer Thinner, which is a highly-flammable solvent. Lacquer is typically sprayed on, within a spray booth that evacuates overspray and minimizes the risk of combustion.
Acrylic varnishes are typically water-borne varnishes with a very low refractive index or high degree of clarity, most often used in fine arts as a fixative.
Various epoxies have been formulated as varnishes or floor finishes whereby two components are mixed directly before application. All two-part epoxies have a "pot-life" or "working time" during which the epoxy can be used. Usually the pot-life is a matter of a few hours but is also highly temperature dependent. Both water-borne and solvent based epoxies are used.
- Wood finishing
ReferencesISBN links support NWE through referral fees
- Flexner, Bob. 1993. Understanding Wood Finishing: How to Select and Apply the Right Finish. Emmaus, PA: Rodale Press: Emmaus. ISBN 0-87596-566-0
- Flick, Ernest W. 1999. Printing Ink and Overprint Varnish Formulations (Paint & Coatings). Berkshire, UK: Noyes Publications. ISBN 0815514409
- Feller, Robert. 1985. On Picture Varnishes and Their Solvent. Washington DC: National Gallery of Art. ISBN 0318186993
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