A BRIEF ESSAY ON HISTORIC FURNITURE VARNISHES AND RESINS
The word “varnish” is a broad term that is often used to describe any type of surface coating. Some period documents referred to varnishes as “lacquers” or “lakkers”, an expression that most likely resulted from attempts to imitate true Oriental lacquer with Western varnishes. Historically, there were three distinct types of historic varnishes: spirit, essential oil, and fixed-oil varnishes.
Spirit varnishes consist of various resins (see Varnish Resins below) dissolved in a solvent, usually alcohol. They dried fast and hard, usually retaining very little solvent in the varnish films. Other solvents were occasionally used such as naphtha but the resins that dissolved in naphtha, such as rosin, were considered inferior.
The films formed by these varnishes were not very durable, they did not stand up to heat and they formed brittle films. The most important uses for these varnishes were as a colorless or water-white varnish on articles where the amber color of other varnish types would be unwanted.
The resins usually encountered in spirit varnishes are copal, sandarac, dammar, shellac, mastic, Venice turpentine and elemi.
Essential Oil Varnishes
Essential oil varnishes were made from resins (see Varnish Resins below) dissolved in a solvent, usually turpentine. The varnish dried by solvent evaporation, though some solvent remained in the resins for long periods of time. They formed tougher, more durable and thicker films than spirit varnishes.
The resins usually found in essential oil varnishes are copal, dammar and Venice turpentine.
Fixed oil varnishes were made by dissolving a resin (see Varnish Resins below) in a drying oil, most often linseed oil. Often, turpentine was used as a diluent and other resins were often incorporated. The advantage of these varnish films was that they were the toughest, most durable and formed the thickest films of all varnish types. The disadvantage was that they dried very slowly. Unlike the first two types of varnishes, the fixed oil varnishes did not lose their solvents to evaporation but the oils remained as part of the film and polymerized. Drying oils do eventually dry but the process may take weeks or months. To speed up the drying process, driers such as lead and other heavy metals were added. The polymerization process could also be hastened by the addition of heat during thefirst steps of the varnish making process. Sometimes heat was necessary to “crack” or “run the varnish” the molecules of the resin so that they could go into suspension in the oil. The heat, in combination with the addition of heavy metal driers, helped to further accelerate the drying process. The resins usually found in fixed-oil varnishes are hard copals and animes but often small amounts of other resins were added as plasticizers.
Resins are often confused with gums and many historical documents and recipes used the terms interchangeably. Specifically, resins contain terpenes and are produced by coniferous plants. Resins are soluble in hydrocarbon solvents such as turpentine and sometimes alcohol. Gums derive from plant saps and most often contain polysaccharides (sugars). Gums, such as gum Arabic, are soluble in water.
Shellac: not a true resin as described above, it is an organic exudation from the Laccifer lacca insect found in Asia. It is soluble in alcohol and comes in numerous grades and colors ranging to a deep brown-red in the unrefined grades like seedlac and garnet to an almost colorless pale resin in the most highly refined grades like pale dewaxed shellacs. Shellacs were commonly used in spirit varnishes. This resin was available in Europe as well as the North American colonies from the early 1700’s but initially it seems to have been used as a sealer, often under paints or varnishes, rather than as a topcoat. Shellacs were used in French polishing a method of applying shellac that first appeared in the early 1800’s.
Copals: a resin from ancient, extinct Dipterocarpaceae trees. This semi-fossilized resin was historically dug from the ground. The resin ranges from soft copals, soluble in alcohol, to harder Zanzibar copals, which need heat to fully disperse them, to amber, a fully fossilized resin. Copals are found around the world. Used in both spirit and fixed-oil varnishes.
Kauri: a resin from the New Zealand Dammar Australis conifer. This resin needed to be “run” or melted to dissolve into oils and turpentines. It was often confused and used as a substitute for copals.
Venice turpentine: a resin from a conifer tree (Larix or larch), Venice turpentine is a soft resin with the consistency of thickened honey soluble in turpentine and naphtha. It should not be confused with the solvent turpentine, which is a liquid distillation of a pine tree resin. Used as a plasticizer in fixed-oil varnishes.
Anime: A name for a resin found in Central or South America, it is a copal resin.
Benzion or Gum Benjamin: not a true gum (gums are water-soluble) but a resin, benzoin is a resin from the Stryax trees of Southeast Asia. It is a soft resin and is soluble in turpentine and naphtha.
Mastic: a resin from the Pistacia tree growing in the Mediterranean region. Mastic is soluble in turpentine. Mastic has been used for medicinal as well as utilitarian purposes for several thousand years. Soluble in turpentine and other hydrocarbon solvents, mastic was first used as a picture varnish in the West in the early 1800’s. It is unclear how often it was used in furniture varnishes.
Sandarac: a resin from a cypress tree (Tetraclinus) growing in the Mediterranean region. Sandarac is soluble in alcohol and was most often used when pale or “water-white” varnishes were needed.
Elemi: a soft resin from the Canarium tree found in Southeast Asia. Elemi is soluble in turpentine and was often added to oil resin varnishes as a plasticizer. This resin is classified as a recent resin since it was not fossilized.
Dammar: a resin from the Dipterocarpaceae tree. This resin is classified as a recent resin since it sometimes was dug from the ground, but most often it was collected from incisions in living trees. It is soluble in turpentine and other hydrocarbon solvents. Dammar first appeared as a picture varnish in the early 1800’s and it is unclear how much it appeared in furniture varnishes.
Rosin: the resins left when pine tree sap and wood is distilled into turpentines. This resin dissolved in turpentine and rosin varnish was considered useful only as a cheap coating for low quality or utilitarian products. Rosin was often produced as a by-product of the naval stores (pitch, pine tars and turpentines) industry.