The Traditional Tools Group Inc.

Note: This is a classic reprint of an article originally published in two parts: in NEWS #029, June 1996 and NEWS #030, August 1996.

1. Types of Oils.

This note takes a look at the non-mineral oils used in woodwork and without exception we find that they all belong to a class of compounds known as Triglycerides.

Almost certainly the characteristics of triglyceride oils which are useful to us in woodwork are their ability to form thin films (like their mineral cousins, the lubricating oils used in machinery) and their drying characteristics (not exhibited by mineral oils).

A not very chemically useful classification but nevertheless useful from our point of view, is to look at triglycerides in terms of their drying characteristics. It is usual to divide them into drying, partially drying and non-drying types.

Drying oils when exposed to the air in thin films, convert to a firm and non sticky film; partially drying oils form a skin much more slowly and the film remains tacky almost indefinitely; non-drying oils remain oily and do not form solid films when exposed to air in thin layers.

The key concept here is of course the emphasis on thin films. The structure of triglycerides is such that oxygen which is a vital ingredient in the drying process cannot easily permeate either the liquid itself or a resulting dried film, hence only the surface will dry effectively and any excessive thickness or build-up will tend to remain liquid or at least tacky below an initial skin.

We can now understand why potential disasters happen when too heavy a film of oil is applied to the wood surface. Once the timber is saturated with oil (or at least the top layers of the wood), any excess oil just sits on the surface and skins over, the liquid or tacky layer below can now never harden and the overall impression is of a “gummy” surface which is resistant to most solvents, clogs sandpaper and gums up your edge tools as you try to remove it. Certainly, the drying oils exhibit a range of drying characteristics: some dry quickly, some slowly, some to a leathery tough film, some to a hard film and to understand why, and what particular oil we should be using for which particular effect, we must again turn to the subtle chemical differences which distinguish the individual members of the triglyceride family.

2. Drying of Oils.

The forming of a hard film on drying oils is not really a drying or evaporative process at all. Actually the vapour pressure of triglyceride oils is extremely low and there is minimal evaporation; the process we are observing at the film surface is one of polymerisation or linking together of the molecules into long chains which entangle and bind the matrix together. Careful analysis suggests that the actual level of polymerisation may be quite low but the few long chains which do develop entangle the long unpolymerised triglyceride strings in a kind of molecular log jam.

The fatty acid chains making up the triglyceride molecules and which possess drying characteristics are said to be chemically unsaturated, that is they have some double carbon-to-carbon bonds and are not completely "saturated" with hydrogen.

Under certain conditions, these carbon-to-carbon double bonds may use their extra bonding capability to link up with adjacent molecules in a similar state and hence the cross-linking and polymerisation process begins. This is a greatly simplified explanation but contains the essential ideas for understanding the process.

Fatty acids which are highly unsaturated in a very regular fashion e.g. double bonds every second carbon atom polymerises at a much faster rate and are said to have a conjugated structure. Eleostearic and licanic acids are examples of conjugated unsaturated fatty acids and make up over 80% of the triglyceride types in tung oil and oiticica oil respectively. It is the presence of these conjugated components which gives these two oils such strong drying characteristics.

Polymerisation occurs by two fundamental mechanisms, application of heat and absorption/reaction with oxygen.

Heating triglyceride unsaturated oils tends to regularise or drive their structure towards a conjugated one and then link together or polymerise the resulting strings. In the case of tung and oiticica oils which are already highly conjugated, the polymerisation process proceeds more rapidly but there is another reason for heat treating both of these oils. In the natural state, the conjugated oils in both tung and oiticica slowly convert to a solid higher melting point form and initial heating to 200-250°C for about half an hour prevents or slows this conversion.

Heat induced polymerisation is of course a bulk polymerisation and as you would expect, increases the viscosity or thickness of the oil. It does however accelerate the process of oxygen polymerisation which occurs when the triglyceride is spread as a thin film.

