Is a natural combination of lime and fluoric acid, and the workable variety is peculiar to Derbyshire, where the art of turning it is carried to great perfection. The most costly varieties are the deep blue and purple, found only at Castleton in that county. Fluor being an aggregation of crystals, all having a fourfold cleavage,* is very difficult to turn, as the laminae are easily split; few even of the best workmen can turn it into very thin hollow articles; the following is the process.
The stone is first roughed out with a point and mallet. Then heated till it will readily frizzle yellow resin which is applied all over it; this penetrates about one-eighth of an inch and holds the crystals together. It is next rough-turned, and a little hollowed; it is again heated and resined, and turned still more into form, then it is bound round with a thin wire, and again resined, and so on till it is sufficiently thin to show the colours, it is then resined for the last time, and polished in the same manner as marble; but the process is more difficult, and ultimately but very little resin remains in the surface of the work. The only tool used is the steel point.
The blue colour of fluor is often so intense that the works marble in hardness, the soft wear away too readily, and the hard too slowly, so as to prevent a plane uniform surface from being produced; some judgment is therefore required in their selection.
* This is easily seen in the cubical crystals of common fluor spar, of which any of the angles can bo very readily split off with the penknife, leaving a triangular facet; the eight angles give four pairs of parallel cleavages, and when sufficiently pursued they convert the cube of fluor into the octohedron, its primary form.
cannot be wrought thin enough to show it: when this is the case the stone is very gradually heated in an oven until it becomes nearly red-hot, when the blue changes to an amethystine colour. Great care is required, for if suffered to remain too long the colour would entirely disappear. The white and lighter kinds of fluor are not worth one-tenth of the value of the blue, but arc wrought in the same manner for commoner works.