Greetings all from warm, humid NSW Australia,

I found this article in the May 12th 1955 Model Engineer magazine (I'm a 
tad behind in
my reading backlog!!!)

Was sumbitted by 'Bud' of Oakland Ca.

'Procure some sulphuric acid of about 1.250 specific gravity...........
Procure an earthenware or glass pan large enough so that the files can 
be placed flat on the
bottom..........
Clean the files with a file card or wire brush. I use a needle to flick 
off any metal particles
stuck in the grooves.
Place the files in the pan and pour in the 1.250 S.G. acid solution 
until they are covered.
Or the acid  can be poured in first .......
Very shortly small bubbles will form on the files as the acid eats into 
the grooves.
If this does not happen within five minutes the acid is too strong. 
Simply pour in
more water (!!!!!!) and stir.
The entire secret of sharpening files is to use weak sulphuric acid, 
since weak acid
eats steel, whereas strong acid does not.
Leave the files in for a couple of hours and then feel them to see if 
they are sharp.
Very dull files that are almost smoothe should be left in for eight hours.

When the files are really sharp, remove from acid and wash thoroughly in 
water.
When they are dry they will have a slightly brownish tinge and unless 
given a coat of oil
will rust rapidly................

(Safety with acid mentioned here re pouring acid into water not water 
into acid, it will
eat clothes with the exception of wool, fumes are dangerous)
Bicarbonate of soda (baking soda) solution will neutralise sulphuric acid.
1 tablespoon per quart of water solution is useful.

The 1.250 S.G. solution can be reused. Pour the acid solution out of  
the pan into
a glass jar leaving the black residue in the pan to be washed out.....

Do not take files out too soon. It is better to leave them in too long 
than not long
enough. They may feel sharp but are actually not etched deep, and if 
taken out
too soon will become dull after a short time.

After you have sharpened the same file for 20 years (!!!!!) it will 
become so thin
that you can bend it around in a circle (!!!!!!) '

So, dear readers, that's it, all you ever wanted to know about acid 
sharpening
of files.
I make this disclaimer:-
I have merely written in part someone else's ideas on the subject.
I take no responsibility for any accident, mishap or injury caused to anyone
using this information. ACID IS DANGEROUS!!!!

Peter B

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