There are several methods employed in industry to achieve this prepolymerisation besides plain heat treatment or heat bodying.

a) Boiled Oil

The addition of metallic salt of organic acids to drying oils increases their drying characteristics appreciably. The chemical reason for this is somewhat complex but in essence it is probably primarily a catalytic effect which accelerates the oxidation process in film drying. The metal salts are added to heated oil while air is bubbled through in order to keep the metallic salts in suspension until fully dissolved. The hot oil froths during this operation and resembles boiling although the temperature is far too low for real boiling to occur. The advantage of this method is that there is only limited prepolymerisation occurring during the absorption of the metal salts and so the viscosity of the oil does not increase markedly.

The drying time of linseed oil “boiled” in this way is about a tenth of “raw” unmodified oil.

b) Organic Catalysts

The addition of small quantities of aromatic organic catalysts or even terpenes such as terebene tend to catalyse oils to the faster drying conjugated structure.

c) Blown Oils

Bubbling air through oils at temperatures ranging from 40° to 150°C oxidises and polymerises the oil in a similar way to ordinary heat bodying treatment although because the temperatures required are lower, the process is cheaper.

Blown oils produce less resistant films than standard heat-bodied oils because of their greater content of oxygen and in addition the resultant films are more susceptible to solvent attack.

In general, the more prepolymerised a film is before we allow it to oxidise, the faster and harder dried film we achieve but its increased viscosity means that the film will tend to be thicker (and hence more visually obtrusive) and will not penetrate and hence bind to the surface structure of the wood as well as a thinner oil. In addition, a less prepolymerised oil will still contain a high proportion of bound liquids in the dried film and hence it will be more plastic and tougher.

Human oil such as occurs as a component of sweat and as natural hair oil is also a triglyceride oil almost entirely of the Oleic-Linoleic group, unsaturated and with medium to weakly drying properties. Those tools which have no other finish on their wooden handles other than human sweat are actually finished in a series of slowly dried thin triglyceride oil films!

3. Deterioration of Oil-Based Films

a) Increasing Polymerisation.

As we have seen, an oil film can be technically "dry" with a very significant portion of its triglycerides still unpolymerised. It is this property which lends toughness and elasticity to the film. One of the primary reasons why linseed oil has played such a significant part in the paint industry is its advantageous balance of polymerisable to non-reactive triglycerides.

Ultra-violet light is one of the greatest enemies of dried oil films as it accelerates and continues the process of polymerisation, causing the film to become increasingly hard and brittle with time. If the substrate on which the film has been deposited moves, such as a timber surface which expands and contracts according to the ambient relative humidity, then the film will eventually become too brittle to allow these dimensional changes, will crack and lose adhesion.

The formation of volatile by-products of polymerisation also leads to a weight loss and eventual thinning of the film so that the surface appears to have “dried out”. As an example, linseed oil films when exposed to intense ultra-violet radiation over an extended period can lose up to half their original film thickness.

b) Action of Water and Alkali.

During the ageing of films, acidity develops from the oxidation of the unsaturated fatty acid chains and it is this surface acidity which decreases the resistance to water and alkali degradation. (Interestingly enough this tends to be an advantage in paint where the acidity helps the oil to 'Wet' the pigments.)

Conjugated oils such as tung absorb much less oxygen to achieve satisfactory dryness and hence initially they have a resistance to water and alkali which is superior to non-conjugated oils such as linseed. However, they eventually absorb the same amount of oxygen as the non-conjugated types and hence their water and alkali resistance decreases with time to be almost indistinguishable from the non-conjugated oils.

With respect to all triglyceride films, oils which have been partially polymerised by heat treatment are superior to the “raw” oils when it comes to water and alkali resistance. The heat treatment reduces the level of oxidisable molecular bonds significantly and hence reduces the final amount of oxygen which can be absorbed in the drying process.

c) Discolouration.

Yellowing of dried triglyceride films is generally in direct proportion to their degree of unsaturation. Oils which are generally less unsaturated than linoleic, such as soybean or poppy seed are reasonably fee from yellowing.

Yellowing does not appear to occur in a perfectly dry atmosphere or at low temperatures. It is accelerated by infra-red and visible red light but retarded when exposed to ultra-violet light.

4. Conclusions.

The above fairly qualitative description of the chemistry of drying films may help in deciding what oil to use in what situation (or indeed when not to use a drying oil) whether the oil should be “boiled”, “raw”, heat-bodied or “blown” and what might be the possible effect. In all cases many applications of exceedingly thin films over reasonable periods of time will produce superior results to fewer thicker films.