Varnish is a transparent liquid used to protect wood, metal, and other materials from air and moisture, and to improve their appearance. A varnish leaves a hard, glossy film when it dries.
Clear varnishes protect the surface of wood while allowing the natural grain of the wood to show through. Varnish stains contain dyes that change the color of the wood but still bring out the grain. Varnishes used on metal are sometimes called lacquers. Lacquers help prevent corrosion without dulling the metallic appearance. Varnishes are also used to protect insulating wires, masonry, and paper from moisture.
Varnishes can be baked on objects in ovens at temperatures of 150 °F to 400 °F (66 °C to 200 °C). Such baking improves the wearing quality of the varnished object.
Types of varnish.
There are two main classes of varnishes, spirit and oleoresinous. Spirit varnishes are made of chemicals called resins. The resins are dissolved in a quickly evaporating solvent such as alcohol. These varnishes dry when the solvent evaporates. Shellac is a common spirit varnish. Other spirit varnishes include Japan, dammar, and pyroxylin lacquers.
Oleoresinous varnishes are mixtures of resins and drying oils that are heated and dissolved in turpentine or petroleum products. These varnishes dry in two ways, by evaporation of the solvent, and by the hardening of the resin-oil mixture when it combines with oxygen. Oleoresinous varnishes withstand outdoor conditions well. Spar varnish, used on the wood exterior of boats, is an oleoresinous varnish.
Making varnish.
Both natural and synthetic resins are used in varnish. Natural resins come from living plants and fossil plants. Shellac, dammar, and rosin are common natural resins. Synthetic resins include such chemical compounds as phenol-formaldehyde, urea-formaldehyde, alkyd (glyceryl phthalate), and cumar.
In making spirit varnishes, the resin is dissolved by churning it with the solvent. Small amounts of heat are sometimes used to speed the dissolving process. After the resin is dissolved, the varnish is refined by filtering and is then ready for use.
In making oleoresinous varnishes, the oil and resin are cooked in closed kettles that hold 5,000 gallons (19,000 liters) or more. The mixture is kept at a temperature of 450 °F to 700 °F (230 °C to 370 °C) until it reaches the desired body (thickness). Then the mixture is cooled and thinners are added. Some natural resins will not dissolve easily in oils. These resins are heated to temperatures of 482 °F to 662 °F (250 °C to 350 °C) to break them down into smaller, more soluble molecules. This process is called pyrolysis.
Drying oils are added to the varnish to hasten the drying time. Linseed oil was probably the first oil to be used in varnishes. Many other drying oils such as perilla, tung, dehydrated castor, soybean, and fish oils are used today. Compounds of lead, cobalt, or manganese are often added to quicken drying.
Varnishes are named by giving the number of gallons of oil that have been mixed with 100 pounds of resin. For example, a 30-gallon tung-oil kauri varnish is made with 30 gallons of tung oil and 100 pounds of kauri resin. Varnishes used on outdoor surfaces contain more oil than those used indoors.
