Guitar Finishes
From the beginning, wood was the natural choice for most tools and useful objects. It was plentiful, easy to work and available in all useful sizes. It can be thin and sharp, soft and flexible, heavy and dense, just about anything that’s needed. It made good wheels, boats, weapons, shields, siding, roofing and furniture. It could be permanent or mobile.
If wood is worked into a useful shape by hand and then used regularly; as the surface is worn smooth, it feels better – smoother, softer and ever more familiar to the touch. The oils from our hands (and from the plants and foods that come in contact with our hands) slowly are worked into the natural pores of the wood fibers. The oils and the smooth wood surface combine to a soft luster. The more we use it, the better it feels. As natural oils are absorbed into the surface, the wood is prevented from drying out too much. The oils also help to seal the wood fibers from absorbing water – the wood object becomes more stable and better resists cracking and checking from dryness, swelling from moisture, or degrading from freezing or heat.
This has been true since man fashioned his first tools.
Lacquer
At some point, we found other ways to protect wood and to bring out its natural beauty. The exact time for the beginning of man-made wood finishes is not known, but there are beautiful examples of lacquer finishes from China in the 4th century BC. Some archeological digs in China suggest the use of lacquer as long as 8,000 years ago.
The idea was to create a protective surface between the elements and the wood. The simple concept behind lacquer finishes is the even application of a resin suspended in a liquid that evaporates away, leaving the hardened resin bonded to the wooden surface.
The varnish tree (toxicodendron vericifluum), indigenous to China, yielded a resin known as urushiol: a mixture of various phenols and proteins suspended in water. Urushiol is relatively slow drying. As the water evaporates, the resin sets-up by oxidation and polymerization: creating a fairly tough surface with a glossy sheen. It only cures well in a warm, humid environment as several things are happening at once: First, the water evaporates slowly allowing the phenols to oxidize and polymerize as a result of an interaction of an enzyme: laccase. The end result is a hardened surface.
The transparent nature of this process made it perfectly suited to applications over wood. The grain and figure of the wood is highlighted and enhanced. The gloss finish is durable and resistant to damage from water, acid or abrasion. The process required a great deal of skill and knowledge for the formulation of the finishes and their proper application. It was not without hazard: the fresh tree resin from the T. Vericifluum causes an allergic reaction in people: urushiol-induced contact dermatitis and requires great care in handling. The Chinese apparently treated this with shell fish.
As with many such skills, they are closely guarded and the secrets carefully protected. If developed in the 4th or 5th century BC in China, the process was kept secrete for hundreds of years.
In Korea and Japan, at about the same time, examples of similar finish processes appear. There is no agreement between scholars (that I can find) as to whom is responsible for the original discovery or whether the processes were stolen from one source or another. The secrets are well kept and we may never know who was the first or exactly when and where it originated.
Once lacquer was developed, various powders or dyes could be added for translucent or opaque color. Early examples used iron oxides for red or black coloration. Powdered cinnabar was used to create the traditional red lacquerware from China. Some Chinese wooden plate-ware fashioned in 900AD appears in rich red color today. Trade in lacquered objects throughout the Middle East included coffins, plates, furniture and musical instruments.
Following the success of the use of coloring additives, other options were tried. Some for decoration like gold and silver powder or flakes; and others for their practical qualities. To create the finish for the Chinese musical instrument: the Guqin, lacquer was mixed with deer horn powder or a ceramic powder to increase its surface strength to better withstand fingering.
The Evolution of the Lacquer Process:
Japanning: Usually used in connection with Japanese Lacquerware. The resin is sap from the Lacquer tree (indigenous to Japan). As the products with this finish found their way to the West in the 1700s; Europeans developed several processes to imitate the smooth lustrous result. The process became known as ‘Japanning’ and consisted of several coats of varnish, each of which was heat dried and polished.
Nitrocellulose Lacquers
In the 1920s with the rise of the manufacturing production line, there was a strong need for a quick-drying finish system that would protect all sorts of items including metals and all manner of wood used for mass production. Oil drying varnishes simply could not be applied fast enough and took too long to dry. The solvent based lacquers were fast drying but not always suitable for exterior exposure. DuPont discovered that a solvent based lacquer containing nitrocellulose provided the needed finish durability with the ability for rapid spray application and very short drying time. Nitrocellulose is a resin derived from the nitration of cotton (or other cellulosic materials). The very fastest drying time could be obtained with Japan Black. As a result, Henry Ford (referring to available colors for the Model T) is believed to have said: “You can have any color you like, as long as it is black”.
