Although smooth files have many more teeth, they are not proportionate in labour ; since more strokes can be made in the same time, as they are of less magnitude. In cutting a flat side, about inch broad, of the bastard cut fineness, a quick workman will make about 300 strokes, and as many teeth in one minutt.
The smaller files are generally cut by women and chil dren, who very soon acquire great dexterity.
The file-cutter, whatever may be the degree of fineness of the file, depends much more upon his feeling than his eyes. Indeed their eyes are frequently directed to other objects while the chissel and the hammer are going at the full rate.
When one tooth is formed, the edge of the chissel and the surface of the file being both very smooth, the former is pushed up against the back of the first tooth, which can be much better felt than seen. By this succession of stroke and motion of the chissel, to feel the last tooth, the work is performed, althorigh the eye is at a considerable distance from the work.
When the files are cut, the next process is to harden them. This is effected by heating them to redness, and quenching them in cold water. Some previous steps are taken to prevent the action of the oxygen of the atmosphere upon the file when red hot, and a peculiar manner of im mersing the file in the water, which we.shall more parti cularly dwell upon.
The preparing process has been improved within these ten years, so far as regards economy. The files were, be fore that time, first smeared with the residuum of ale bar rels, commonly called ale grounds, and then covered over with common salt in powder, which was retained merely by the adhesive nature of the ale grounds. They were now dried before the fire. The files were now taken once or twice and heated in a smith's fire, made of small soaks, frequently moving the file backward and forward, in order to heat it uniformly red hot. At this period the file gives off a white vapour from the surface, which is the salt in the act of subliming. The surface appears at the same time covered with the salt in a liquid state, which, like a varnish, preserves the surface from the oxygen of the at mosphere, during the time it is red hot. The file is now held in a perpendicular position, and the immersion in the water commences at the point, slowly depressing it up to the tang, which should not be hardened. All files are clip
ped in a perpendicular direction. Those, however, which have a round side and a flat one, are moved also in a hori zontal direction, with the round side foremost. ' Without this precaution, files of this shape would warp towards the round side. This arises from the flat side having been more hammered than the round side, which is formed by the concave die, and does not acquire the same density which the hammer gives.
It is common after hardening to temper most cutting instruments. Files, however, are never tempered at ad by the maker. Nor any but rough and the Ilstard-cut files tempered by those who use them. If these were not in some cases tempered, the points of the teeth would break, and the file would do but little service.
When files are hardened, they are brushed with water and coak-dust. The surface becomes of a whitish-grey colour, as perfectly free from oxidation as before it was heated.
In applying the salt as above directed, a very great pro portion of it is rubbed off into the fire and is lost. The consumption of salt used in this manufacture at Sheffield alone, amounted to about 10001. annually. The economy with which it is now used, hag reduced this quantity to less than 3Q01. This saving is effected by mixing ale grounds and the salt together, the salt being in such proportion as just to be taken up by the aqueous part of the grounds, which should not exceed 31b. of salt to 1 gallon of ale grounds. The files require only to be smeared thinly with the mixture, which, when dry, adheres firmly to the surface, till the salt fuses. The manner of heating files for hardening has been also improved. Instead of putting the files singly into a conk fire, a fire-place or oven is formed, into which the blast enters. Two iron bearers are placed on the upper part of the cavity to support a number of files at once ; these are heating gradually while the work man continues to select the hottest, and in a hotter part of the fire gives them the full degree of heat required for dipping them into the water.
Some manufacturers pretend to possess secrets for har dening, by introducing different substances into water, such as sulphuric and muriatic acid. The quantities, how eve•, are so small, that if those bodies could be shown to Possess any such nualities, the effect must be trifling.