What comes after "oil"?
This is the third article in our Beginning Finisher Series. In the last issue we looked at oil finishes. Our primary focus was on the “drying oils”, specifically Tung Oil and Boiled Linseed Oil (BLO). We examined the difference between drying oils, non-drying oils (such as olive oil), and semi-drying oils such as soya (soybean) and walnut oil. Finally, we looked at Mineral Oil, a non-drying oil refined from petroleum. In the process we made the point that oil finishes are not (marketing hype surrounding Tung oil notwithstanding) very “durable”; they have virtually no resistance to heat, moisture, abrasion and household chemicals. If you want greater “sheen” and better resistance to heat, abrasion, household chemicals, solvents, and water (both liquid and vapor) then varnish or an oil/varnish blend will be a much better choice. An oil/varnish blend (a.k.a. “Danish Oil”, “Antique Oil” and similar products) is simply a mixture of more or less equal parts varnish, boiled linseed oil, and mineral spirits—again, make your own and save. While not as durable as varnish alone, oil/varnish blends are significantly more resistant to the above hazards than are oil finishes.
In this issue we will turn out attention to varnish. What is varnish, and if varnish isn’t oil (as so many seem to believe) then why do we talk of linseed, Tung, and even soya oil in our descriptions of varnish? I’m glad you asked! Answer…you can’t make varnish without oil—oil is one of the two primary ingredients used to make all varnishes. But, because varnish is made from oil and oil is necessary in the formulation of varnish, does not mean that varnish is oil. Let’s consider a simple analogy. Bread is made from several ingredients; flour, salt, sugar, eggs, milk, yeast, etc. Each of these ingredients is easy to identify before all are combined, put into an oven, and baked. However, when the loaf of bread is removed from the oven it is much different than it was when it went in—it has been changed; modified if you will, by the cooking process. We can no longer identify the various individual ingredients used by the baker. Each has combined with the others forming a new product that we call bread. Bread did not exist before the ingredients were combined and baked. Bread is the result of the baking process—the individual ingredients, while contributing to the final product, are gone; they were modified to produce a new compound called bread. Even though the volume of flower is the most important contributor to a loaf of bread, we do not go to the grocery store to buy a loaf of flower. It should also be pointed out that if one simply combines the ingredients but does not put them in the oven to bake, and thereby be modified, bread will not be produced. All we will have in that case is a rather useless lump of combined ingredients called dough.
Varnish was discovered in the middle ages when the German monk, Theophilus Presbyter discovered that linseed oil, heated in a closed container, would combine with fossil resin (amber) to produce a polymer (though Presbyter did know what a polymer was) that we call varnish. Presbyter’s varnish, precisely because it was a new compound (a polymer formed by the cross-linking of oil and resin molecules) was far more resistant to moisture, heat, solvents, and abrasion than was the linseed oil from which it was made. Think about this last sentence for a moment. If varnish really is oil—just oil with a bit of resin mixed in—then it would be no more durable than the oil from which it is made. The resin would simply be floating about in the oil. The oil would not be modified by the resin. Returning to our bread analogy, just as combining flower, milk and other ingredients does not produce bread unless the ingredients are baked, neither does simply mixing oil and resin create varnish unless the componants are modified by the addition of heat. Bread is produced only when the ingredients are modified at the molecular level by baking. In the same way, varnish is not made by simply adding resin to oil. To make varnish, the oil and the resin must be modified; they must be cooked so that the molecules of each combine (polymerize) to form the new molecular structure that we call varnish. It is this cross-linking of oil and resin molecules that makes the varnish so much more durable than the oil from which it is made.
The oil used by Presbyter, and by far the most common drying oil still used today, was linseed oil. Other oils, however, can be used and each, like the choice of the flour used in baking bread, imparts its own unique properties to varnish. Tung oil, for example, is used in the manufacturer of most marine varnish because tung oil imparts superior moisture resistance compared to other oils. By itself however, tung oil has virtually no moisture resistance. In fact, the finish film produced by tung oil is virtually indistinguishable from the finish film produced by boiled linseed oil (BLO). It is only as an ingredient in the manufacturer of varnish that its moisture resistant properties are realized. An additional property of tung oil is that it is somewhat more color stable than linseed oil. However, over time, tung oil, like linseed oil, will oxidize causing the finish to darken. The lightest color finish is achieved with varnish made from soya (soybean) oil. Varnish made from soya oil is lighter going on and darkens far less over time than either linseed oil varnish or tung oil varnish.
