To remove varnish from an old panel painting that has yellowed and varnish it again
Take some white soap and ashes strained through a sieve, and soak all together in water. And with a sponge, take some of the said ashes and soap and rub the painting with it. And when you see that the old varnish has been taken off, throw a bucket of water at the painting to clean it, then leave it for a quarter of an hour in the sun to dry and revive the colors. Once your painting is dried, you can apply your turpentine varnish.
at left bottom margin of folio 004r
Make sure the colors do not come off.