The hard exterior of both castings and forgings rapidly deteriorates the cutting edges of the turning tools; while, in addition to their hardness, the surfaces of most castings are much impregnated with the sand of the mould, which is as destructive acting upon the tools after the manner of a grindstone. The materials therefore undergo some preparation, before they are submitted to the action of the tools in the lathe.
Iron castings that are sufficiently in excess of the finished size of the work, sometimes have the exterior removed from those parts that are to be turned by the hammer and chipping chisel, page 850, Vol. II. This method however, is neither suitable nor convenient in all cases, and when inapplicable, the exterior of the casting may be cleansed from the sand with acid. Iron castings of large size are plentifully wetted from time to time during a few days, with a mixture of about one part of sulphuric acid to three of water; dabbed on with a kind of mop, made of strips of cloth attached to the end of a stick, or bound together by a twisted hazel or willow rod, as a handle. The castings are afterwards well washed with water and scrubbed with a stiff birch or wire broom, to remove the sand from the corroded surfaces. The "pickling," unless carefully carried out, will however still leave portions on which the sand remains sufficiently to be objectionable; and if their size be not too large, it is a much better practice to completely immerse the castings for about twelve to twenty-four hours, leaving them suspended or standing in a bath of the above mixture, about three times diluted, the castings being well washed and scrubbed on their removal.
Brass and gunmetal castings when treated in the same manner, page 375, Vol. I., are placed in a bath of nitric acid, mixed with from four to six parts of water. They may also have the sand removed from their surfaces with an old file. After the cleansing by either method, brass and gunmetal castings are generally held upon the anvil, and equally hammered, to increase the density of the metal to improve its elasticity and tenacity.
The hard external scale left by the hammer and fire upon forgings, should be reduced with an old file or removed upon the grindstone; more especially from about the edges or those parts where the cut will be commenced, that at its first entry, the cutting edge of the tool may only encounter the clean metal. Steel bars or forgings are annealed or thoroughly softened as a first step; the process being to heat them to redness and then excluding access of air, to allow them to cool as gradually as possible. After the heating the forgings to be annealed are sometimes left in the center of the fire, which is banked or carefully closed all around them, the entire mass being left undisturbed until it has become perfectly cold. Small objects liable to be lost in the fire, may be enclosed in a sheet iron box or tube. Larger pieces placed in the naked fire, when uniformly heated to a dull red heat, are also sometimes withdrawn and placed on a deep layer of thoroughly dried or charred sawdust, or charcoal dust; they are then at once closely covered up with the same material to the thickness of three or four inches, the whole being left undisturbed until cold. When annealed, the exterior of the steel forging is cleansed with the file or on the grindstone, as with those of iron. Many engineers carefully anneal all small forgings whether of iron or steel, and afterwards pickle them, as with cast iron, but for a longer period. This practice is followed to entirely remove the coat of black oxide, driven hard into the surface in the forging, and its adoption causes the edge of the tool to remain in cutting condition for a much longer period than would otherwise be the case.