While reactive oils (linseed oil and tung oil) use measures in millennia, varnishes are the "new" kids on the block. Varnishes have only been around for several hundred years.
Varnishes are a combination of resin, drying oil, and solvent. Historically, varnish components were made of completely of natural components: The resen was pine sap, the solvent was turpentine (which is distilled pine sap), and the drying oil was very often linseed oil. Of course, other componets were also used. Ultimately, modern varnishes have little in common with the varnishes of yesteryear, although some are still in use. For more information, see the resources at the bottom of the page.
Varnishes come in several variants. Some are beyond the scope of this document, as I don't use them. Bear in mide that books have been written on this stuff, and this barely scratches the surface. Again, for more information on those, see the resources below.
Spirit Varnishes - Spirit varnishes are two component varnishes. One is a solvent (typically alcohol or naphtha) and the other is a resin. Resins like copal, rosin, and dammar have been used, but the most common, by far, is shellac. Although listed as a varnish in days of yore, shellac has actually moved into an "independent" class by itself. With such volatile solvents that flashed off so quickly, the film finish tended to be very thin, and considered inferior.
Essential Oil Varnishes - Essential oil varnishes are not unlike spirit varnishes, with the exception of the type of solvent. An example would be turpentine. Slower evaporative solvents allowed for thicker films that were generally considered better. The down side was that solvents would become trapped and linger for long, stinky periods... think about Grandma's paste wax that smelled like "old people" furniture.
Fixed Oil Varnishes - Although they've been around for hundreds of years, these are the modern day varnishes. For all practical purposes, they are essential oil varnishes with the addition of a reactive oil. A solvent (traditionally turpentine, or mineral spirts more recently), a reactive oil (linseed, tung, walnut, etc.,), and resin combine to make fixed oil varnishes. As the solvent evaporates, the reactive oil will polymerize to form a superior film finish.
A Couple of Specialties
Within the fixed oil varnishes, there are a couple of specialty types that deserve a mention.
Spar Varnish - Oceans are absolutely the most inhospitable place on Earth for wooden boats made to float on them. Wind, sunshine, rain, salt water spray, temperature extremes, and wood movement all add up to a harsh environment that wood simply doesn't do well in. Enter spar varnish. Spar varnish was developed to better protect and extend the service life, and reduce the maintenance requirements, of ship's spars. Taking advantage of the superior film properties of fixed oil varnishes, plasticizers were added to make the film more flexible to deal with the movement, and UV inhibitors were added to deal with the constant beating from the sun. Spar varnish is great for outdoor pieces, and the UV inhibitors not withstanding, it's great for indoor pieces (like dining tables and bathroom cabinets) expected to have a "hard" life.
Wiping Varnish - Wiping varnishes are simply fixed oil varnishes that have been thinned to a viscosity like, or only slightly thicker than, water. It makes them very easy to apply with lint free rags. It also makes it much easier to control the amount of varnish going on with each additional coat.
A Final Thought...
Varnish "has it all." Although I rarely use it as an everything to every situation, it can be. Varnish can penetrate, build as a film, protect, and enhance the look of wood. Alone, or in combination with other parts of a process, varnish is a very good finish.
A Safety Note - When curing, reactive oils produce heat. This exothermic reaction can cause discarded rags and materials to spontaneously combust. Spread out rags to let them dry outside. It is rumored that King Tut was wrapped hard and put away wet... 3500 odd years later, they discovered his charred remains.
A Brief Essay On Historic Furniture Varnishes and Resins. 2017.
Williamsburg Art Conservation Inc..
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