A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
Having drawn your Design on a Sheet of fine Paper, well stretch'd in a Frame, lay over it a Skin of Pitch and fine Brick-dust, well incorporated together, covering this again, with a Lay of Spanish White and Masticot: This done, proceed to Colour your Design; and lastly to Varnish it, using to this Purpose, some old Oyl, clear and defecate, but of a good Body: After which, there remains nothing, but to stick it to a Glass; which must be flat and very smooth. 'Twill however, be the better way, to take a square piece of Earth well vitrified, laying over it the mixture of White and Masticot; afterwards Painting it, applying the Varnish, and covering it with a Chrystal; but first it will be necessary, that your Painting be well dried in a Stove, after which you may Varnish it with Nut-Oyl and Amber, or barely with Nut-Oil, taking care that it be well purified, and thicken'd in the Sun*.
* N. B. The Art of Painting in Enamel, invented not long ago, refers very naturally to this Head; and as it is now managed, is preferable to that here discribed by the Author.