Zecchi is now selling the medium I recommend pre-mixed to save painters the trouble of making their own. The recipe is the usual: 1 part Canada balsam cut with 1 part turpentine, and then that mixture is added to 2 parts Zecchi sun-thickened linseed oil. The item number is 3882 ” Medium Antichi Maestri” and the price is €12 for a 125ml bottle and €22,50 euro for the 250ml bottle. (They ship abroad too, check out their website for more details).
I tried to get them to name it after Theodore de Mayerne or to call it the “Cecilian medium” as Charles Cecil developed a similar version (adding mastic varnish) based on his reading of de Mayerne’s manuscript on 17th century painting techniques but no dice.
‘Old Master Medium’ is so trite.