The substitution of stone for common brass or gold bearings, was prompted by the inevitable wear of the holes from frequent cleaning, and the abrasion of the pivots, produced by the accumulation of dust with viscid oil; the pivot being cut away, or the hole opened too large. So long as the verge and cylinder were the prevailing escapements, the necessity for jewelling was not so strongly felt, except in the balance holes. The introduction of the lever escapement brought with it a better watch,—capable of more accurate time, but demanding an improved construction.
An Italian, in 1723, first introduced the practice of using stone for bearings. He not only conceived the idea, but was successful as an artisan in making his own jewels; ingenious and skilful as he was, however, he encountered obstacles almost insurmountable.
The art of cutting gems, it is true, was at that time well understood, but no one had attempted to drill a hole in a hard stone fine enough for a properly sized pivot. The watches at that time that were jewelled could boast of nothing more than the balance holes, and they were not pierced to let the pivot through.
It is a very difficult matter to polish a taper indentation in a stone, even with modern appliances, in consequence of the tendency to create a tit at the bottom,—thus throwing the balance staff out of upright. The difficulties in the then state of knowledge retarded the general introduction of stone-work for many years. The Swiss, however, seeing the advantages derived, finally struck out the various manipulations with success. Time and experience gave more skill, and at the present time it is impossible to find a Swiss watch, even of the cheapest class, that is not jewelled in at least four holes. The English trade adopted the art later; but even then it did not become general for many years. Within a generation, only fine English levers were jewelled.
The mere substitution of a harder substance was not the only improvement; other conditions necessary to accuracy were insured. The hole could be made round—the material of such a character that no chemical action could be effected on the oil used for lubrication, and the vertical section of the hole could be made so as to present the least amount of frictional surface, yet still giving a perfectly polished bearing, thus avoiding the cutting of the pivot.
The whole “modus operandi” from the stone in the rough to the last setting up is well worth the attention of the watch repairer, and certainly that of the manufacturer.
Of the materials used in the trade, the first and most important is the diamond, used only in the time-piece as an end-stone—but at the bench all-important, as a means of making the other jewels. The diamond possesses the requisite susceptibility of polish, combined with greatest hardness of any substance known; but this adamantine quality precludes its being pierced with a through hole. Considered chemically, the diamond is pure carbon,—its different varieties differing only in structure—common charcoal, its lowest—plumbago, its intermediate grade. Another variety, called the “black diamond,” or “diamond carbon,” occurs, which is interesting as being a parallel with emery, compared with crystallic sapphire. The form of diamond most in use for mechanical manipulations, is almost always crystallized; yet it will be seen that the agglomerated form of diamond carbon plays no unimportant part in jewelling. As a jewel, no use is made of the diamond, other than as an end-stone. Marine chronometers, in which the balance will weigh from five to nine pennyweights, are almost invariably furnished with a diamond end-stone, set in steel. Yet, hard as the substance is, it is often that a pivot will cut an indentation in its face. The cause of this apparent anomaly is to be found in the structural character of the gem, and its value. The lapidary, saving in weight as possible, does not care, in “Rose Diamonds,” to pay attention to the lines of cleavage. If the face of the stone makes a slight angle with the strata of the jewel, there occur innumerable small angles of extreme thinness—the pivot, coming in contact with any of these thin portions, may fracture it, and the fragment, becoming imbedded in the tempered steel pivot, becomes a drilling tool. In our experience we have had marine chronometers sent for repair, that have lost their rate so much as to become utterly unreliable from this cause alone—the pivot having produced an indentation of the stone, creating more friction, and thus destroying the accuracy of the instrument.
As a general rule, the rose diamonds sold for this purpose are sufficiently good for general work. In a very fine watch or chronometer the stone should be selected with reference to its polish on the face, and its parallelism in the lines of cleavage. The diamond, however, gets its great importance from being the only agent we can use in working other stones. Without it the whole art of jewelling would not be practicable. The various steps are all connected some way with diamond in its different shapes. “Bort,” the technical name for another variety, is merely fragments of the stone that have been cleaved off from a gem in process of cutting, or gems that have been cut, but found too full of flaws to become of use for ornamental jewelry purposes, the cost depending on the size, varying from $5.50 to $18 per carat. This “Bort” is used as turning tools—the larger pieces being selected and “set” in a brass wire and used on the lathe, in the same manner, and with the same facility, as the common graver. For tools, even the diamond is not of equal value—a pure white and crystalline in structure generally being too brittle (though hard) to endure the work. Among the workmen the “London smoke,” a clouded, brownish stone, is most prized—it possessing the twofold qualities of toughness and hardness.
Another form of “Bort” comes in the shape of a small globule, sometimes the size of a pea; it is crystallic, and when fractured generally gives very small, indeed minute pieces of a needle shape. These are carefully selected, and form the drills with which the English hole-maker perforates the jewel. These drills, when found perfect, for soundness, form, and size, are very highly prized by the workman, as the choice of another, together with the setting, will often take a vast deal of time and labor.
“Bort” is also used in the making of the laps or mills with which the jeweller reduces the stones to a condition for the lathe and subsequent processes. For this purpose such pieces as are not fit for cutting-tools, or drills, are selected. A copper disk, having been first surfaced and turned off in the lathe, is placed on a block or small anvil; each piece of stone is then separately placed on the copper, and driven in with a smart blow—care being taken that no place shall occur in the disk that does not present, in revolution, some cutting point. It would seem impossible to retain the diamond fragment, but it must be remembered that the copper, being a very ductile metal, receives the piece; the first rubbing of a hard stone then burnishes the burred edges of the indentations over every irregular face of the diamond, leaving only a cutting edge to project. The rapidity with which such a lap, well charged, will reduce the hardest stone, is somewhat marvellous. It is the first tool used in jewelling, and so important that a more detailed and explicit description of its make will be given when the process of manufacture is treated upon.