In applying the stone to the mill, it is placed flat on the surface, and firmly pressed with the ends of the fingers and thumb applied on the back of the wooden disk, the upright stem passing between the fore and middle fingers. If the stone be large, it may, with advantage, at the commencement, be rubbed upon the flat face of the revolving mill with small circular strokes, and at the same time the stone may be slowly twisted round with the fingers, in order to expose it equally to the action of the mill. If the stone be small it must be held quite steady throughout the process, but in order to wear the lap uniformly, the stone is placed in a different position every time that it is rested on the mill. The velocity of the mill employed in grinding should be only moderate, so as just to avoid throwing off much of the grinding powder with the centrifugal force; the progress of the work may be expedited by using a higher velocity, but the emery and water are then thrown off so abundantly as to be very objectionable, and the condition of the work can be less delicately felt. The stone should also be pressed upon the mill with only moderate force, as great pressure is liable to cause the stone to push away the loose particles of grinding powder, and also to wear the mill irregularly, whereas moderate pressure allows the loose particles of emery to roll over between the mill and stone, and the work then progresses more rapidly, and the mill is less injured.
The stone having been made as smooth as practicable with the emery, the polishing is proceeded with in the same manner upon a mill of appropriate material, generally pewter, hacked or jarred, as explained under the head Carnelian, page 1044. and supplied with rottenstone and water. This completes the one side of the stone, and it is then detached by heat from the cement stick; and the same routine is followed with the second side.
If the stone is required to be wrought to a definite shape, as for example an oval, the edge is ground to the oval form before the sides are flattened. For this purpose a corresponding oval is cut out of card to the exact dimensions, and laid upon the stone; the oval is then marked with ink upon the stone, which is brought very nearly to the shape by means of nippers, or flat pliers of soft iron, like those employed for rounding disks of glass preparatory to grinding them into lenses, see page 1265; the nippers are firmly compressed upon the stone, and then twisted sideways to break off small particles. The hardest stones, such as sapphires, will yield to the action of the nippers, although they are scarcely ever used with valuable gems; but if the stones are smooth and rounded like the natural surface of a pebble, the nippers will slide off, and therefore such stones are first slightly roughened to give a hold to the nippers.
The stone having been nipped of the required shape, and nearly to the size, it is cemented upon a stick, the edge being left exposed, and this is then ground square by holding the stick horizontally, and continually twisting it round between the fingers to avoid grinding flat places; when the stone has been thus figured to the required shape, the flat face is ground and polished.
If the stone is required to have a bevelled edge, or chamfer around the face, the stone is first nipped to the form, then fixed on the cement stick, with the side outwards that is to form the back of the stone; the edge is ground square and the back flattened and polished if necessary. The stone is then re-cemented upon the stick with the face side outwards; the face is flattened, and the bevelled edge is then ground by holding the stick at an angle, and continually twisting the stone round to grind the chamfer uniformly. The thickness of the narrow square edge left on the stone, serves as a sufficient guide for practised lapidaries to ensure the uniformity of the bevel, but the amateur will probably find it desirable to mark a line on the edge, and also the face of the stone, to show how far the chamfer should extend.
If the stone is to have a rounded edge, it is first prepared with a bevelled edge exactly as above, and the angle is removed by a rocking motion of the stone upon the flat mill. For this purpose the stick is held underhand, being grasped between the fingers as near to the bottom as admissible, and the stick is continually traversed from nearly the perpendicular position to the angle, at which the chamfer was ground. The wrist or elbow being the center of motion according to the curvature required, and at the same time the stick is twisted round in the fingers, in order to round the edge uniformly. If the stone is to be considerably rounded over the entire face, he preparatory step of grinding the face flat may be omitted, as he stone will be left sufficiently level by the slicer, and the prin-ipal bulk of the material is removed by the chamfer, which serves as the basis or guide for keeping the rounding uniform, assisted during the principal portion of the work by the central part of the stone, not reached by the rounding until near the conclusion of the rough grinding.
If the stone to be rounded on the face be circular, it is rolled upon the flat mill with circular strokes; between every few strokes it is shifted to another part of the mill, and the stick is continually twisted round in the fingers. If the stone be of a short elliptical shape, it is treated in the same manner, except that it is traversed in an elliptical path. In the case of very long ellipses, the two sides of the ellipsis are first ground separately with a rocking motion, and the stick is slightly twisted in the fingers between every few strokes. The ends of the ellipsis are rounded in the same manner, and, lastly, it is smoothed with long, semi-elliptical strokes. The principal guide for the degree of rolling is obtained from an inspection of the progress, but the sense of feeling is also greatly trusted to by working lapidaries. Stones that are flat on the back, and much rounded on the front, are called tallow tops, from their resemblance to a drop of tallow.