[TheForge] Re: Rust Patina Finish

[Mike Spencer]

>>  The untested answer is a phosphoric acid pickle.
>
> ...I thought of nasty acids, which never stop eating through the
> steel. But Phosporic, now that could be something else again.

Very much so. Depending on what you do after pickling, phosphoric acid
leaves more or less of a tightly adhereing coat of iron phosphate on
the metal.  Experimenting with different concentrations of acid and
with flushing, light rinsing, wiping, steel wooling etc. after the
pickle will be enlightening.  The comercial product is a thick
liquid and should be diluted with water.  In a chilly shop, an
immersion heater will help to keep it up in tepid-to-kinda-warm range.

One test piece -- a repouse experiment in 16 ga m/s -- I pickled in
phosphoric and only wiped dry, resulting in a uniform matte grey
finish.  No wash/rinse, no wax or other coating.  It lay on a shelf in
a damp shop for over a decade.  After a few years, neighboring pieces
of sheet metal and bar had turned all rusty.  When I moved my shop and
this piece had been there well over 10 years, it had a few small rust
spots on it.

I've finished several pieces with phosphoric acid pickle, light rinse,
wipe dry, light rub with 4/0 steel wool, Brownell's Oxpho-blue,
carnauba wax.  These have resisted rust (indoors) better than pieces
finished the same but without the phosphoric pickling step.

And phosphoric acid is wayyyy less dangerous to work with than
HCl/muriatic, battery acid/H2SO4 or nitric.  Of course you don't drink
it (er, well, actually, you may, in dilute form in some cola-type soft
drinks :-), nor put it your eyes or nor soak your hands in it.  But if
you get a bit on your skin, you *don't* get instant blisters or have body
parts slough off while you're running to the sink or shower.


- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                 						   /V\ 
mspencer@tallships.ca            						  /( )\
http://home.tallships.ca/mspencer/						  ^^-^^

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