Watch what happens when 200 years of varnish are removed.
Painting restorer Philip Mould shared this video of his work. The brown discolored areas are covered in varnish, which has protected the oil-based paint for 200 years. Over time the varnish becomes discolored and must be replaced.
The delicate job is possible thanks to the gel-like goop that Mould spreads over the varnish. This agent dissolves grime and the old varnish without eating through the paint underneath. It is unclear what ingredients Mould used to make the agent, and is probably the result of a lifetime of trial and error in his field.
The process is long and tedious, but the result is vivid color that no one has seen for hundreds of years.