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| Shiny fittings on the newly-painted front door, the result of hours of polishing. © Eugene Brennan |
Painting
is something I tend to procrastinate on and life is too short for
cleaning brasses regularly. I do a good job of cleaning the hardware on
the front door every two or three years when I paint it, but then I only
polish the brasses a couple of times before I lose interest and have
more pressing chores to attend to. After three years of non-cleaning, a
thick layer of oxide builds up on the fittings which takes several hours
to remove with wire wool and Brasso. (Brass is an alloy of copper and zinc, so the
tarnish is zinc oxide and copper oxide). There may be other compounds
making up tarnish such as acetates (The green patina on domes of public
buildings and the Statue of Liberty, known as verdigris, is copper
acetate. This forms naturally or is created artificially by using
acetic acid on the copper sheeting, pre-construction). Anyway to cut a
long story short, from my school chemistry I knew that acid dissolves
oxides and turns them into salts. Someone gave me half a 5 litre
container of Ecokem phosphoric acid they didn't want. Phosphoric is used
for cleaning rust (iron oxide) off iron and steel and descaling lime build-up from sanitary fittings. So I tried the stuff
and it did a reasonable job of quickly removing the tarnish.
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| Phosphoric acid, a product used for descaling. © Eugene Brennan |
This is a
faster method than using wire wool or Brasso, but I'll use the latter to
further clean and polish the brass (Brasso contains a mild abrasive). I
noticed the brass had turned pink and suspected that the acid had
reacted to a greater extent on the zinc in the brass alloy, dissolving
it and leaving the copper behind. My suspicion was confirmed by Grok.
After washing the acid off, the brass turned black and brown when it
dried. Not sure what produced that chemical reaction. Will have to ask
Grok for a follow up.
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| The acid dezincified the brass, leaving copper behind. © Eugene Brennan |
Edit: Grok
said it's copper oxide, formed by the rapid oxidation of porous copper
after dezincification by oxygen in the air or rinse water. It transpired that the dezincification was superficial and I used Brasso to finish the job, leaving the fittings perfectly shiny.
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| Grok confirmed the dezincification. © Eugene Brennan |
