For this purpose, bars or pieces of iron are buried in powdered charcoal, covered up in a vessel, and kept at a red heat for a greater or less number of hours, according to the object desired. There are niceties in the process, which it is not necessary to explain, that produce the peculiar quality of steel, as distinguished from cast iron. If the operation of heating the iron in charcoal is continued too long, or the heat is too great, the iron becomes cast steel, and cannot be welded; but if it is not melted in the operation, it can be worked with the hammer in the same manner as iron. In each case, however, it has acquired the property upon which the keenness of the knife depends; and the chief difference between the cast steel, and the steel that can bear to be hammered is, that cast steel takes a keener edge, but is more easily broken. [Illustration: Shear and Tilt Hammers: Steel-manufacture.] The property which it has acquired is that of bearing to be tempered. If it be made very hot, and plunged into cold water, and kept there till it is quite cooled, it is so hard that it will cut iron, but it is brittle. In this state the workman brightens the surface, and lays the steel upon a piece of hot iron, and holds it to the fire till it becomes of a colour which he knows from experience is a test of the proper state of the process. Then he plunges it again into water, and it has the degree of hardness that he wants. The grinding a knife, and the polishing it, even when it has acquired the requisite properties of steel, if they were not done by machinery, would cost more than the whole price of a knife upon which machinery is used. A travelling knife-grinder, with his treadle and wheels, has a machine, but not a very perfect one. The Sheffield knife-maker grinds the knife at first upon wheels of immense size, turned by water or steam, and moving so quickly that they appear to stand still--the eye cannot follow the motion. With these aids the original grinding and polishing cost scarcely anything; while the travelling knife-grinder charges two pence for the labour of himself and his wheel in just sharpening it. [Illustration: File-cutters.]