How Oils Dry

 Oils dry by the process of oxidation not vaporization.  Painting mediums can contain some volatile solvents that will vaporize but the actual drying of the oil paint is by oxidation.  There are two different stages of oxidation:

1. Polymerization of the drying oil to form long fatty acid chains.
2. The crosslinking of the fatty acid chains.

The first stage of drying (polymerization) is the most critical.  During this stage the oil requires access to large volumes of oxygen as the oil is polymerized.  This large addition of oxygen to the paint film increases the volume/weight of the paint film causing it to expand.

The second stage or crosslinking dominated stage practically never stops.

One way that has been used to explain oil drying is to think of a plate of spaghetti.  First stage of drying is oxidation polymerizing the drying oil into long fatty acid chains represented by the spaghetti.  The second oxidation stage of crosslinking the long fatty acid chains can be thought of as slowly, over decades, gradually gluing each point that they touch.

The oil paint film is initially very flexible, as the plate of spaghetti.  Over time with increased crosslinking, gluing the spaghetti together, the paint film becomes considerably less flexible.

As oils dry they expand increasing the volume of the paint as much as 25%.  Pigment particles are moved farther apart resulting in the paint film becoming more transparent as it dries.  This is one of the unique features of oils.

The methods by which oils dry can make the paint film susceptible to cracking.  Initially during polymerization when the volume of the paint film is increasing.  Cracking during this stage is generally a result of the methods used by the artist.  Generally as long as the more flexible paints are painted on top of the less flexible, often called fat over lean, this should not be a problem.  When it does occur it generally starts to show some signs within the first year.

Premature varnishing can result in the varnishing cracking as the paint film expands, particularly if damar varnish is used.  There are modern alternatives to damar that should be used (future post).

Oil paint films become more brittle with time as the crosslinking continues over the decades, if not centuries.  As the paint film becomes less flexible it becomes more vulnerable to cracking because of changes in temperature and humidity.  With modern air conditioning and heating this is less of a problem if the painting is properly framed (future post) and hung (future post).  This can remain a problem when transporting or shipping paintings and displaying older works out of doors.

Drying time (future post) will be dependent on the medium used and the paint thickness.  Modern linseed oil used for most oil paints has very few impurities compared to the linseed oil used in the past resulting in faster more uniform drying.  This means that many of the old rules of thumb are not valid for modern paints.  However, even with modern paints and ideal drying conditions a painting should never be varnished until a minimum of three months after the last oiling out (previous post).  This is the earliest and it could take a year or more depending on the thickness of the paint.