Dictionarium polygraphicum. The method of drawing the spirit of the vitriol of Venus, which has a wonderful blue, and how to separate the caput mortuum for tinging of glass.

Dictionarium Polygraphicum:
Or, The Whole Body of Arts Regularly Digested.
Vol I.
London: Printed for C. Hitch and C. Davis in Pater-noster Row, and S. Austen in St. Paul's Church Yard. MDCCXXXV.
1735
Put a pound of this vitriol into a glass retort strongly luted, the lute being dry, set the retort in a sand-furnace, fitting to it a very large receiver, as directed for aqua fortis; this done, kindle the fire, and continue it gentle for four hours to prevent a too excessive heat, which would drive out the spirits impetuously, and so burst the receiver, whereof great care must be taken not to spoil all. As soon as the spirits ascend like white clouds, improve your fire by degrees, until they disperse and your receiver clears again and cools, and all the spirit comes together: then let the fire go out of it self, and after twenty-four hours, unlute the joints, take away your receiver, and put the liquor it contains into glass-bottles, and stop it very close with glass-stopples to prevent air, which if they could draw, would disperse it all by exhalation. This choice liquor has that noble blue which affords us wonderful tinctures, and other inestimable operations, as well as such surprizing effects in physick, as cannot be equalled.

But to return to our caput mortuum of this precious vitriol, which has occasion'd us to give its preparation, and is what we make use of for this fine water-colour, or egmarine on glass, yo. find it in the retort, out of which the white spirit was distill'd, whereof we have already treated. To get it, you must break the retort; then reduce it to powder with a mixture of zaffer, and so tinge your crystal of an admirable sea-green colour.

We were mistaken in saying besore, that the caput mortuum must be expos'd to the air before you pound it with the zaffer, for that is not altogether incumbent in tinging of glass, tho’ this exposing of it cannot but add something to the luftre, for it draws thereby with a certain magnetick property, the occult spirit of the air, and so from a black, of which it was before, becomes of a pale blue colour, and partly assumes what is lost by distillation and extraction of the spirits, so you may save a great deal of time and pains by this preparation, to your no small ad vantage in expediting the matter.