44. - In Lapping Penknives and Small Articles, it is more usual to charge the wheel whilst it is at rest, by rubbing on it a lump of emery cake, made of emery compounded with suet chopped fine and rendered down, and mixed with a very little wax, sometimes the dressing is rubbed in with the agate or bouldering stone, and as before explained, to fine the lap, at the conclusion the head is rubbed off and it is smoothed with the agate.

When the lap is coarse and the work is pressed heavily it produces a white colour on steel, and when the lap is fine and the work is pressed lightly, and gradually drawn from the one end to the other it gives a black polish - to attain this end the emery is worn down fine with the work, and afterwards with the bouldering stone, and the effect of the emery is still more deadened by putting a little bees'-wax on the face of the lap, the smoothness of which is tried with the finger before applying the work.

45. - Comparative Durability of Laps. - The metal wheels and grinding tools are from several reasons highly advantageous, as in the first place they admit of being fashioned with more exactness, and they longer retain that exactness than the natural stones and the compositions previously referred to. For instance when grit stones are in use, desintegration is constantly and rapidly going on, as in the course of work the particles of the stone are rubbed down and torn out; so that the abrading surface is incessantly changing, by the gradual exposure of the part of the grindstone previously beneath. Much care is required to keep the edge of the stone circular and of the precise form required.

With the cement wheels, this progressive change as constantly, although more slowly occurs, from the abrading and structural materials being mingled.

46. - On the Action and Durability of Laps. - Metal laps are under very different circumstances from grindstones or cement wheels, as the metal constituting the lap has no cutting power in itself, but only derives it from the particles of emery which become embedded in its surface and act as the teeth of a file. Other particles of the emery lie continually between the metal lap and the article to be ground, and separate the two; these grains have a partially rolling motion and, in all probability, have a tendency to grind both the work and the lap also. When the emery is crushed very fine, or that it is wasted, so that the lap and work come nearly in contact, the abrasion becomes so much reduced that fresh emery is generally thrown on to restore the action, and this again separates the lap and work; which therefore rarely come into absolute contact. It must not be supposed, however, that although the metal is generally more cohesive than stone or cement, that it is not at all worn away, as the metal laps are likewise depreciated in form, but in a much slower degree than the cement wheels or natural stones.

47. - Metals employed for Laps and their Respective Purposes. - In the selection of the metals for laps, there is much of prejudice, and speaking generally it may be said the softer the metals the more readily do they retain the grinding powders, but the sooner are they worn out of form. In the following tabular view the more usual metals for laps and their purposes are given.

Brass is used by Opticians, with fine emery and water for smoothing lenses and specula.

Cast-iron is used by Glass-grinders, with coarse sand for roughing; Opticians, with sand or emery for rough grinding; Engineers and machinists, with emery and water for general purposes, in metallic construction; Diamond polishers, for polishing the facets of diamonds for jewellery; the iron laps or skives are charged with diamond powder.

Copper is used by Engineers and machinists, with emery and water for general purposes, in metallic construction. Copper is considered to retain the emery remarkably well;

Lapidaries, with flour emery for grinding small and hard gems, and for cutting facets;

Glass grinders, with emery for fitting stoppers into bottles;

Glass engravers, with emery for their small disks and tools.

Lead, generally alloyed, is used by Engineers and machinists, with emery and water, for metallic construction generally; Cutlers, with emery and oil, for fine grinding or perfecting the forms of cutlery prior to polishing the pieces; Lapidaries, with emery, first coarse and then fine, for grinding and smoothing most stones, except some few of the hardest, which require copper; Lapidaries, with rottenstone and water, for polishing most of the stones, except a few of the hardest, which require hard pewter or copper; Lead, mixed with a variable quantity of antimony or alloy, like type metal, is much used by engineers and mechanicians for laps.

Pewter is used by Gold cutters, for cutting and faceting gold and silver, to which a most splendid lustre is given by means of crocus, which is generally rubbed into the lap with the burnisher;

Watchmakers, with crocus or red stuff as above, for polishing some of their brass and steel works;

Lapidaries, with emery for fine grinding, and also with rottenstone for polishing, - pewter being selected for those small and hard stones, for which lead is too yielding.

All these artizans select in preference the metal of old pewter plates, which consisted of pure tin with a minute addition of copper. Some of the modern pewters appear to be tin and lead in nearly equal parts, and are much the same when used for laps, as lead hardened with a little antimony, which is much less expensive. See articles Pewter, vol. i., page 284, and Lead, page 277 of the same volume.

Tin may be considered as being applicable to all the purposes of the genuine old plate pewter, which is now difficult to be met with.

Zinc, alloyed with tin, which is much harder than tin or pewter, is said by Mr. Gill to be employed by the Geneva jewellers in lapping gold and silver works.