Oils in Finishing | Popular Woodworking Magazine
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Find out why some oils heal better and what makes an oil suitable for finishing.
ORis one of the most important ingredients used in finishing products. In addition to being a finish on its own, oil is an important component in paints, polyurethanes and furniture polishes; a primary binder in stains, enamels and pore fillers; a plasticizer in lacquers; and a lubricant used together with sandpaper or abrasive powders to level the finishes and rub them on a uniform shine.
Despite its importance, oil is poorly understood. To make sense of this and understand how the different types differ, a little technical knowledge is useful.
The nature of oils
You have probably noticed that some oils remain liquid forever while others become sticky after a while and still others dry completely after a day or two. The explanation is that some oils have more reactive sites than others, and it is in these sites that the oil molecules crosslink and cure, usually with the help of oxygen.
Oils that never dry out have very few or no reactive sites, oils that stick have some reactive sites and oils that dry completely have sufficient reactive sites to make it possible.
Oils have three large families: mineral oil, vegetable oil and synthetic oil.
Mineral oil is distilled from petroleum and is always a straight chain hydrocarbon without reactive sites, so the mineral oil never dries.
If you apply mineral oil to the wood, it continues to penetrate the wood until it can go further. So what happens after each application is that the surface dries slowly and loses its intense color until eventually there is not enough oil in the wood to keep the surface in a semi-permanent oily state. (Washing the surface removes this oil, of course.)
Vegetable oils are pressed from seeds and nuts and make up most of the ingredients in the finishes. They are composed of a glycerol molecule with three attached fatty acids. This compound is called “triglyceride”, a term that you are probably most familiar with in the context of blood tests and what is more or less healthy to eat. (Animal fats have the same chemical structure as vegetable oils, so they are also triglycerides.)
There are many different fatty acids, each containing zero to four reactive sites per molecule. Sometimes all three fatty acids attached to a glycerol molecule are the same, but they are usually mixed, so the best way to calculate the number of reactive sites in a given oil is to use averages.
Oils with fatty acids containing on average a zero to one reactive site for fatty acid are not crosslinked enough to be dry. These oils, including olives, castor and coconut, are called “non-drying” oils.
(Oils without reactive sites are also called “saturated” and are not healthy to eat because your body cannot break them down. Your body breaks down the oils in their reactive sites and the oils that have them are called “unsaturated”.)
Oils with an average of one or two reactive sites for fatty acid dry better, but it takes a long time and it is often necessary to heat the oiled surface to facilitate drying. Even so, these oils can still remain sticky, so they are called “semi-drying” oils. Examples include nut oil, soy (soybean) and safflower.
Oils with fatty acids containing an average of two or more reactive sites achieve complete cure, albeit slowly, and are called “drying” oils. The most common examples in the finish are flaxseed oil and tung oil. Hardening occurs more quickly when heat is applied and also when metal dryers are added. These dryers (often sold as Japanese dryers and composed of cobalt and manganese naphthenates) are catalysts that accelerate the introduction of oxygen into the oil.
The difference between flax seed oil and tung oil can be explained quite easily by counting the reactive sites on their fatty acids.
Flaxseed oil has an average of about two fatty acid reactive sites while tung oil has nearly three. In addition, the reactive sites in the tung oil are better arranged for hardening. Hence tung oil hardens faster than linseed oil and is noticeably more water resistant when hardened.
Keep in mind that the flaxseed oil we are talking about is raw flaxseed oil, i.e. without added dryers. When metal dryers are added, producing “boiled” linseed oil, the product hardens faster than tung oil, which is never sold with added dryers. The addition of dry flaxseed oil dryers does not make the final cure more water resistant. Makes the oil dry faster.
Vegetable oils such as flaxseed oil and tung oil can be “cured” to make them dry faster, harder and more shiny – resembling paint more than oil. The method is to heat the oil to about 500ºF in an oxygen-free environment (in inert gas) until the oil starts to gel, then cool it quickly.
Since there is no oxygen, the reactive sites on the fatty acids crosslink directly (without oxygen atoms in between), therefore the characteristics of the oil are modified. When the partially hardened oil is then exposed to the air, it completes its drying very quickly, considerably faster than the paint.
Synthetic oils generally do not have reactive sites, therefore they do not dry out. Silicone oil is probably the best known in the world of finishing. This oil resists very high temperatures, therefore it is often used to lubricate machinery. It is also much more resistant than mineral oil and has a lower refractive index, which creates more depth in the finished wood. So silicone oil is often used in furniture polishing.
These transparencies have received a bad reputation because the oil is so smooth that it causes the disappearance of the finishes forming “fish eyes” or craters. But these transparencies are very popular with consumers.
The fish eye can be eliminated by better cleaning the surface with solvents or detergents, adding a little silicone oil (sold as a fish eye eliminator or smoothie) to the finish to reduce the surface tension or sealing with shellac, which usually not influenced.
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