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. XCII. The Way of preparing Natural Crystal.

WE have already given a very fine Preparation of Natural Crystal in the second Book, Chap. 76, yet we will give you here the principal one, which is its Calcination, and which we will explain at large.

To perform this Calcination well, Take Natural Crystal the fairest you can get, since that is the Basis for Artificial Gems, it is no matter whether it be in great or small pieces. Fill with these Pieces of Cry∣stal a large Crucible, cover it with a Cover made of the ••ame Earth, which must be somewhat broader than the Mouth of the Crucible, for fear Ashes or Coals tumble in, which you must take particular care of. Then set your Crucible in burning Coals in a little Furnace, and when your Crystal is well heated, cast it into a Vessel of cold Water, the more Water there is, so much the better will the Calcina∣tion be performed, because the cold of it is so much

the greater. Then take it out of the Water, and dry it in an Earthen Ladle, then put it into the same Crucible to be heated again, taking care to cover it well: Then cast it again ••nto fresh cold Water, and repeat heating it and quenching it thus twelve times, changing each time the Wa∣ter. You may know if your Crystal be well calcined, for it will easily break and crumble: If there appear in it any black Veins, you must take care to take them all clean out, by breaking them, and do this till only the White remains behind; then it is well prepared.

After you have well dried your Crystals thus cal∣cined, grind them to an impalpable Powder on a Marble or Porphyry Stone, by putting a little on at a time, and fearce it well through a fine Silken Sieve.

And since we use this Powder of Crystal for all Artificial Gems whereof we are going to treat, your best way will be to keep a good quantity by you, which you may always have recourse to in Work∣ing.

If you design to succeed in this Art very well you must not use ordinary Fritt of Crystal, how good and fair soever it be, nor Chalcedony, nor Tarso, nor any other Stones; for the Glass made of them is far less fàir and resplendent than that made of Natural Crystal, which has the most lustre, and approaches nearest precious Stones, as we have already remark∣ed in the preceding Chapter.