To produce a finer edge than is possible with the grindstone, the oil-stone is brought into requisition, the shape of the oil-stone being varied to suit the shape of the tool. Three kinds of oilstone are in general use, Turkey stone, Arkansas stone, and Washita stone, the latter being softer and of inferior quality to the two former. The best quality of Arkansas stone is of a milky white color, of very fine and even grain, and very hard, being impervious to a file; but there are softer grades. An oil-stone should be of even grain throughout, so that it may wear even throughout, and produce a smooth and unscored edge. Arkansas stone is rarely obtainable in lengths above 6 inches, on account of the presence of fine seams of hard quartz, which wears less than the stone, and forms a projection that scores the cutting edge of the tool, and the same applies to the Turkey stones.
For tools fully hardened and not tempered the hardest oilstones are best; but for tools that are tempered, as tools for woodwork, a softer grade of stone is preferable, since it will cut the most free.
When an oil-stone has worn out of shape it may be dressed on a grindstone; but if a flat surface is required it is best to true it by a piece of coarse sand-paper laid upon a flat true surface.
The action of an oil-stone is to smooth the surfaces; but while doing this the oil-stone itself forms what is termed a wire-edge, which resembles a feather-edge, except that it is smoother and more continuous. It is caused by the weak edge of the blade giving way under the pressure with which it is held to the stone. To reduce the wire-edge as much as possible the tool is pressed very lightly to the oil-stone during the latter part of the stoning, and is frequently turned over. If the motion of the tool upon the oil-stone is parallel with the line of cutting edge, the wire-edge will be greater than if the line of motion were at a right angle to it.
Again, the strokes performed while the cutting edge is advancing upon the oil-stone produce less wire-edge than the return strokes, hence the finishing process consists of a few light strokes upon one and then upon the other facet repeated several times. Now let it be observed that, the wire-edge will never be turned toward the facet last oil-stoned, and cannot be obviated by the most delicate use of the stone; but after the stoning proper is finished, the operator will lay one facet quite level with the face of the stone, and then give to the blade, under a very light pressure, forward diagonal motion, and then perform the same operation with the other facet upon the stone, the last facet operated upon being usually the straight and not the bevelled one. To still further reduce the wire-edge for very fine work, the operator sometimes uses a piece of leather belt, either glued to a piece of wood, as upon the lid of the oil-stone box, or some attach it at each end to projecting pieces of wood, while yet others lap the tool upon the palm of the hand. In giving an edge to a razor, the process may be carried forward in the usual way by means of straps, the first strokes being long ones made under a slight pressure, the strokes getting shorter and the pressure lighter as the process proceeds, until at last the motion and contact are scarcely perceptible.
Fig. 2068.
When, as in the case of plane blades and carpenters’ chisels, the area of face is large, it is advantageous to grind the face somewhat concave, as in [Fig. 2068], so that the heel and the point only of the tool has contact with the oil-stone, thus reducing the area to be stoned and steadying the tool, because, the area being small, the heel as well as the edge may be allowed to rest upon the oil-stone without unduly prolonging the stoning operation.