after-yellowing, yellowing definition

Yellowing is the gradual discoloration of a piece of artwork.  The process can result from a number of causes including the use of oils/varnishes susceptible to yellowing, a build-up of dirt on the piece that becomes attached to the varnish and/or by the surplus of linseed oil used on the piece.  Fortunately, the majority of oil paintings can be issued a new coat of varnish after the old varnish is taken away, thus bringing back the original's colors.