The degreasing has nothing to do with preventing explosives.
Therefore the amount of crease is not enough on a file.
The acids are strong enough to do there own decreasing. However the
reason is, that you want to etch the surface of your rather expensive
file very even. If the files are greased or otherwise dirty, parts
the acid will react later to the dirty steel than the clean parts.
The teeth of the file will not be of the same height after etching.
The mixture I gave was from a book of 1936. And even I heard that
this mixture goes back to the 19e century.
It has an deep etching effect on carbon steels. It eats the metal
also very good in the grooves. I have tried other mixtures and acids,
they will work, but this is fast and effective. And some mixtures
only eat the top of the teeth away, and that is not what we want.
Be aware, the amount of acids is based on 100% strength. And need to
be upgraded if you have less stronger acids. Less use of water. It is
dangerous to work with HNO3 stronger than 60%. Stronger will fume and
even react with water vapour.
But strong acids are nothing for the ignorant among us. When you
mix, take safety precautions and always first water than add the
acid. And always mix slowly.
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