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
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
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.
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.