The main difference is the binder -for oils, it's usually a simple mix of pigment into linseed oil (or safflower oil). There may be some other filler, especially in student-quality paint, but we don't need to go into that right now.
Acrylic paints are much more complex. The binder is an acrylics emulsion (a product of the petrochemical industry and available from only two manufacturers in the world. According to the rep for Golden Acrylics paint; these are BASF and 3M). Along with the pigment, there may be a humectant in varying amounts which help manufacturers even out the difference in drying times as different pigments will dry faster than others.
Another difference is in the way they cure once the paint has been applied: Oils cure by oxidisation -the linseed oil pulls in oxygen and hardens over time. Acrylics dry by evaporation and polymerisation. In other words, as water leaves the emulsion, polymers form creating a film of plastic containing the pigment. This can happen very quickly -a thin layer of acrylic paint can be dry within minutes. Even a thin layer of oil paint will remain workable for an entire day; great for working wet-into-wet, scrubbing out mistakes and creating a flow and softness in the artwork. It could be said that oils never dry; they just continue the process of oxidisation until crazing occurs and the paint detaches itself from the support. Neither of us will have to worry about that for our own masterpieces, though, as there are 600 year old paintings still in good order and many future conservators who'll desperately need employment. It's just too early to say how acrylics paints will fare over the next few hundred years. I suspect, judging by all the plastic crap floating around in the sea, that they'll outlast humanity, or even the lifetime of the universe.
Because acrylics dry so quickly, the artist has to take a different approach to painting than with oils; often building up layers of paint. You can achieve a high degree of precision very quickly but you may lose out on all that lovely oleaginous softness.
Dilution and clean-up after painting in oils needs a solvent, such as turpentine, Sansodor or Zest-it. Solvents are often volatile materials, which give off pungent fumes, especially turpentine and white spirit, so they should only be used in a well-ventilated studio (that includes Sansodor, which might not have much of an odour but is still a volatile chemical). They can also be a fire-hazard. For acrylics, we use water for dilution and clean-up.
Acrylics can be used to imitate other mediums such as watercolour (by dilution of the paint with water) and oils (by use of acrylic mediums and retarders or painting extremely fast!). They never quite reproduce the spontaneity of either -mostly because they dry so fast and so thoroughly. Whatever about imitation, though, acrylics is its own thing and comes into its own by its inherent versatility.
What's best about acrylics is the sheer range of paints, additives and textures that can be bought or created. Craquelure, gel mediums, iridescent colours, process colours, high-build mediums, impasto mediums. They can also be applied to a whole array of substrates other than canvas: paper -gessoed and unprepared, wood, glass -a seemingly unending variety. They're such fun to use.
However, there really is nothing like the ease of application and the flow of oils. In comparison, acrylics seem to drag, especially when starting a painting on new canvas or paper.
Another difference is that when using acrylics, there's a colour shift as the paint dries. Acrylic paint dries darker than the out-of-the-tube paint. According to the same Golden representative, this is because of the refraction of light through the water in the acrylic emulsion. There is no such shift with oils. Oils has an inherent lustre, even if sometimes areas seem to be a little matt and dry-looking. Acrylics always dries matt. However, you can mix gloss medium into acrylics paints to get around this. You can also varnish your painting, of course!
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| Acrylics with a dry-brushed texture |
A very general guide I use for myself is: When you want to paint a painting quickly; use oils; If you want to hang a painting quickly; use acrylics.
For the artist interested in selling their work; there's another factor you may want to consider. There seems to be a hierarchy of value in mediums. Oil paintings seem to have a higher innate value than acrylics paintings. I know this is ridiculous but it appears to be true. Let's forget for a moment about the completely arbitrary perceived value of the artists themselves. Although acrylics have been around since the 1950s, oils have been around for many hundreds of years and tradition mostly trumps novelty in painting sales.
Some final points:
- You can paint in oils over acrylics but never the other way around.
- You can't mix oils and acrylics paints and expect a good or lasting result.
- After using acrylics, clean your brushes immediately; once the paint is dry on your brush, it's very difficult to remove.
- Of all the painting media, oils is the most forgiving: start with oils.
- In either case, studio waste should be treated with care and disposed of properly. Some of the pigments in all kinds of paint are toxic.