The materials from the mineral kingdom may be divided so far as regards these pages into two groups; the earthy, and the metallic.
The metallic minerals, consist in general of metallic oxides, combined with a larger quantity of some base, such as silex, clay, or sulphur, which are the most common mineralizers; the cohesion of the mass has in general to be overcome by heat, which destroys the affinity of the component parts, and allows of the separation of the metals in various ways. Of these processes the author will have scarcely anything to say; but the metals themselves, when so obtained, will be treated of at some length in the succeeding chapters.
The earthy and crystalline mineral substances are less frequently worked by the amateur, than the metallic, and therefore they will be noticed rather briefly, and in the order of their hardness, as derived from the following table. The instructions on a variety of processes of grinding and polishing these earthy substances, will be found in the chapters of Vol. III., to be exclusively devoted to that subject under its numerous modifications, as applied to the materials of the three kingdoms generally; both as the exclusive means for the production of form, and also for the embellishment of surface, or polishing.
Lead, Steatite or Soapstone, Meerschaum .
Jetetc. . .
Calcareous spar .
Platinum, Gum Metal. . .
Apatite . . .
Soft-Iron . . .
Silex . .
Sandstone, Sand . . .
Hardest Steel . . .
Sapphire . .
Ruby and Corundum . . .
Cuts all substances . .
The annexed table exhibits the relative degrees of hardness of the several substances in the estimation of the mineralogist; thus talc may be scratched by gypsum, gypsum may be scratched by calcareous spar,the last by fluorspar, and so on throughout; in the next column are named some of the minerals, metals and other substances of similar degrees of hardness; and the last column contains the number of minerals, which in respect to hardness arc ranked under each of the ten grades.
In the several practices of working these numerous substances, structure must also be taken into account, or the mode in which their separate particles are combined; thus hardened-steel, quartz, granite, and sand-stone, are each included under the number 7. The particles of the steel, however, are much more firmly united than those of the glassy crystalline quartz, which is far more brittle; and still more so than the aggregations of crystals in the granites; the last may be wrought by sharp-pointed picks and chisels of hard steel, which crush and detach, rather than cut the crystals; and although sandstone consists almost entirely of particles of silex cemented with silex, still as the grains of the sandstone are but loosely held together, it may be turned with considerable facility with the tools used for turning marble, and which is the every day practice in turning the grindstone. Whereas granite which contains from half to three-tilths felspar, a substance softer than silex, and porphyry which consists of crystals of felspar embedded in a base of felspar, cannot be turned with steel tools at all.
Several mineral substances are formed by the successive deposition of their component parts in uniform layers, as in mica and slate; or in alternate depositions, as in the Yorkshire flags or sandstones. Mica may in consequence be split into leaves even so thin as the one 50,000th of an inch; it is used by the optician in mounting objects for the microscope, and is often misnamed talc; slate may be split into very thin leaves of considerable size; and those sandstones which result from the recomposition of granite, arc most readily split through the layers of minute scales of mica, which being lighter than the other ingredients, are deposited in separate layers.
Many hard substances, as the agates, carnclians, and flints, show neither the crystalline nor lamellar structure, and break with a fracture termed the conchoidal, of which the broken flakes, of glass, flint, and pitch, may be taken as familiar examples.* Hard crystalline gems, on the other hand, are formed of laminae, arranged in various directions, and may be readily split by the hammer and chisel through their natural cleavages or joints; but in most of the earthy minerals, grinding is resorted to for obtaining the ultimate and defined shapes, the consideration of which methods are for the present deferred. Should none of these processes be resorted to by my readers, they will at any rate serve to explain the broad features of the respective influence of the mineral materials, (amongst others) upon tools; which is undoubtedly an important link in our subject, and one full of general interest and variety, from the diversity of the methods which are pursued in such of the useful and ornamental arts as require these natural mineral substances, that include both the softest and hardest solids with which we are acquainted.
The hard mineral substances are mostly attached to the lathe, by resinous cements, as driving them into hollow chucks, like pieces of wood or metal, would endanger their being broken, from their crystalline nature. The soft cements consist of about half a pound of resin, one ounce of wax, and any fine powder, often the fine dust from the stones that are turned; pounded brickdust and coarse flour are used, and pitch also enters into the composition of other kinds. Shell-lac either alone or mixed with half its weight of finely-powdered pumice-stone is sometimes employed; and fine sealing-wax, which is principally shell-lac, is used as well as many other kinds of cement. The stone is in general warmed to the melting point of the cement; but sometimes the latter is melted by friction alone.