To harden a tap, heat to a cherry-red, in a clear and not a blazing fire, or, what is better, heat in charcoal, holding the tap by the square end; dip it endwise in the water, immersing the whole of the threaded part first, and holding it still until the plain part is of a very low red; slowly immerse the remainder, holding it still, when fully im-mersed, until it is quite cold. Then brighten the flutes, and temper as follows: Heat a piece of tube (with a bore about twice the diameter and a length one half that of the tap) to a bright red heat, take it from the fire, setl it up vertically, and hold the tap in the centre of the tube, with the plain part the tap in the tube and the thread part projecting. Revolve it in this position sufficient time to heat the plain end about as warm as the hand can bear then keep revolving the tap and moving it endwise back and forth through the tube until the thread will color to a deep brown and the shank to a brown purple. If any unevenness appears in the color, hold the parts having the lighter color a little longer in the tube, or cool the part coloring too deeply by applying a little oil to it. Perform the whole process slowly, taking the tap from the tube to retard it, if necessary.