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  Sharpening Irregular Tools by Scientific American, August 8, 1856  

When an angle or cutting edge has become rounded off, or dulled, a removal by any means of a coating from the surface, will restore the sharpness of the angle.

This principle has been made to some extent available in a number of ways, one of the most obvious of which is the sharpening of old files by heating them to redness in a common forge fire, and plunging them in cold water - in other words, by heating and hardening them over again.

There is nothing in the hardening itself which is at all superior to that originally imparted to the file by its manufacturer, indeed it is generally much better hardened by the professed file maker than any smith can subsequently do it ; but the files so treated are very generally improved by the operation, on account of their superior sharpness.  One effect of the heating of the steel in the open air is to oxidize a thin coating, or in ordinary language, to "raise a scale " on the whole surface, and this increases the sharpness of each tooth of the file.

The operation is, of course, of no service unless the teeth have become very dull, and it is inexpedient to employ this means of rejuvenating more than once.

The cut annexed, Fig. 1, explains very clearly how the removing of a uniform coating can increase the sharpness of a dulled edge.

Fig. 2 shows the same effect applied to the teeth of a file, and also shows why it is not well to attempt to repeat it several times, owing to the gradual obliteration of the teeth, even if the scale could be made of exactly uniform depth in each instance.

The successive lines, A A, B B and C C, show the outline of the surface after one, two and three repetitions of the process.  In practice there are two objections to this method of sharpening files, aside from the difficulty of hardening them properly: these are the irregular depth of the oxidation, which gives a very rough and imperfect form to the teeth, and the decay or rotting of the steel.

Steel may be worked an indefinite number of times if it is well hammered at each operation ; but when, as in this method of sharpening files, the metal is more than once heated an d hardened without hammering, its cohesion becomes enfeebled, and after several repetitions of the process it cracks into fragments.

A better way of producing a similar effect on fine articles is to corrode the surface by the application of diluted acid.  This is subject to the same evil of irregularity and roughness as the other, but produces no bad effect on the character of the metal.

In fact, it has been affirmed to improve the quality of poor or imperfectly hardened steel.  Any of the acids which bite steel can he employed, but sulphuric or vitriol is generally preferred, using only about one part acid with from ten to twenty parts water. 

Knives after being thoroughly cleansed of grease and allowed to lie some half an hour in this bath receive a smooth fine edge with very little whetting; and although the action is rather too slow and feeble to be generally applicable for renovating files, it may be used with very good effect on sickles and the like toothed cutters. The work should be very thoroughly rinsed in pure water after its removal from the acid, and then dried.



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