How Oils Dry
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:
Polymerization of the drying oil to form long fatty acid
The crosslinking of the fatty acid chains.
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.
stage or crosslinking dominated stage practically never
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
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.
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.
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
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.
(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.