Back in 2002/2003 I picked up a series of postings to one of the canal internet newsgroups in response to the query - Any trade secrets on cleaning brass mushroom vents and portholes for lasting shine????????:-
From: Bob Conduct (email@example.com)
1. If they're not corroded it's just down to elbow grease and Brasso, (or, if you can obtain it, Day & Martin's Metal Polish is better than Brasso).
2. If they're corroded use "Renov'Lib" from Liberon Wax Co Ltd on 01797 367555. More abrasive than Brasso and with a Phosphoric Acid content to dissolve the corrosion it's the magic cleaner that antique dealers use.
3. If they're very corroded use 32%w/w Hydrochloric Acid on 000 or 0000 steel wool, followed by a good wash (that's both the mushrooms AND you :-)) followed by 1.
Nothing preserves the shine satisfactorily except the elbow grease bit!. However Liberon sell a light drying oil called Jade Oil designed for preserving the surface of brass that's been "antiqued". It might work on polished brass - haven't tried it.
To prevent corrosion over the winter use clingfilm or Vaseline. Vaseline is messy but far more effective. You only need a very thin film - almost invisible - and consequently easy to clean off in the Spring. To achieve this on a cold end-of-season day just warm the mushroom briefly with a gas blowlamp and spread the Vaseline over the mushroom with a cloth pad. The Vaseline film comes off easily with the first Brassoing of the spring.
Finally don't buy those huge tins of Brasso - it goes off quite rapidly and ceases to be very effective. (I know someone who bought a 5 Litre tin but had to throw more than half of it away!).
From: Richard Clark (firstname.lastname@example.org)
Common table salt in water, with citric acid (or lemon juice) is very effective, but do it carefully, as it goes for the metal very quickly. Do it in a series of applications, washing off and inspecting the surface each time. End off with Brasso or equivalent, for shine. Hydrochloric acid is nasty stuff. Richard Clark
From: Paul Little (Paul@sosmix.demon.co.uk)
Try Duraglit. It comes in an Orange tin (a bit shorter than a drinks can) with a roll of wadding impregnated with the stuff inside. Simply tear off a bit, wipe on, leave for a moment, wipe off. Works a treat with no elbow grease whatsoever. Really, I'm surprised noone else has mentioned it. It saved me an awful lot of time during RAF Basic Training.
From: Chris Deuchar (SBZCD@szn1.agric.nottingham.ac.uk)
There's no such thing as a "lasting shine" with brass - for this you need chrome :-)
Even lacquered brass will tarnish in a few months and then you have to get the lacquer off to polish again.
Once heavily tarnished, use Amways cleaner or Bar Keepers Friend. Slightly less efficiently, use salt and vinegar. NOTE WELL, though, that ALL these require to be washed in COPIOUS amounts of water afterwards AND polished immediately upon drying with brasso or whatever.
This 'post' is mainly about scale on steel but sets out the chemistry involved before coming round to brass.
Actually Ron I thought that you with your "chemistry" background would come back with a comment on the kettle de-scaler removing tarnish and corrosion but leaving the copper colour! Can you tell me what the chemistry is here? I actually use a light solution of a Fernox de-scaler (because I have a kilo of it) which contains sulphamic acid.
I don't mind. ;-) "Chemist" mode is never far away - put 2-4 chemists together and within 60 seconds they'll all be talking "shop" - if we didn't like chemistry we wouldn't do it (pay ain't that great!), so we all find it interesting.... (sad I know...)
Anyway, descaling.... A few facts.
1. Water is "hard" due to (a) temporary hardness (bicarbonate) and (b) permanent hardness (sulphates and chlorides) - Note soft water (from a tap) is still "hard", just not as hard as others.
2. Scale comes from the salts in water mainly from (a) - unless you are boiling the water down (I hope not...).
3. Thus the scale is composed of calcium and magnesium carbonates (which arise from the more soluble bicarbonates, which lose carbon dioxide as the water is heated).
4. Any strong acid will react with the carbonate to give the calcium (or magnesium) salt of the "new" acid and more carbon dioxide (hence it fizzes when you do so).
5. *However*, most calcium salts are insoluble, so one much pick an acid that will give a soluble product, otherwise you turn one scale deposit into another...
6. Vinegar (acetic acid ) works slowly (acetic acid is not very strong, but nearly all acetates are soluble).
7. Hydrochloric acid works wonders, but dissolves all the steel in sight....
8. Sulphuric acid is stronger again, but not so corrosive to steel, but calcium sulphate is plaster of Paris....
9. Citric acid is good, but long term exposure to metal is not good IIRC
10. Sulphamic acid (aminosulphonic acid) is therefore the usual choice and it is the normal ingredient in all powder descalers you buy in the shops. Its strong enough to quickly dissolve the carbonate, but leave the steel alone.
11. Don't get smart and use distilled water in your central heating - it will cost a bomb, and distilled water doesn't like being pure - it dissolves almost anything in sight (very pure water will even dissolve glass! - not a lot but enough to be not pure any more, hence it's always sold in plastic bottles)
12. Fernox (as you say) has sulphamic acid (thus prevents scaling up) and also IIRC has other ingredients which inhibit any corrosion on the steel - put a plain old steel (not stainless) nail in a beaker of fernox and it don't rust *at all*
13. Tarnish on brass is due to simple oxidation of the brass, giving it an dull oxide coat. Anything that will dissolve the oxide off will work fine - and an acid (of any kind) is ideal - hence my comment on coke which has a 'shed' load of phosphoric acid in it.
14. If the copper/brass has gone green, then it's a hydrated oxide and/or hydroxide coat; again acid is the trick.
15. I wouldn't use too strong an acid on brass (strong ones might attack the non copper component) - copper is untouched in all acids, unless they have oxidising properties as well - thus it won't last 5 minutes in nitric acid.
16. All this cleaning and polishing, of course, makes your brass get slowly thinner and thinner....
There. You wished you've never asked now... ;-))