Comment by Daub
> I wonder whether it's still worthwhile to replaced a yellowed varnish varnish on old painting, just to be sure that it doesn't degrade further (with the assumption that historical varnish are somewhat lesser than modern one, which I really don't know if it's the case).
I would say that we should just live with the difference. There is a near guarantee that our experience of paintings as they exist today will always be significantly different to how they would have been experienced when they had been made, regardless of what we do. The factors are:
- Yellowing of oil/pigment/varnish. This is pretty much inevitable, even in modern paints.
- Fugitive colors. Most high-color pigments tended to fade significantly over time. Degas was once seen crying in front of a Delacroix, upset at how badly the colors had faded, even in his lifetime (It might have been Manet... can't locate the relevant anecdote online).
- Excessive varnishing. It used to be so that paintings received regular varnishing by their owners. Over time, the layers of varnish would build up to a ridiculous degree.
- Lighting. For me this is probably the most annoying factor. A general rule is that paintings should be lighted in the same light that they were painted. The values go to hell if a painting is lighted too brightly. There are many so-called hanging experts, who always light too strongly, even in respected national collections.
Semi-related anecdote: The first time I went to the Rodin museum I was struck by how badly one of the busts had been lighted. Whilst the guards were not looking, I moved it to a more agreeable position. I came back one year later to find it was still in that position.
As for using modern varnishes... I am not expert, but 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. On top of that would sometimes be laid stand oil, again derived from flax. If this continuity is broken, all manner of problems might arise, most of which would be long term issues such as cracking and flaking.
> 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.