Siccative oils

1.  Introduction -drying oils and oil films
Siccative oils are the drying oils used in artists' oil based paints.  If you paint with oils, you may have noticed that there are a large number of oils available.  Some oils are from different plant sources, while others may have been treated or extracted in different processes.  If you start looking through the available information on oils for oil painting, you'll find that different oils have different properties that are important to painters.  Two key properties, often discussed, are yellowing and drying time.  However there is also a third important property to consider - oil film hardness (and toughness).

Yellowing in oils is complex, and involves a good bit of Chemistry - we'll tackle yellowing in a later post with more background information covered.

Drying and film hardness are correlated to the amount and types of polyunsaturated fatty acids in the drying oil.  So the next logical questions are :
What are polyunsaturated fatty acids?
What is their role in oil film formation?

Linoleic Acid:  A Polyunsaturated fatty acid with 2 unsaturations.  This is a molecule commonly found in drying oils, also called siccative oils.

Oil paint as a polymer - drying

1.  That slooow drying oil paint
Oil paint can take a long time to dry.   When it's thinned with turpentine the turpentine evaporates quickly and the paint "dries", but sometimes that layer can still be smudged.  What is it about oil and oil paint that makes it stay "wet" and liquid for a long time and then - voila!  It starts to thicken and then dries to a hard film.

Think about the liquids you use everyday; water, isopropanol (rubbing alcohol), possibly even solvents like acetone and ethyl acetate.  Most of these liquids evaporate fairly quickly.  Even a drop of water doesn't take days and days to dry.  When these everyday pure liquids dry *poof* they're gone.

2.  Typical solutions and their drying
Of course it's possible to take something solid, for example salt or sugar, and dissolve it in a liquid like water.  When the liquid water evaporates, solid salt or sugar is left behind.  In contrast, when oil paint is dissolved or thinned with turpentine or another solvent, the solvent evaporates and the pasty paint is left behind -- and then the paint slowly hardens over time.  Step back and think about this - oil paint has an unusual property.

Solid particles (gray balls) dissolved in a solvent (blue "water molecules").  Instead of particles you can also imagine that the gray balls are molecules of something that would be a solid without the water dissolving it.

Confused about artists' materials? 1. A very short and easy technical history of oil paint

Have you ever wondered what goes into artist grade paint? If you've ever shopped for artist's oils or acrylics you may have noticed that there's a lot of information surrounding those little tubes. Even right in the store, the paint displays are full of ratings, claims and data to take in. Dig a little deeper; look into articles and books for artists, and you'll find advice on how the individual colors behave, information describing good and not-so-good ways to use the media that mix with the paint, and lots of factoids and explanations about why this is so.

Does all of this information seem confusing? Sometimes contradictory? As if it points everywhere except towards the solid answers you need? If you've found yourself confused trying to make sense out of everything you've read, heard and been told about artists' paint, it may be because a lot of the information out there is out of date, or extrapolated from materials that don't behave like your paint, or simply well-worn paint mythology. Your confusion is not a lack of insight or knowledge; in fact it's a sign of a solid scientific intuition and curiosity taking hold.