Originally Posted by Scott Methvin
Very interesting thread. I wish you had included a handmade sample of Kremers' genuine vermillion. I have been using that pigment for many years. One of the first things that came to mind in this comparison was how my paint is not really tube-able because it hardens in the tube after maybe a week. My guess is that quality paint makers use certain things to mitigate this hardening that pure raw linseed and vermillion pigment together create. True naples yellow also hardens up quickly in its' pure state with RCP linseed oil. Maybe these added ingredients are the darkening element?
Thatís interesting that oil paint made from Kremerís vermilion pigment hardens so quick after itís made. I have several vermilion paints including Natural Pigments that uses only pigment and oil and have not experienced any hardening in any of the tubes. It may be possible that Kremerís vermilion pigment may be contaminated (litharge)? Have you contacted Kremer?
The darkening agent that appears to cause vermilion oil paint to darken based on my tests is ultraviolet light or sunlight. I have vermilion pigment (PR106) from KAMA pigments. I placed a small amount of vermilion pigment only, between glass slides and placed it outside where it received partial sun throughout the day. In about a week and an half it darken significantly.
On the opposite side of the slide not facing the sun the vermilion retains a great deal of its brilliance. Under magnification the dark side shows a speckled landscape of red and dark areas. The affected pigment particles affected by the light donít appear to contaminated particles away from the light. If anything the binding oil to some degree appears to slow down the darkening process of vermilion when exposed to long periods of strong daylight. It may be that the more cured the binding oil is, either through additives or just a lot of time the more previous vermilion is to strong daylight/