(Chapter 21) Footnote 53:

ex massa vel cera, probably referring to the materials used in casting hollow metal artifacts by the lost wax process. Cf. Engl. mass, “a lump of raw material for moulding, casting, sculpture, etc.” ( OED), and see chapter 9, note 11 (on p. 39 line 10 of 1543 Fabrica). The clay or plaster is moulded; wax is then laid over the moulded form and detailed prior to application of a second layer of clay or plaster. For a contemporary account of this process, see Vanoccio Biringuccio’s Pirotechnia (Venice, 1540, trans. 1943/1966 by C.S. Smith and M.T. Gnudi), Book 6 Chapter 4 “On Moulds for Statues to be Cast in Bronze.”