Making and Knowing
A minimal edition of BnF Ms Fr 640 in English Translation

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Varnish for paintings

Take a pound of Venice turpentine and heat it in a pot until it simmers, put in half a pound of the whitest turpentine oil you can find and stir all together well on a charcoal fire and take it off immediately after, and it is done. But if you find it too thick, add more oil, whereas if it is too clear, you can thicken it by putting a little turpentine. And so you will give it whatever consistency you want. It could be made without fire, but it is more desiccative when heated. It is appropriate for panel paintings and other painted things, without distorting the colors or yellowing, and dries both in the shade and under the sun, overnight, and during the summer as well as in the winter. It is usually sold 15 sols a pound.

at left bottom margin of folio 003r

You need a little more turpentine than turpentine oil to thicken the varnish, which you need to apply with your finger in order to spread it thinner and less thick because when it is thick, it turns yellow and gathers [together]. Varnish is not used to make paintings shine, because it just takes the light out of them.

at middle bottom margin of folio 003r

But it is used to enhance colors which have soaked in and to keep them from dust. Mastic varnishes does not resist rain, whereas oil [varnish] and rosin varnish do.