Comment by masklinn
> AFAIK one factor that accounts for the amazing longevity of old paintings was the compatibility between their many layers. The canvas, the primer, the paint and the medium were all derived from the same plant: flax.
AFAIK that's largely untrue in the general case:
- Canvas painting only appeared in the 15th century, and took a while to spread, paintings before the 17th century (16th in some locations) tend to be on wood panels e.g. raphael and da vinci painted almost exclusively on board.
- Canvas is almost always coated with gesso which at the time would use rabbitskin glue as binder, no flax there.
- While flaxseed was the most common drying oil (in europe), walnut, poppyseed, and safflower, were also in use. And additives were usually mixed in to manage the viscosity of the paint, so even in the "best" case it's not like the paint would be just pigments in linseed oil unless it's a very early oil painting, which wouldn't have been on canvas.
- As for varnishes, not only was flax not the only drying oil for oil varnishes, varnishes could also be "spirit" varnishes (with a resin dissolved into a solvent like alcohol or turpentine), waxes were also sometimes used as or in varnishes.
True… what I said only applies to canvas paintings. I should have made that clear.
The longevity of murals is easy to account for: the paint is applied to wet plaster, in that way becoming part of the wall. That is why the murals of Pompeii survived.
> Canvas is almost always coated with gesso which at the time would use rabbitskin glue as binder, no flax there.
You are right about the rabbit skin glue, but wrong about the gesso. As I recal, traditional Gesso is a mix of glue plus titanium white powder and is very brittle, generally unsuited to a flexible support such as canvas. A canvas painter would more likely use something flexible like a mix of pigment and rabbit skin glue, or pigment and egg protein or pigment and oil.
I thank god for modern primers. Using modern primers, I can prime a canvas in two days. An oil based oil primer could take months to dry.