Roman \"varnish\"

[Medicus matt]

Aetius Amidenus writing in the late 5th C apparently describes using linseed oil (presumably thickened by heat) to preserve encaustic paintings; not seen the actual text myself though.
Earliest proper process I've got is from the Mappae Clavicula (oldest extant manuscript being early 9th C) which gives the following:-

How to treat a painting so that it cannot be destroyed by water.
Coat a painting in the sun with the oil called castor oil, and it is fixed so tightly that it can never be destroyed.