Fire is kindled in the firebox below and the heat gradually raised until furnace and boxes are cherry-red in color. This heat is maintained for seven to eleven or more days, depending upon the hardness desired, i.e., the amount of carbon they desire absorbed. The furnace is closed and allowed to cool slowly, which requires another seven or more days. Upon unpacking the furnace the bars are found to be brittle and of a steely fracture instead of the soft malleable material which was put in. They have become high carbon steel. Expert workmen are able to judge very closely the hardness of the steel by looking at the fracture and they sort the bars in this way, piling bars of similar hardness together. [Illustration: HUNTSMAN COKE-FIRED CRUCIBLE FURNACE—MODERN TYPE ] Bars thus made show many blisters on the surface and the steel became known as “blister steel” on this account. The reason for these blisters was not discovered until along about 1864, when the well-known English metallurgist, Percy, proved that the blisters were caused by the chemical action of carbon on the slag contained in the wrought iron. The gases formed produced the blistering of the bar. That this is the explanation is proved by the fact that bars of mild steel or iron without slag do not blister. Blister bars heated to a forging heat and drawn out under the hammer or rolled into bar steel are known as “spring steel”, or “plated bars.” As in wrought iron manufacture, a cutting to length, repiling, heating, welding and again drawing down by hammering or rolling produces much more homogeneous and reliable steel. Piled and reworked steel of this sort became known as “shear” steel because blades of shears for cropping woolen cloth were always made in this way. Many of us will recognize in the cementation process an extended “case hardening.” Case hardening is very largely resorted to by iron and steel workers, who in a few hours can give a hardened and long-wearing thin outer layer of steel to a piece of iron or soft steel after it has been forged or machined into the desired shape. [Illustration: SIEMEN’S GAS-FIRED CRUCIBLE FURNACE—REGENERATIVE SYSTEM One pair of Checker-work Chambers, k. h., is being heated by the hot outgoing flame and waste gases while the other pair is heating incoming gas and air. They are worked alternately. ] This shear steel was largely made and was quite satisfactory, until, as described before, Huntsman, a Sheffield clock maker, conceived the idea of melting together in a pot or crucible blister bars or bars of shear steel. This he did to equalize the carbon content and give uniformity of product which had never been attainable through the cementation process alone. From that date (1740) to this the crucible process has undergone only minor alterations and to-day it produces the highest grades of steel which we have. Practically all of the high grade tool steels are produced by this process.