I was once told by an artist that he had read an account in which da Vinci said that the greatest pieces of art that he (da Vinci) had ever seen were those that were revealed in the water stains of interior walls and ceilings of old buildings. Did he really say this? Where can it be found?
I know this is a super late reply, but hopefully it's still helpful! This is from his Treatise on Painting:
"This is the case if you cast your glance on any walls dirty with such stains or walls made up of rock formations of different types. If you have to invent some scenes, you will be able to discover them there in diverse forms... these are like the sound of bells, in whose tolling, you hear names and words that your imagination conjures up.
Don’t underestimate this idea of mine, which calls to mind that it would not be too much of an effort to pause sometimes to look into these stains on walls, the ashes from the fire, the clouds, the mud, or other similar places. If these are well contemplated, you will find fantastic inventions that awaken the genius of the painter to new inventions."
(This is modernised language from the 1797 translation). So, not quite "the greatest pieces of art", but a kind of reverence for them.