The down-side of nitrocellulose lacquer is two-fold: The solvents are highly flammable and very toxic. To make matters worse, lacquer grade nitrocellulose is closely related to the slightly higher grade that is used for explosives. Fortunately, after about a month of curing, the ingredients are no longer toxic.
Acrylic Lacquers
In the period immediately following World War II, tremendous advances were made in the field of plastics. One result during the 1950s was the development of acrylic resins: synthetic polymers that were colorless and transparent thermoplastics derived from acrylic acid. These new resins had the distinct advantage of not requiring buffing to obtain a shine. Used in lacquer, these were very quick-drying and became the finish of choice for the auto industry – until the development of polyurethane.
Polyurethane Lacquers
Otto Bayer and his team developed the chemistry at the I.G Farben Laboratories in 1937 in Germany: this was particularly important because he needed a clear path around the patents owned by Wallace Carothers on polyesters. The polymers in polyurethane are formed by a step-growth polymerization by the reaction of a monomer (containing at least 2 isocyanate functional groups) and another monomer (containing at least 2 alcohol groups). The result is a wide variety of useful and durable product from soft foams to hard bushings – depending on the formulation.
Polyurethane finishes are a two-part system for a rapid chemical cure. These finishes proved to be tougher and more weather and chemical resistant that the acrylic lacquers. The down-side includes toxicity in the chemical formation of the monomers and the toxic off-gassing in the final process, as well as no easy way to repair damaged finishes.
Polyester Finishes
Developed and patented by Wallace Carothers in the 1930s, polyesters are thermoplastic or thermoset. Their use as a wood finish has grown due to their thixotropic property when spray applied to bridge and fill open grain and a high-build film thickness. Once cured, they can be polished to a very high-gloss and durable finish.
These qualities make polyester finishes a natural choice for mass-production, low-cost musical instruments. The high-build film thickness works against the theory of thin finishes to maximize the soundboard’s ability to transmit vibrations. The long-term flexibility, so desirable for furniture products, also works against the needs of the soundboard to vibrate with the damping effects of a thick finish.
As a chemical cure product, there is no easy way to repair this type of finish.
Catalyzed Lacquer
Catalyzed lacquer falls between the application qualities of nitrocellulose lacquer and the durability of varnish. Catalyzed lacquer is a complex finish composed of urea formaldehyde or urea melamine and an alkyd that has some nitrocellulose resin added (to make it handle like normal lacquer). The addition of an acid catalyst starts a chemical reaction that forms a durable finish.
Catalyzed lacquer comes in 2 versions:
- Pre-catalyzed lacquer – the components are premixed, either by the manufacturer or at the store where you buy it.
- Post-catalyzed lacquer is a two-part system that you must mix, following precise ratios. Once the catalyst has been added, these lacquers have a fairly short shelf/pot life.
Water Based Lacquer
Water-based finishes contain some of the same ingredients as varnish and lacquer – notably polyurethane (alkyd and acrylic) – but water is used to replace many of the flammable and polluting ingredients. Because the resins don’t normally dissolve in water, they must be chemically modified or forced to work with water.
Water-based lacquer is usually made with either an acrylic resin or an acrylic polyurethane mixture. As with varnish, the addition of the polyurethane makes the resin tougher and more scratch resistant, but without the same solvent and heat resistance as its oil-based lacquers.
Shellac
In India and Thailand, an insect that is about the size of an apple seed: Laccifera Lacca, secretes an amber colored resin-like substance called: ‘Lac’. The insect lac was believed to have been introduced to India from Persia (Iran). The word ‘Lac’ is derived from the Sanskrit ‘lakha’. During their reproductive cycle, the insect feeds on sap, leaves and twigs of certain indigenous trees. They then secrete a resinous substance to form the cocoon around themselves to incubate and protect their eggs. The cocoon, containing the bug remains and a section of the twig it was formed on, is called ‘Sticklac’. The sticklac is then refined to removing the bug remains and the twig. The original process was to refine the dye that gave the cocoon its amber color. The natural dye industry was very large throughout the Middle East and Asia for all of early recorded history. Use of the lac dye is mentioned in a Roman volume on Natural History written by Claudius Aelianus in 250 AD. Natural dyes remained valuable commodities until the mid 1800s when the invention of aniline dyes by an English Chemist named Perkins brought the natural dye industry to an end.
