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. CCLIV. How to make Spherical Concaves, and Convex Glasses, commonly called Burning Mirrours.

BEFORE we discourse of the Metal Mirrours, we will shew how to do such of Glass: The Use of these Glasses is to unite the Sun-Beams, and so kindle a Flambeau, Wood, or any other Combusti∣ble Matter. By them Metals also may be dissolved in a little time as easily as in a Crucible on a Furnace, or at a Forge.

The Whole Mystery of making them, is to have the Moulds of a Round Shape, otherwise they have but a very weak Effect on the Sun Beams; the Moulds must be so exactly made, as neither side shall differ from the other.

To make the Concave Glass, you must have the Mould Convex, and the Convexity thereof must be made by a Sphere, according as you have it greater or less; and 'tis from this Sphere the Convexity of the Mirrour must be taken: As for instance, Take a Sphere of what bigness you please, divide it equal∣ly, and also one of the Hemispheres in three equal parts, by Planes parallel to the great Circle, the Convex, Segment shall then be the sixth part of the whole Sphere, and the Measure of your Mirrour: To do this you may have recourse to the Works of Archimedes, Iohn Baptista Porta, Kircher, and many other Authors.

If you would make the Mirrour a Convex Glass, you must have the Moulds Concave, and these you may do two ways, thus: Take the two Concave Sides of the Mould, and closing them together equally, as the Founders do their Frames, pour in through the Mouth of the Mould your Crystal Metal, letting it fill the Mould, and afterwards cool. Another way is, to take two Concave Mirrours, and joyning their Faces, solder them well all about, only leaving a small Orifice, through which you may fill it with some Aqua Vitae, and so stop the Hole, and frame them with Wood, or Metal: This sort of Mirrour, has a more ready Influence on the Sun Beams then any other; we'll say something of the Effect there∣of when we come to speak of the Metal Mirrours, but upon the whole Matter you must have these Glasses all very well polished.

These Burning Glasses may be made Parabolick, or Spheroidal, and such have still a better Effect than the Spherick: You must proceed in Mould∣ing them as with the former; you must observe a just proportion in doing them; for when they are too much raised, they are hindered by their Deepness from having a good Effect; and up∣on this depends the Whole Nicity of the Art.