A Treatise on Diamonds, and Precious Stones/Chapter 2/Section 11
SECT. XI.
On cutting and polishing precious Stones.
The cutting and polishing of precious stones forms the business of the lapidary, an art materially differing from that of the diamond cutter.
The lapidary's apparatus[1] consists of a bench and several mills, adapted to the various operations he has to perform. They receive their motion from a wheel, which is turned with the left hand, by means of a crank. The mill used for cutting or rubbing down the stones, is a horizontal plate of soft lead[2]; its surface is supplied with wet emery powder, which becomes indented in it by the pressure of the stone.
When a stone is to be polished, the lead mill is removed and replaced by one of pewter,which is first prepared by what is termed hacking. This operation is performed by holding the edge of a knife perpendicularly upon it, and turning the mill in contrary directions, until a rough surface is produced. Rotten-stone is then applied to it, with a little water, and the stone is held on the mill until it has received the polish required.
Soft stones, as Opals, Peridots, &c. are polished on mills covered with cloth, and oxide of tin is used with a little water, instead of rotten-stone.
Gems that require nicety in cutting, whether in steps or brilliant, should be given to an experienced workman. The operation is commenced by holding the stone between the fingers until it is rubbed down into the shape required, first forming the table, next the bizel and girdle, and lastly the collet-side, This done, the stone is cemented to a round stick, four or five inches in length. The ribs and facets are formed by holding the stick in an inclined position on the mill, and supporting the other end in a notch on the gin-peg: this instrument is an upright peg, fixed in the bench by the side of the mill, containing holes or notches, one above another, to receive the end of the stick: thus the angle the facet makes with the girdle is greater or less, as the stick is inserted in a higher or lower notch. The peg not only tends to keep the hand steady, but is the cause of the facets being cut true, and always forming the same angle. In step cutting, it is particularly useful; otherwise it would be difficult to place the rows of steps parallel to each other.
To assist the lapidary, quadrants, angles, &e. have been formed with mathematical exactness, in which the stone is placed to be cut and polished; but our workmen have rejected their use, considering them only fit for novices, and depend wholly on tact and a well-practised eye.
The slitting of precious stones, which is a very important branch of the lapidary's business, is performed by a horizontal mill of thin iron, properly hammered, on the edge of which diamond powder is placed, and pressed into it so as to form teeth. Some nicety is required in corning the mill. The best method is to put half a grain of diamond powder in a watch glass, with a few drops of oil, and, with the point of a pen, to place a minute quantity ona small circular instrument, called a Gigger, which is then held against the edge of the mill, while itis gently turned round. By this means the diamond powder is equally distributed. Practical lapidaries usually put a small quantity of powder on a block of steel or a smooth stone, with a drop of oil, and rub it with a muller to separate the particles; they then apply it to the mill with the finger, and set it in the edge by holding a piece of glass against it, at the same time turning the mill round gently.
Oil must be copiously supplied during the operation of slitting; and if the mill be properly corned, it may be worked a whole day without requiring any additional diamond powder. Lapidaries use Oil of Brick, on account of its great fluidity; but as its smell is extremely disagreeable, warm olive oil may be substituted.
THE END.