In Times of Desperation – Making Artist Oil Paints Dry Faster
Most artists (including myself) have been under the pressure of exhibition deadlines. Oil painters, unlike acrylic medium artists, face the problem that work can take weeks to dry, which causes a dilemma if your deadline for delivery is in three days time, or even tomorrow.
If you know that you have a deadline to work to, certain decisions can be made prior to paint touching canvas. However, once paint is applied to your support, you are relatively limited (but not helpless) in your options. I will cover both situations below, though the simple advice is to plan before acting.
About Oil Paints – the curing process
Most painters think oil paints dry like acrylic paints. They don’t, which is why techniques for forcing the drying of acrylic paints don’t work. So, to start, we are going to have a little lesson on what goes on when an oil painting ‘dries’
Unlike acrylics (which dry primarily by evaporation of solvent), oils don’t dry – they cure by a chemical reaction. This is why getting the hairdryer out (as so many people on the internet have attempted) has little effect, as opposed to on acrylics (though forced drying of any paint coating can create other problems).
So how do oils cure? Basically by a process the same as plastics are made – smaller molecules (monomers) link together to form a larger molecule (a polymer). In this case the monomers are fatty acids (such as linoleic acid) in oils such as linseed, and many other plant oils. If this cross-linking (think of a strand of wire formed into a mesh fence) did not occur, our paintings would never cure or become ‘dry’ to the touch.
What causes this curing? It is an auto-oxidative process which means oxygen molecules from the air react with the short monomers in the oils to crosslink them together. The more accessible the oxygen molecules, the faster the drying – which is why thick impasto paintings, in which oxygen must permeate through the upper paint layers, can take a disproportionately long time to cure compared to thin layers.
Temperature generally affects the rate (speed) of any chemical reaction. Hence, leaving your painting in your car during the heat of summer will cause it to dry noticeably faster than one in an unheated NY studio during winter.
Siccatives, and certain metal compounds (notably compounds transition metals such as cobalt, manganese or nickel) can be formulated to accelerate the curing process, acting as catalysts to speed up cross-linking. This is one of the reasons why colours such as cobalt blue or manganese blue will dry much faster than others such as cadmium yellow.
Alkyds also use a curing process similar to oils but include different monomers which cross-link more quickly. Unfortunately, while you can include alkyd mediums (such as Liquin®) to paint prior to application, you cannot do anything with alkyd mediums once the paint has been applied, other than wiping it off and starting again with alkyd materials. Attempting to apply a coat of alkyd medium over your painting may actually make things worse – you may accelerate drying of a very thin surface layer which may then make it harder for oxygen to travel to the lower layers; you could end up giving a thin layer painting the same drying characteristics as a thick layer impasto painting, and could cause cracking due to different film layers drying at substantially different rates.
What to do if you have not yet applied paint to your support
Alkyd paints (such as manufactured by Winsor & Newton, which I favour if using alkyds) are generally touch dry by the next day. This means that a dry to touch (but not fully cured – and certainly not cured sufficiently for varnishing) painting could theoretically be delivered by the afternoon of the next day if you work in thin layers. Thicker applications might take 2-3 days.
If you work in a cold studio, assisting curing by placing in direct sunlight, or using a heater to maintain the temperature around 20-30°C (68-85F) would assist the curing process.
Using alkyd mediums (e.g. Liquin®) can further accelerate drying (though see also comments below on ‘fat over lean’)
If you prefer to work with tradition oils, the use of alkyd mediums can significantly speed up curing. Instead of a week, you might be able to achieve a curing time (to dry to touch) of 2-4 days, though some experimentation is required. You might need a 50:50 ratio of paint to alkyd medium. Fortunately as alkyd mediums are ‘mediums’, as opposed to solvents, you can use any ratio of paint to medium satisfactorily (though it is always best to visit the manufacturer’s website to check the recommendations for any medium).
Siccatives are typically drying agents, often based on the compounds of the transition metals mentioned above. Manufacturers such as Pebeo/Fragonard produce such agents though some can be highly coloured and unsuitable for use with delicate pastel colours. Modified poppy oils may contain siccatives, but usually only enough to give this slow drying oil the faster drying time of linseed oil, but not faster. If you have not used siccatives before, experimenting with them with a critical work with impending deadline is not generally a good idea except in desperation – the use of alkyd mediums is a generally more foolproof method.
What to do if you’ve already painted your work with traditional oils?
Here your choices are limited. Unless you have a special recirculating oxygen tent with heatlamps (which introduces flammability issues), your options are few.
Heat and air circulation will help, as will light (there is some information suggesting that sunlight also helps accelerate the process). Some plein-air painters have noted that leaving the painting on the back shelf of the car in summer has greatly accelerated curing. In the studio you might have to improvise with a fan heater blowing with a gentle heat (lowest setting) on a work placed in direct sunlight. It won’t work miracles, but could take 1-2 days off the curing time of traditional oils.
What if the traditional oil painting is dry to touch but you need to make some further additions or changes? Depending on how ‘cured’ the lower layers are (dependent on time, temperature, and paint thickness) using either traditional oils with a suitable alkyd medium might be an option. Many alkyd mediums are manufactured to be quite flexible, and may avoid cracking problems associated with applying a faster drying layer over a semi-cured lower layer. However, I suggest you carefully read the recommendations of the manufacturers of such mediums.
A final alternative – go paint something else in alkyd oils, and save the uncured work for a future exhibition. Also make a mental note to prepare better next time, or to commit to alkyd oils when time constraints are dire.
The ‘Fat over Lean’ rule
I hate ‘fat over lean’ as a saying. It was fine in Renaissance times where you had limited choice of mediums, but is meaningless in todays world of alkyd mediums and paints, siccatives, etc.
While I prefer ‘slow over fast’ as a general rule of thumb (i.e. top layers should dry slower than lower layers), this rule needs to be modified when we are using fast or accelerated drying techniques. The preferred rule is ‘flexible over less flexible’.
Fortunately many alkyd mediums intended to be added to paints are quite flexible, which means they are less likely to crack if a faster curing alkyd coat is applied over a slower curing coat (which is why they are often used in glazing coats). Winsor & Newton specify that their alkyd mediums act like oil in the 'fat over lean' rule (see link below) and thus even fast drying Liquin modified layers can be applied over 'leaner' layers. HOWEVER, there is always a risk when pushing the boundaries and while you might get away with this when you are in a desperate situation, it is still not good practice to paint faster curing materials over slower curing coats unless that slower coat is substantially cured, or you have verified by technique by experimentation with the particular paints and techniques you use.
Remember that this article is talking about what to do in exceptional circumstances, and where chances and shortcuts might be warranted. It is recommended that you read the data sheets published by the manufacturers of any additives or mediums (Winsor & Newton is especially good at this, and has many plain English articles on the practical side of their use) prior to using them. I also suggest that you experiment with different mediums and techniques when time permits, so you are already familiar with your options when a situation arises. Remember that most so-called traditional artists were as much scientists as painters, continually experimenting with paint media (most hand made their paints) and techniques so they were familiar with what worked well, or didn’t work satisfactorily in different situations.
As painting with oils is as much an art (pardon the pun) as a science, I cannot guarantee that the recommendations herein will necessarily work for you. You must use them at your own risk, determining yourself whether the end justifies the risk.
http://www.winsornewton.com/resource-centre/product-articles/article-drying-times-for-oil-paint/
http://www.winsornewton.com/resource-centre/hints-tips-and-techniques/oil-colour/mediums/#fat




