Yellowing of Mediums

It is well known that oils have a tendency to yellow as they oxidize into a film.  This can be most commonly observed on any tube of paint where a bit of oil has separated from the pigment and has flowed out around the cap and has dried; it's a rather normal occurrence, but sometimes the yellowing of the pure oil can be startling.  You have to wonder if this will be noticeable in a completed painting.  Thank goodness the answer is no.  Even if you over saturate paint with medium, any yellowing from the additional oil can be reversed by allowing the paint to absorb UV light from the sun.  This is not true for resins like damar.  Below are some swatches of titanium white and a tint of ultramarine blue mixed with common drying oils as well as alkyds.  Each color swatch was mixed with a generous, but consistent amount of medium throughout.  For the last row I allowed the pure medium to form a film on top of the white acrylic gesso.  This test was then left to dry in complete darkness for 1.5 years; the results can be seen in the first image below.   As you can see, some mediums resisted yellowing much more than others.  The only inconsistent outcome was with the pure walnut oil.  On it's own, it didn't appear to yellow at all.  I think this must be an error because both the white and blue swatches containing walnut oil did in fact show yellowing.  I believe it is safe to say walnut oil will not yellow as much as linseed oil, but more so than stand oil.     

After I photographed the results of these tests samples left in the dark, I allowed them to sit in a north facing window sill for about 1 month where they received indirect light.  Then, I photographed them once again with the same camera settings and lighting as before.  The second image shows how yellowing can be reversed by exposure to sunlight. The time spent in the light really helped clear up both the paint swatches and oil films.  Even the worst examples improved greatly.        

Brands of Oil Mediums Used 
- Utrecht Alkali Refined Linseed Oil
- M. Graham Alkali Refined Walnut Oil
- Utrecht Stand Oil
- Grumbacher Sun Thickened Linseed Oil

The purpose of this study is to show that short term yellowing of oil paint due to the absence of light can be reversed.  Yellowing may come back if the paint is stored again in the dark for a prolonged period of time, but will not likely return to the un-bleach appearance.  However, the best method to minimize yellowing is to limit the amount of medium added to paint.