But rather than representing a frozen piece of time, all paintings are in flux. From the moment it's created, a painting begins to deteriorate; dirt accumulates on its surface, its colors lose their luster and its paint cracks and flakes.
Conserving precious artworks is the job of the restorers at London's Courtauld Institute of Art. Their work is a combination of science and, appropriately, art, as they combine cutting-edge technology and painstaking manual techniques.
"Inevitably, pictures will need to be cared for and those works that have been cared for, we retain them, we benefit from them and we can still see them," said professor Aviva Burnstock, head of conservation and technology at The Courtauld.
"Things that have been very neglected -- we've lost them. That's why conservation is important."
What lies beneath
At The Courtauld they use techniques including X-ray spectrometry and infrared photography to reveal more of a painting than can be seen by the naked eye -- with sometimes surprising results.
"X-rays will penetrate all the way through the painting, so you can see aspects of the whole thickness of a picture and sometimes you can see the frame and the nails that have been used to hammer the canvas in, and sometimes you can see the reworkings in paint -- so you can see things you can't see on the surface," Burnstock told CNN.
"You can do an infrared photo on a specially adapted camera," she added. "You might see something beneath the varnish --- you might see drawing under the paint layer, you might find a picture under another picture, or a drawing ... that's been covered up with a completely different picture."
Ultimately, though, the restorers are doing battle with a force they cannot hope to beat: time.
Burnstock says the conservators use materials they hope will last at least 100 years, but eventually, even they will deteriorate. And despite technological advances, it's impossible to completely restore an artwork to the way it originally looked.
"A painting will change from the moment it's made so there's no chance of restoring it to the way it looked when it was first made," said Burnstock.
"But you can appreciate how it might have looked by doing the research that's needed and present it in the best way it can be presented."