The greasy substance left over after the refinement and separation of the lac dye was discovered to have similar properties to urushiol. An English writer visiting India in 1590 observed the use of shellac as a protective coating for woodwork. This was his observation (modified to read more easily):
“They take a piece of Lac of what color they will, and as they turn it, as it takes its shape, they spread the Lac upon the whole piece of wood which presently, with the heat of the turning melts the wax so that it enters into the crests and cleaves unto it, about the thickness of a man’s nail: then they burnish it with a broad straw or dry rushes so cunningly that all the wood is covered, and it shines like glass, most pleasant to behold, and continues as long as the wood is well cared for: in this manner they cover all kinds of household stuff in India”. ( London, Sir I. Pitman & Sons, LTD)
Shellac was not widely accepted as a furniture finish in the West until the early 1800s. Wax and oil finishes had been the finish of choice throughout the West until that time. The benefits, and the gloss finish, became the norm in the early 1800s and remained popular until the development of nitrocellulose lacquer in the 1920s and 30s.
One rather unique characteristic of shellac is that it is entirely non-toxic. Even today, it has the following uses:
- Pharmaceutical – as a pill coating so it doesn’t dissolve until, entering the lower intestine.
- Food coating – it is used to coat apples so they look shinier.
- Confections – protective candy coatings and glazes – Reese’s Pieces for example.
- Hats – to stiffen felt.
- Electrical – mixed with marble dust to glue incandescent bulb metal bases to the glass.
For woodworking, shellac is sold in flake form and then dissolved in ethanol. There are many grades and colors of shellac flakes from the highly refined Kusmi and Golden Bysacki, to the standard grade TN (Truly Native) – hand processed in India from sticklac. The reason shellac is sold in flake form is that the flakes will last a very long time without breaking down but once mixed with ethanol has a limited shelf life. Once dissolved in ethanol, the shellac begins a chemical transformation known as esterification that eventually will render the mix as a sticky gum that will not dry. For this reason, it should be used shortly after mixing. Shellac flakes will dissolve in denatured alcohol, methanol, butyl or propyl alcohol. Each has a different rate of evaporation and therefore affects the mix ability to spread evenly under brush applications. Methanol is highly toxic and therefore, not the best choice. It was widely used in older applications and sometimes referred to as wood alcohol or methylated spirits.
Some of shellacs greatest advantages include qualities that apply to musical instruments:
- Non yellowing
- Excellent adhesion
- Very hard – it doesn’t dampen the vibration of a soundboard
- Good sealer – stops the transfer of moisture vapor
- Easy to repair – Since shellac dissolves in alcohol, new shellac melts into old, allowing seamless repairs
- Easy to remove – Shellac dissolves in alcohol
- Shellac can be applied by padding, brush or spray
- Non-Toxic – FDA approved for food utensils and children’s toys
Varnish
The word ‘Varnish’ derives from the Latin: “Vernix”: meaning odorous resin. The Latin ‘vernix’, however, comes from the ancient Greek: “Berenice” which was the original name for the area that is now known as ‘Benghazi’ in Libya. It is believed that the first varnish finishes were developed from the trees of the forests that once covered that region.
The earliest varnishes were formulated from tree sap (pine sap) and a solvent applied by brush for a golden, glossy finish. As the term ‘Varnish’ applies to a broad variety of finishes, we will try to stick to traditional definitions.
Traditionally, varnish is a combination of a resin, a drying oil and a solvent (or thinner). In the simplest formulations, varnish dries by the rapid evaporation of the solvent, allowing the resin to harden. In more complex formulations, as the solvent evaporates, the drying oil and resin can undergo a chemical change and cure to a hardened state. Chemical cures between the oils and oxygen are called autooxidation.
