The art of glass shewing how to make all sorts of glass, crystal and enamel : likewise the making of pearls, precious stones, china and looking-glasses : to which is added, the method of painting on glass and enameling : also how to extract the colours from minerals, metals, herbs and flowers ... : illustrated with proper sculptures / written originally in French, by Mr. H. Blancourt, and now first translated into English ; with an appendix, containing exact instructions for making glass-eyes of all colours.

CHAP. CCXXXI. To make strong Cement to mix with Lapis-Lazuli, to separate the finer and better Stuff from the other.

ONE cannot so easily part the finer Lapis-Lazuli from its grosser parts, without making use of this Cement to unbind the parts: Take four Ounces of very pure and clear Venice-Turpentine; six Ounces of Rosin of the Pine, six Ounces of Grecian-Pitch, three Ounces of very good Mastick, three Ounces of fresh Wax, an Ounce and half of Linseed-Oyl cleansed, as shall be directed in Chap. 233.

Put the Turpentine into a new glazed Earthen-Pot, very clean, to dissolve over a slow Charcoal-Fire, and continue stirring it with a Wooden-Spatula, throw into this by degrees, the Rosin of the Pine, in small pieces, and stir it still very well; thus put in suc∣cessively the Pitch, the Mastick in Powder, and last of all the Wax sliced small, stirring all continually about to mix and incorporate: Take great care of your Fire, least the Cement should blaze, or burn, all the Ingredients being hot of themselves, and combustible: Having well incorporated them, pour in the Linseed-Oyl, stirring it as before, and so let it boil gently for a Quarter of an Hour.

To try whether the Cement be enough, drop some of it off the Spatula into a Vessel of cold Water; if it spread 'tis not enough; but if it do not, 'tis sufficiently boil'd; so take it off. Or else you may wet your Fingers, and take a drop thereof, roul and draw it out in length; if it snaps and breaks of it self, 'tis a sign that 'tis enough; take it off and pour it boiling hot into an Hypocrass-Bag steeped before

in hot Water; take care to let it go all through into a Vessel of cold Water; and for the better security, squeeze it along from top to bottom with two flat Sticks, that none may remain in your Bag; afterwards work it well with your Hands, till all the Water be drained from it, and because being hot it may stick to your Fingers, you may anoint them with some of the Linseed-Oyl.

The Cement being thus prepared, keep it in a Vessel of cold Water, shifting your Water every Day, or every second Day, and by this Method you may keep it for ten Years.