This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
The mending of defective places in grindstones is best done with a mass consisting of earth-wax (so-called stone-pitch), 5 parts, by weight; tar, 1 part; and powdered sandstone or cement, 3 parts, which is heated to the boiling point and well stirred together. Before pouring in the mass the places to be mended must be heated by laying red-hot pieces of iron on them. The substance is, in a tough state, poured into the hollows of the stone, and the pouring must be continued, when it commences to solidify, until even with the surface.
The stone should not be left with the lower part in the water. This will render it brittle at this spot, causing it to wear off more quickly and thus lose its circularity. It is best to moisten the stone only when in use, drop by drop from a vessel fixed above it and to keep it quite dry otherwise. If the stone is no longer round, it should be made so again by turning by means of a piece of gas pipe or careful trimming, otherwise it will commence to jump, thus becoming useless. It is important to clean all tools and articles before grinding, carefully removing all grease, fat, etc., as the pores of the stone become clogged with these impurities, which destroy its grain and diminish its strength. Should one side of the grindstone be lighter, this irregularity can be equalized by affixing pieces of lead, so as to obtain a uniform motion of the stone. It is essential that the stone should be firm on the axis and not move to and fro in the bearings.
Complaints are often heard that grindstones are occasionally harder on one side than the other, the softer parts wearing away in hollows, which render grinding difficult, and soon make the stone useless. This defect can be remedied completely by means of boiled linseed oil. When the stone is thoroughly dry, the soft side is turned uppermost, and brushed over with boiled oil, which sinks into the stone, until the latter is saturated. The operation takes about 3 to 4 hours in summer. As soon as the oil has dried, the stone may be damped, and used without any further delay. Unlike other similar remedies, this one does not prevent the stone from biting properly in the oiled parts, and the life of the stone is considerably lengthened, since it does not have to be dressed so often.