- Resin varnish – dries by evaporation of the solvents, though there is still a cure rate between the oils and resins
- Acrylic and waterborne varnishes – have extended cure periods after the water has evaporated
- Oil, polyurethane and epoxy varnishes – have extended cures after the solvent has evaporated and as the components chemically interact.
The drying time of different varnishes can be sped up by exposure to an energy source such as ultraviolet light or direct sunlight. Adding heat can also speed up the rate of cure to some varnishes.
Organic solvents, organic oils and certain resins (all used as binders) can be highly flammable in their liquid state. All drying oils, some single component polyurethanes and some alkyds produce heat while curing. This explains why oil soaked rags and paper can ignite, long after application, if piled and enclosed where the heat cannot easily dissipate.
The Traditional Components:
- Resin: Mostly the sap of trees, including: kauri gum, amber, rosin, copal, sandarac, balsam, elemi, etc.
- Drying Oils: Linseed oil, walnut oil, tung oil, etc. All versions contain large amounts of polyunsaturated fatty acids.
- Solvent (Turpentine or thinner): Organic turpentine, mineral spirits, white spirits, etc.
More Modern Components:
- Resin: Synthetic resins like phenolic.
- Drying Oils: benzoin, mastic, alkyds, etc.
- Solvents: alcohol or petroleum based solvents, etc.
The first clear varnish was developed by the Valspar Corporation in 1906 and was advertised as “The varnish that won’t turn white”.
Drying oils like linseed oil and tung oil, though not true varnishes, will cure by the exothermic reaction between oxygen (in the air) and the polyunsaturated fatty acids in the oils. These cure much more slowly than the simple process of evaporation of a solvent. They add considerable advantages to the mix: flexibility, long life, better bond, moisture resistance, acid resistance, UV resistance, stability, etc. The formulation needs to take all these into consideration.
Alkyds are chemically modified vegetable oils. These are engineered to speed up the cure rate and function well in a wide variety of harsh outdoor conditions. They may also include ultra violet (UV) absorbers. This can extend the life of the finish and help keep it glossy longer.
Other Types of Varnishes:
- Spar Varnishes: also known as marine varnishes, were developed to be elastic and water resistant. The concept was that sailboat spars bend a great deal under the load of their sails. The finish must not become brittle and crack while bending or it would allow moisture through. Flexibility and weather resistance is everything – gloss retention and long life are secondary. This is the perfect example of a product engineered for a very specific purpose. This would not be a good choice for musical instruments as the lack of hardness would tend to absorb vibration and therefore dampen sound.
- Polyurethane Varnishes: These are formulated from various polymers that are intended to chemically cure after the original water has evaporated. They cure by reaction with moisture in the air. These finishes generally have a thick film build that is well suited for floor applications and other high-abrasion resistance situations. They are generally not favored for furniture or musical instruments.
- Acrylic Varnishes: Normally a water-borne varnish with good UV resistance and a high degree of clarity: clear finish. Their primary drawbacks are that they are difficult to repair and do not soak into the wood and therefore do not bond as well as oil varnishes.
- Two-Part Varnishes: These are epoxy varnishes that rely on the chemical cure between two agents that create a hard finish. True polyurethanes are two-part systems. These may have limited bond as they do not soak into the wood and are hard to repair as they cannot melt into additional finish due to the fact that they have been permanently cured by chemical reaction.
- Conversion Varnishes: These are generally used for furniture and cabinet work to achieve a rapid cure and very hard finish that is clear and does not yellow easily. It is a two-part system: a resin (blend of amino resin and an alkyd) and an acid catalyst. As the mix cures, it gives off formaldehyde (toxic and a known carcinogen), and is very difficult to repair.
Tung Oil
Also known as ‘Chinawood Oil’, Tung Oil is created from the refined oil of the dried nuts of the Chinese Tung tree.
The word: ‘Tung’ is an ancient Chinese reference to the heart. It also is the name for the heart shaped leaves of the Tung tree. Tung fruit grows in clusters of 4 or 5 nuts. The trees have very particular climatic requirements but can be found in China, Paraguay, Argentina and parts of Africa.
