Oil paints have been used for centuries in most of Europe’s greatest paintings, together worth far more than even Apple’s entire assets. It is therefore surprising how recent and incomplete is our understanding of how they actually work.

Many with extensive experience of oil painting still believe that some component in the oil medium, most commonly based on linseed oil, evaporates over time and allows the paint to solidify. This is completely untrue: it has been known for over a century that the oil medium undergoes slow oxidation (effectively low-temperature chemical burning) which links together the already large molecules from which it is composed, resulting in a polymer (‘plastic’).

It is true that thin layers of paint applied with ample solvent can become apparently touch dry quite quickly, particularly in the heat, but the oil paint in those layers requires months or years to undergo proper drying. Those modern books on painting practice which get this bit correct may still get the details wrong, though: one states that oxidation of drying oils generates carbon dioxide gas which must be released from paint layers. If that was true, our paintings would fizz, bubble, and blister. It is also not true that ‘water-soluble oil paints’ work differently; in fact they are water-miscible, and rely on conventional drying oils to consolidate the paint layer. They are most definitely not watercolours!

Understanding the drying of oil paint is key to understanding one of the rules in oil painting, that of applying ‘fat over lean’. It too is widely misunderstood, and many sources, printed and online, interpret the rule in ways which could result in serious structural problems in your paintings, including early severe cracking, or even delimitation of sections of the paint layer. Here I try to explain why the rule exists, and how you can apply it to your painting practice so that your works get the best chance of living long and looking good.

Drying oils

Oil media, as naturally occurring fats, consist mainly of a mixture of triglycerides, complex molecules consisting of a ‘spine’ of glycerol, to which are bonded three fatty acids, hence the ‘tri’ prefix. Fatty acids are made up of a long chain of carbon atoms, a couple of oxygen atoms at one end, and many hydrogen atoms. The most important fatty acids in oil paints contain 18 carbon atoms in their chain, so are commonly known as ‘C18’ fatty acids.

The physical and chemical properties of different oil media are largely determined by the fatty acids which they contain. Some, such as palm oil, liquefy at higher temperatures, so would only be suitable as part of the complex medium for oil sticks or bars, or in encaustic paints. For oil paints, the most important property is whether the oil will ‘dry’ by slow oxidation. As a rule, oils which contain little in the way of unsaturated fatty acids remain liquid and do not oxidise to become solids. The most important of the unsaturated fatty acids are C18:2 linoleic and C18:3 linolenic acids. Oils are deemed siccative, that is capable of drying, if they contain 66% or more unsaturated fatty acids.

The most important siccative oils used in paints include linseed, safflower, poppyseed, and walnut. Perilla, soya bean, and tung oils have also been used as they will dry. Sunflower seed oil is semi-siccative and should not be relied on for drying, and castor, rape, and olive oils do not dry. The composition of natural oils varies considerably with different crops, conditions, and processing: sometimes a batch of oil which should be siccative falls short because it is too low in unsaturated fatty acids, something which can affect walnut oil in particular.

How oils dry

When iron rusts, or methane gas burns, that process of oxidation is obvious and relatively simple. When linseed oil oxidises in drying, the process is protracted, complex, hidden from our eyes, but leads to the most important process of polymerisation, in which molecules bond together to form a solid layer. Discovering exactly what happens has not been an easy task, and is probably still incomplete in some details.

A series of slow chemical reactions starts to take place at the unsaturated bonds on the carbon chain of the fatty acids. These may be started by the presence of a metal catalyst, such as cobalt, manganese, lead, copper, or iron, which can initiate early reaction and thus promote the drying process. Through a series of complex chemical reactions, the triglycerides gradually change into a mesh of cross-linked molecules, a polymer which you could call ‘polyglyceride’, like a plastic.

Note that unlike some oxidation reactions, no carbon atoms are removed from the fatty acid and combined with oxygen to produce carbon dioxide. As polymerisation proceeds some volatile compounds are produced, and those are gradually lost to the air, accounting for the distinctive smell of drying oil paint. But unless something goes drastically wrong, the only ingredients required are a drying oil and oxygen, and the only significant products are the polymerised oil layer and small amounts of volatile by-products. In some oil paintings, polymerisation can take as long as 80-100 years to complete.

Cross-linking to form a polymer happens at an unsaturated bond, demonstrating the importance of having sufficiently high proportions of unsaturated fatty acids, in particular C18:2 linoleic acid and C18:3 linolenic acid. Apart from the presence of metal catalysts, key factors in promoting rapid and strong cross-links include a thin paint layer, to allow oxygen to be absorbed from the air, and the absence of more superficial layers which will block the absorption of oxygen. This explains why oil paint dries at its air surface relatively rapidly, as that gets the most oxygen, becoming dry to the touch long before deeper paint has started to solidify.

Oil paint not only becomes significantly stronger as it polymerises, but it can also change dimension, typically shrinking as the fatty acids link up and volatile by-products evaporate. So a good firm heavy impasto can start to collapse in on itself as it dries in depth, and a layer of still wet paint underneath an already dry layer is going to set up stresses in the rigid upper layer as the lower shrinks. Those stresses are likely to result in physical failure of the upper layer, seen as cracks in the paint surface.

