Varnish
[4] 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.
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.
All drying oils, certain alkyds (including paints), and many polyurethanes produce heat (an exothermic reaction) during the curing process.
Thus, oil-soaked rags and paper can smolder and ignite into flames, even several hours after use if proper precautions are not taken.
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.
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.
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.
Originally, the term "varnish" referred to finishes that were made entirely of resin dissolved in suitable solvents, either ethanol (alcohol) or turpentine.
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 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.
Violin varnishing is a multi-step process involving some or all of the following: primer, sealer, ground, color coats, and clear topcoat.
The refined resin is sometimes available as a translucent solid and is then "run" by cooking or 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.
Some violin finishing systems use vernice bianca (egg white and gum arabic) as a sealer or ground.
The resins include amber, dammar, copal, rosin, sandarac, elemi, benzoin, mastic, balsam, shellac, and a multitude of lacquers.
These were believed to impart special tonal qualities to musical instruments and thus were sometimes carefully guarded secrets.
The source of shellac resin is a brittle or flaky secretion of the female lac insect, Kerria 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.
Other natural color shades such as ruby and yellow are available from specialty pigment or woodworker's supply outlets.
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.
[12][13] 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.
Even after more modern synthetic resins did become resistant, a true spar varnish maintained its elasticity above other virtues, even if this required a compromise in its UV-resistance.
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.
Exterior use of polyurethane varnish may be problematic due to its heightened susceptibility to deterioration through ultra-violet light exposure.
All clear or translucent varnishes, and indeed all film-polymer coatings (e.g. paint, stain, epoxy, synthetic plastic, etc.)
Acrylics have the advantage of water clean-up and lack of solvent fumes, but typically do not penetrate into wood as well as oils.
Various epoxy resin systems have been formulated as varnishes or floor finishes whereby two components are mixed directly before application.
