What's under the yellowed crust of varnish on renaissance paintings

Art dealer and BBC presenter Philip Mould posted this video showing restoration work on a centuries-old painting. It's more vigorous than you might expect: a solvent tailored to the varnish but safe for the paint, and the resulting slimy mix simply wiped off to reveal surprisingly clear, vibrant color.

Mould hasn't shared the secrets of what method is being used. Turpentine is sometimes used with another solvent, but that doesn't appear to be what's happening here. No matter what method is employed, it takes a good deal of skill to remove the varnish and not have any impact on the actual painting underneath.

Details about the featured painting aren't abundant. Mould later clarified that the "woman in red" is 36 years-old and was painted in 1618, according to an inscription.

Below is a digital restoration of the Mona Lisa. The varnish and paint are reportedly too chemically similar to attempt the job with current techniques.