Written records, including references from Confucius, around 400 BC highlight the qualities of Tung Oil as a protective finish for a variety of materials including wood and cloth. It was used to seal water out of masts and sails, as a furniture finish and even to seal the masonry of the Great Wall.
In 1912, the American Ambassador to China shipped Tung trees to the States but the growing conditions did not support them. Eventually (in the 1930s) successful plantations were established on the Gulf Coast and became a substantial source of tung oil until 1969 when hurricane Camille destroyed the plantations and ended domestic production.
As a wood finish, tung oil is a penetrating oil – not well suited to musical instruments as it dampens vibration. The same qualities make it an excellent finish for furniture and architectural woodwork as it preserves the wood’s elastic qualities and is easily maintained and repaired.
Stradivarius – The State of the Art
By the time Antonio Stradivari (1644 – 12/18/1737) crafted his violins, cellos, guitars, violas, harps and basses, the process of wood finishing had evolved to an art. Each luthier of the day had their particular formula for finishing instruments – Each struggled to have the richest color, luster and depth or visual feel. The process of finishing musical instruments was primarily a practical matter: raw wood that is constantly handled becomes caked with dirt and rosin from the horse hair bows. Finishes were essential to protect the wood and allow some degree of cleaning.
The Guild system of passing on the knowledge and practices of the Master to the Apprentice was the order of the day. Stradivari learned his craft as a student to the well known Master Luthier: Nicolo Amati (1505-1578). Common instrument finishing techniques were well understood but allowed personal variation for reasons of aesthetics and quality. Thick finishes were understood to limit the vibration of the soundboards and therefore were not desirable.
But there is a mythology to the 1,101 instruments that Antonio Stradivari created (approximately 650 survived). As the instruments built between 1698 and 1725 are viewed as some of the finest instruments in history for their magical tone, endless research is performed to try to determine how the Luthier achieved his Art. It is known that Stradivari began to modify the designs of the Amati models of his predecessor. The shape and arching was altered, the thickness and recurve of the top, back and sides was more carefully measured and changed and the varnishes were more strongly colored. According to the mythology: Stradivari developed a formula for varnish that so enhanced the tone of his instruments that the secret must be kept from his contemporaries. The story holds that the documented process died with his sons, who burned the records to assure their father’s place in history as the finest builder of all time.
To test the theory, electron microscopy was used at Cambridge University to determine the make-up of the varnish as well as the wood preservatives and the sealers used prior to the final finishing. They concluded that the components were commonly available from the pharmacist shop that existed next door to Stradivari’s studio and were consistent with the common practices of the time.
Joseph Nagyvary, a biochemist at Texas A&M University and part-time violin builder, believes that the special treatment of the spruce tops in combination with the varnish holds the secret to the magic tone of a Stradivari. His research focused on the methods of treating woods for furniture and instruments of the day to withstand rot and insect damage. The process, according to Nagyvary, involved soaking in various mineral solutions. By this theory, the dried, mineral rich tops made ideal stiff and vibrant soundboards.
Nagyvary then researched the varnish formulation. He theorized that oil varnishes penetrate deeply into the wood and the oils remain gummy, dampening the vibrating potential of the soundboard. The Stradivari finishes appear thin, brittle and glassy. He theorized that the Stradivari finishes contained sugars or polysaccharides – creating a stiffer, more brittle matrix that would bond well to the spruce. It was not unusual for documented varnish formulations of the time to have powdered glass, amber or porcelain as additives for stiffness and luster. His theory holds that pectins make good polymers – the component that makes jelly gel.
Who knows? Maybe he is right.
French Polish
French polishing is a process – as opposed to a material. In the world of musical instruments it is often referred to a a finish.
The technique involves applying many coats of shellac dissolved in alcohol in very thin layers. The process consists of using a pad made up of absorbent wadding inside a smooth natural fiber cloth. The motions for application are quick and repetitive. The process is labor-intensive and requires time and patience.
French polishing came into fashion in the 1700s and remained in-use on fine instruments and furniture up through the 1930s when other options were available that required much less labor.
The beauty of French Polishing for instruments is:
- A very thin finish allowing maximum response from all tone-woods.
- Complete control of the finish process
- Easy repair to damaged areas
- High gloss and deep luster for the woods
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