Last week a few of my colleagues were wondering about HOW oiling out or varnishing alters the appearance of oil paint. Some were unclear as to whether or not there is a structural or compositional altering of the paint film itself. I explained that rather than altering appearance in that way, a varnish (or medium used to oil out) merely alters/impacts the surface properties (reflectance/refraction/absorption or light waves). Among the many papers on this subject I was able to find a very clear power point that presents the “how” quite nicely:
Before commenting on some of the slides I’d like to mention that my understanding of light and material interaction comes from a background in physical based rendering, which attempts to simulate correct physical behavior in 3D imaging.
I’ll briefly tough on some of the slides.
“reflected diffuse light without absorption–> Eye sees White” (slide 4) - this is actually a specular reflection on the surface of the pigments themselves. Non metallic opaque surfaces have both a diffuse component, which in this case is red and which spreads in all directions, as well as a specular component, which is always white ie. it reflects the colour of the incoming light. The strength of it depends on the refraction index. (the following includes slide 5.) In the case of the pigments the combined surface is so rough that the specular reflection spreads nearly as wide as the diffuse and is no longer noticeable as such. Pigments are, however, surrounded by oil, which I will refer to as a clear coat. This is a transparent substance with no diffuse and only a specular reflection. This surface is shiny, no matter what. What happens, though, is that when it sinks in, the surface is somewhat shaped by the embedded pigments and becomes rough. Now the specular reflection becomes diffuse and blurry and the reflection of bright light sources get layered on top of the pigments blocking the view onto the pretty colors. This is what makes it all look dull and dreary. In addition one needs to keep in mind that the specular reflection increases the shallower the viewing angle on a surface is. The microscopic bumps in the surface can therefore brighten the specular component. Oiling out or varnishing adds another clear coat with a smooth surface, hence reducing the diffusion of the specular component, allowing a less obstructed view onto the pigments. It doesn’t reduce the specular as such, it just prevents the reflections of bright light sources spreading all over the surface. Due to the similarity in IOR (index of refraction) in the clear coats the initial rough surface of the sunken in layer, now covered by a new clear coat, becomes invisible.
All the other arguments about the effects of refraction are valid, but in my opinion not nearly as crucial in altering the appearance, compared to the effect a clear coat has merely by reducing the diffusion of the specular component of the paint surface. (slide 6) Internal reflections do trap some light, causing some darkening; because it doesn’t escape it won’t hit the retina. This can have the perceived effect of more saturation. This effect should be very small though. Additionally, light reflected as a specular reflection never hits the pigments, also causing some darkening.
(Slide 7) The deeper one looks into the layer of pigments the more saturated they appear can be explained by the pigment’s specular component reflecting more direct light (white) at the surface, and more indirect light, which is tinted by the pigments themselves, deeper down.