The rule

That is the theoretical basis of one of the cardinal rules of oil painting, succinctly summarised in the three words: fat over lean. The purpose in painting oil-rich layers over relatively low-oil layers is to ensure that the lower layers – those with less oil – dry first, and do not impose stress by changing dimension under already-dry paint. So a less snappy but more accurate rendering of this rule is to make choices to ensure that your paint layers dry from the lowest upwards.

One simple but long outmoded way of ensuring that a painting dries from lowest layers up is, of course, to leave each layer to dry before applying the next. Even the slower processes in Renaissance workshops did not permit that, but allowing 2-4 weeks between layers is a real help. Few painters today can afford the time, and few of those who commission paintings are prepared to wait a year or more for delivery. Some practical methods that you can use to satisfy the rule include:

• add a drier or accelerant, such as cobalt, to the lower layer(s);
• keep lower layers thinner, and only apply thicker paint in the uppermost layer;
• add progressively more oil to the upper layers;
• avoid using pigments which dry more slowly in the lower layer(s) (these tend to be the pigments which take up most oil during manufacture);
• add alkyd resin to the lowest layer(s).

Corresponding practices which are likely to cause problems include:

• add a drier or accelerant, such as cobalt, to upper layer(s);
• apply paint more thickly in the lower layer(s);
• add oil to the lower layers rather than the upper ones;
• use pigments which dry more slowly in the lower layer(s);
• add alkyd resin to the upper layer(s).

Tempting though it may be to try to accelerate the drying of upper glazing layers, by using driers such as cobalt or adding alkyd resin, this is bound to increase the risk of cracking in those layers, particularly as they are likely to be thinner than the layer(s) below. Thus your wonderful deep colours accomplished by skilful glazes will in time be marred by extensive cracking.

A common error in advice given to painters arises over the use of alkyd resin additives. Alkyd resin behaves quite differently to a drying oil, and must not be considered to ‘fatten’ paint. Adding drying oil to paint slows its drying; adding alkyd resin accelerates its drying. Thus it is best to consider that alkyd resin makes a paint layer more lean, and its use should be confined to the lower layers. To add alkyd resin to glazing layers is running a high risk that your glazes will crack, when slower-drying lower layers shrink under already-dry and brittle glazes.

My own practice is to add alkyd resin to my underpainting, use paint straight from the tube for middle layers, and to add an oil-rich painting medium to glaze layers, allowing ample time for each layer to become touch dry before painting wet-on-dry over it.

Soap formation

Every household has the most common non-food product of triglycerides: soap. It is unfortunate that, when oxidation and polymerisation of oils fails, they may well end up as soap, a process known as saponification. Not only does this alter the optical properties of the paint layer, but soap is not a strong polymer, and will weaken the paint layers to the point where delimitation may occur. Soap formed within a paint layer also has a habit of making its way to the surface of the painting, in raised bumps or blebs, where it may exude as a gooey liquid and cause serious problems for the conservator.

Commercial soap manufacture adds a strong base (alkali) such as sodium hydroxide to a triglyceride. The base breaks up the triglyceride into fatty acid salts and glycerol, which are usually slippery liquids or semi-solids with surfactant, cleansing and lubricating properties, and catastrophic to the integrity of an oil painting. In oil paints, soaps probably form when oxidation does not take place, but there are suitable metal ions available, possibly with some water.

Recent microscopic analysis of small sections of the paint film from many paintings, spread across a wide range of time from the dawn of oil painting to the twentieth century, suggests that soap particles are not uncommon in oil paintings. In many cases these particles remain small and scattered through one or more layers of paint. When drying goes badly wrong, they can coalesce to form more substantial volumes, and cause visible and structural problems.

Understanding the causes of saponification in oil paintings is only just beginning. Clearly any factors which prevent the drying of a paint layer, such as the use of non-drying oils, the absence of oxygen as a result of early application of superficial impermeable layers, and very thick paint layers, will put that layer at risk of saponification. Metal ions which appear to be capable of causing saponification include nickel, aluminium, and lead; these can be contained in the pigment in that layer of paint, in additives in the paint, or they could migrate from other paint layers or the ground.

At present the best summary of factors which are likely to increase the risk of saponification includes:

• use of non-drying oils such as sub-standard safflower, walnut, or poppyseed, or any sunflower seed oil;
• early incorporation of superficial varnish and other layers rich in resin which could prevent adequate oxygen levels in deeper layers;
• thick paint layer(s);
• nickel, aluminium or lead pigments;
• oil paint additives rich in nickel, aluminium or lead salts, such as surfactants;
• surfactants migrating up from some acrylic grounds;
• water trapped anywhere between the ground and the paint surface, including water-based paint layers in mixed media paintings.

As ever, the above is advice based on research undertaken and published by others far more eminent than me. Painting is risk-based, and none of the above comes with any guarantee of success. Sometimes the most carefully-painted work suffers serious structural failure, and the reasons may not always be obvious. However in the long run, following the fat over lean rule should ensure that more of your oil paintings survive in excellent condition long after you will have ceased worrying about them. I wish you success.