Some well-timed rain this past week granted me an opportunity (and some useful analogs) to discuss “oiling out” and varnishing.
When oil paint is spread out over a surface and exposed to air, it begins a sequence of chemical processes that transform the fluid paint into a strong, resilient film. One of the often encountered visual characteristics of this transformation is a change from a more “specular” surface appearance to a more “diffuse” one. (The apparent magnitude of this change can be affected by several factors, including the absorbency of the ground and solvent use.)
To better appreciate this potential change in appearance, look at the first two images of stones. On the left is an image of the stones during the rainstorm. The center image shows the appearance stones after the water had evaporated. Notice the significant differences in terms of value and color relationships. You see, as the water leaves the stones, the smooth, wet surfaces which provided a more ordered reflection of light (specular) give way to a more rough, varied collection of surfaces that produce a more disordered (or multi-directional aggregate) reflection of light. As these surfaces change, so do the resulting light stimuli on our retina. This example of wet and dry stones is a useful analog for the change in appearance in oils as they undergo a drying or curing process. The surfaces that reflect light here are physically altered so that the aggregate of light reflections can produce a very different stimulus.
When an adequately dry painting is varnished, the once rougher paint film surface is augmented with a relatively transparent film. This film (varnish) returns a smooth surface to the paint film that is capable of providing light reflections very similar to what was found when the paint was “wet.”
Appreciating this dynamic should make clear why oiling out may be necessary when revisiting oil layers that are colloquially “dry.” Trying to match sunken or drying colors or values can lead to significant relationship distortions should the painting be brought back to a uniform, often more specular finish (via a varnish.)
Shown: On the left image, wet stones A, B, and C were chosen for comparison. The center image shows the difference in appearance when water evaporated from stone surfaces. The right image simulates how a varnished painting may appear if stones A, B, and C were worked on (matching to context) while the surface was significantly more diffuse. Graphics below images show potential changes in sample color relationships due to surface changes and the aforementioned problematic editing.
(For those that may like a basic primer on how light interacts with different surfaces, this is a good resource):