The Resin used by Presbyter was fossilized pine sap (amber). Amber is mined from ancient deposits and then, as now, was frequently a component in “spirit varnish”—shellac to which other natural resins are added to improve various properties. The downside of amber is that the supply is limited, making it a very expensive ingredient.
The first man-made resin was phenolic resin. Phenolic resin is made by combining phenol and formaldehyde. Like amber, phenolic resin is a solid so it was easy to incorporate into the varnish technology of the day. Phenolic resin also produces a very hard varnish. This property makes varnish made from phenolic resin an excellent choice where hardness is desirable, for example for table tops, desks, floors, etc.
Alkyd resin appeared in the 1920's. Its name is derived from the two chemicals used in its production—alcohol and acid. Alkyd was the first liquid resin. In addition to its use in the manufacture of varnish, alkyd resin is used to make lacquer and oil-based paint. The primary attribute of alkyd resin is its superior clarity. This property, in my view, makes it an excellent choice, when combined with soya oil, as a finish for fine furniture. Alkyd resin varnish made from soya oil produces a very warm, clear finish that is quite color stable over time.
Finally, in the mid-1930s, urethane resin (polyurethane) was developed. Urethane resin, also a liquid, is very inexpensive to produce, making it the lowest price resin available for the manufacturer of varnish. Polyurethane’s dominant property is its excellent abrasion (scratch) resistance making it the choice for children’s furniture, kitchen tables, and other applications subject to having items pushed across the surface. (Please note; abrasion resistance and hardness are not the same thing! Logic should tell us that it is much easier to scratch something hard than it is to scratch something soft and compliant. But, again, logic and marketing do not walk hand in hand.)
On the down side, polyurethane is somewhat cloudy when compared to the other varnish types. It also exhibits poor adhesion properties, polyurethane does not adhere well (even to itself) without a mechanical bond (sanding). Varnish made with urethane resin is also highly susceptible to UV damage. Further,iIt should never be applied to shellac that contains wax. As a finish, polyurethane varnish did not see wide spread use until after WWII. Since then, lower production costs associated with shelf-space limitations and mass marketing have made it the most dominant varnish in the market place. Polyurethane is not the "best" varnish by any standard of comparison; but, certainly the cheapest to produce thus yielding the highest profit margin. Therin, in my view, the primary resons that it is the most commonly available.
At the risk of bogging down in unnecessary detail, let's take some time to see how these components are brought together to make varnish. It all begins with heating the oil. As the oil is heated in the absence of oxygen various sites along the oil's molecular chain are opened and made receptive to other oil molecules. The oil molecules begin to link with one another forming long chains of molecules in a process called polymerization (see drying oils in the last issue). When the oil reaches the optimum temperature, typically between 400° and 450°, the resin is introduced to the heated oil. The molecules of resin also begin to attach themselves to the oil molecules, thus modifying the molecular structure of the oil. (Hence the term “oil modified resin” seen on many varnish labels.) Soon every oil molecule is linked at various locations to other oil molecules and to molecules of resin creating a great mass of interlocking molecular chains. We call this molecular structure varnish.
The varnish is not usable in this form. It is much too thick to be applied. Thinner is added to the varnish so that it can be spread. Flatting agents may also be added to reduce the sheen from gloss to semi-gloss, satin or dull.
When the varnish is applied, two things occur. First, the thinner evaporates leaving behind the finish film which consists of the solids; the actual varnish produced by cooking the oil and the resin. Second, when the varnish solids are exposed to oxygen in the air the cross-linking of the molecules begun in the manufacturing process runs to completion; the varnish cures producing a durable finish film that is highly resistant to water, water-vapor, household chemicals, solvents, heat, and abrasion. In its cured state, varnish (which we call a reactive finish because varnish cures by reacting with oxygen) can never again be softened or dispersed in thinner.
At this point in our Beginning Finisher Series, we have examined two different finish types: (1) oil finishes, and (2) finishes made from oil. Hopefully, the differences between the two are clear. In the next article we will consider a type of finish that combines the two; a finish we call an oil/varnish blend.