The hardening of a tap or hob is the simplest process of the making, and consists in heating the work to a dull red while in a clean fire of cinders or charcoal, and dipping it into water which has been in use a long time for purposes of hardening and forging. The heat of a plate-furnace is a convenient means for heating the tap if it is a long one, but short ones are easily heated in an ordinary forge fire, if the fire is at least as long as the tap, and the wind is gently administered to avoid bending the work. The bottom of the fire should be nearly flat, that the work may be supported along its entire length, this being a means of preventing the work bending while soft with the red heat. When the proper dull red is attained, it should be rolled into a few pounds of powdered prussiate of potash, if a large tap, and as soon as the powder which adheres is melted, the tap is put into the water while the length of the work is nearly vertical, but a little inclined to the water's surface; this allows the tap to bend while cooling, and tends to prevent it breaking. When in the water, it remains there till cold, when it is ready for tempering. The large numbers of taps and other tools that are broken in hardening result from using steel which has not been tested, or moderated with softening if it is found to be too brittle; there is no risk of breakage if the steel is not too solid, and the expense of breaking trial pieces of steel previous to the making is nearly nothing when compared to the breakage of tools after being lathe-turned, screwed, or planed. All trial pieces that are broken previous to the making of a tool can be used for other purposes.

The degree to which taps and hobs are tempered is the same for all sizes, and is indicated by a light golden tinge which appears on the surface if it is properly polished with flour emery cloth after hardening. All the flutes require polishing, and the entire surface of every flute; only by such means can it be known whether the tap or hob is equally tempered. The heat for tempering is slowly applied, that the interior of the tap may not remain too hard after the cutting edges are tempered; and the means of imparting the heat is a thick tube which is as long as or longer than the work to be tempered. The diameter of the hole in the tube is about three times the diameter of the tap, and the tube is heated to a yellow heat and allowed to remain in the fire while the tempering is being performed, if small taps are to be tempered, but for large ones it is necessary to bring the tube from the fire, and stand it with its length in a vertical position upon an iron block having a hole, or upon two blocks, so that a space shall exist beneath the tempering tube. While the tube thus remains, the tap to be tempered is suspended with a pair of one-sheave pulley blocks; and slowly moved up and down while in the hole, the head being gripped with a forge tongs having angular gaps and thick handles, which are tightly held with a coupling ring. It is necessary for the head of the tap to be equally hard with the other portions; consequently, it may be requisite to lower the tap until its head is at the bottom of the tube-hole, and allow it to remain there till the tong-grips are properly heated.