Although the term lapidary work, may seem to be applicable to all the various modes of working or finishing stones, it is restricted to the cutting, grinding, and polishing of gems and small stones, and some other materials, principally for jewellery, or mineralogical specimens.
The lapidary never employs abrasive materials in that which may be called their natural or unprepared state, in the manner that the grindstone or oilstone are employed for restoring the edges of tools; but he uses the several abrasive materials in a pulverized form, and upon revolving disks of metal and other materials by way of vehicles, thus constituting artificial grinders, which he denominates as mills; thus we have the slitting mill, the roughing mill, the smoothing mill, and the polishing mill, all generally of metal; but for soft stones the smoothing mill is sometimes a plain disk of willow wood or mahogany. The polishing mill is sometimes composed of a spiral coil of list placed on edge like the leaves of a book; sometimes of bristles like a brush, or of wood covered with buff leather, which several apparatus are fully described under the head Wheels, in the Catalogue of Abrasive Processes at the commencement of this volume.
The general succession in which these mills are employed by the lapidary for substances of different degrees of hardness, is also briefly explained in the catalogue under the three heads, Alabaster, Carnelian, and Sapphire, stones that differ considerably in hardness, and have therefore been selected as general examples; under each head is appended a list of such gems and other substances as are worked by the lapidary in a similar manner. The principal peculiarities in the methods of working other stones, are also mentioned in the catalogue under their respective names, as Agate, Amber, Avanturine, etc.
The general remarks offered under these heads, have greatly abridged the observations to be submitted in the present chapter, which will be confined principally to a description of the apparatus, and the details of manipulation, which are nearly alike in working corresponding forms in either hard or soft stones; the principal difference being that the polishing mills are composed of hard or soft materials, according to the degree of hardness of the substances to be polished.
The apparatus commonly employed by the practical lapidary will be first described, followed by some account of the methods of producing the more usual forms met with in lapidary work, and the modifications in the apparatus generally employed by amateurs for similar purposes will be subsequently adverted to.
All the mills of the lapidary revolve upon vertical spindles or axes, so that the disks travel horizontally, which is just the reverse of the position employed by the cutler, and also by the glass and gold cutters, although the two latter classes of artizans are in the frequent habit of working analogous forms, and in some few instances, as in the case of cutting facets on amber beads, the gold cutters are considered to excel the ordinary lapidary.
Flat and rectilinear works are, in all their stages, ground and polished upon the broad flat surfaces of the lapidary mills, which revolve with moderate velocity, and the work is held almost stationary, much the same as in lapping flat works in metal.
Convex works are roughened on the ordinary flat roughing mill; but they require to be continually rolled about to bring every part in quick succession into contact with the mill or disk, as holding the stone at rest would inevitably wear down a flat place. Convex works in soft substances are in general smoothed on a wooden disk, which, from its elasticity, and also from its surface wearing slightly rough, or fibrous, yields a little to the stone, and does not meet it so rigidly upon one mathematical line as the unyielding metal disk. The list mill, from its pliancy, is also very well adapted to convex works, and is commonly used for glass; the leather and brush mills are also occasionally employed for rounded works.
Concave works necessarily require mills that will penetrate into their cavities; the rounded edge of the disk is in this case used somewhat as a glass-cutter would grind a transverse flute, except that the work is held at an angle instead of parallel to the axis. But when the cavity is required to be spherical, or curvilinear in two directions, the grinder is required to be of a bulbous form, and of the suitable diameter for the required curvatures, and the grinding and polishing tools must be all turned to the same diameter.
In Germany and other parts of the Continent, where large quantities of common lapidary works, such as seal handles, are executed, water-power is generally employed for driving the mills, which for these works are, in some cases, mounted upon horizontal spindles, and the edges of the mills are then principally used. The cutting of diamonds has been slightly noticed in vol. i., page 176; as there mentioned, the facets are cut by cementing two diamonds upon the ends of two sticks, and rubbing them together. The facets are afterwards polished upon an iron skive or mill, charged with diamond powder; considerable pressure is exerted upon the stone, and in Holland, where the greatest quantity of diamonds are cut and polished, horse power is generally employed, or the mills are driven by one or two men. In the latter case the driving-wheel is about 6 feet diameter, and instead of being mounted vertically, and driven by a winch handle, as usual in this country for similar purposes, the driving wheel is mounted to revolve horizontally, upon a vertical spindle, having a crank of small radius, to which the motion is communicated by a connecting rod leading to a wooden frame, that swings horizontally upon pivots at the one end, like an ordinary gate, and has at the other end two upright handles, by which it is pushed alternately backwards and forwards.