2. =Sharp Toe- and Heel-Calks.=—The outer calk is split and a small steel wedge welded in. It is then laid upon the edge of the anvil, indented and sharpened from within to without in such a manner that the calk shall be thin from the branch to the ground, and the outer surface shall be in the same vertical plane as the outer edge. If a calk is narrow from its base to its end, and at the same time without flaw, it does not need a sharp cutting edge. The inner calk should never be sharpened except the ground be very slippery. The cutting edge of this inner calk stands at right angles to the length of the branch, and its outer corner should then be rounded to prevent its injuring the opposite foot (Figs. 159, 160). [Illustration: FIG. 159.] [Illustration: FIG. 160. Outer and inner heel-calks sharpened.] For horses used for heavy draft purposes a toe-calk is welded to the shoe and sharpened. For this purpose we use only steel (toe-steel), which is easily welded to the shoe and remains firm. Toe-calks and steeled heel-calks are tempered, in order, as much as possible, to lengthen their period of durability. This method of sharpening is the oldest and most wide-spread, and is employed on the shoes of all horses of which we require more than light service. [Illustration: FIG. 161. Left fore-hoof sharp-shod: _a_, toe-calk bevelled from in front; _b_, outer heel-calk directed lengthwise with the branch; _c_, inner heel-calk, half sharp and directed transversely to direction of the branch.] Hoofs are easily damaged or even ruined by frequently repeated sharpening of the shoes, because every time this is done the shoes must be removed and replaced. 3. =Shoeing with Screw Heel-Calks.=—Any ordinary flat shoe not too thin and narrow at the ends of the branches can be changed to a shoe with screw heel-calks by punching holes in the ends of the branches and cutting a thread in them. [Illustration: FIG. 162. Ground-surface of the end of a branch of shoe, showing (_a_) hole and counter-sinking for a screw-calk (one-half natural size).] The screw heel-calk holes are made either by punching or boring. The punching is done by means of an almost cylindrical hammer-punch, afterwards finishing the holes by driving through them a round punch which tapers from the middle towards both ends. On the ground-surface of the shoe the hole is moderately counter-sunk (Fig. 162, _a_), so that after the thread has been cut and the calk screwed into place the shoulder of the latter will rest on the counter-sinking. At present nearly all screw-calks are made by machinery, either of iron or toe-steel. The former is too soft and therefore not sufficiently durable; the latter, however, is quite durable when the calk is properly hardened (tempered) by heating to a cherry-red, sticking the head of the calk as far as the tap into a bed of moist sand, and allowing it to slowly cool.