Mechanically polishing brass that's pitted?

[Dave Plowman (News)]

And wear old clothes...

[Dave Plowman (News)]

Soak in artificial (non brewed) vinegar first - that removes the oxidisation which is pretty hard. Makes subsequent polishing easier.

[The Natural Philosopher]

salt probably or sweat.

[Andy Dingley]

Watch out for brass over diecast zinc too.

For Screwfix I either check in the shop first, or I buy just the one of something before buying a batch. They mix solid brass and brass over zinc, yet describe them randomly in the catalogues as "brass", "brassed" and "polished brass finish".

Not many castings (like doorknockers) are going to be brass on steel, i.e. magnetic. That's more likely for things that are pressed from sheet, such as mirror plates and lock escutcheons.

[Andy Dingley]

What's the practical likelihood of that happening on a door knocker?

Perhaps your household is better run than my own, but I'm afraid we have such terrible trouble in finding parlourmaids these days there's no likelihood they'll be able to polish the brassware and starch the antimacassars before tea.

[spamlet]

So did I, and then it rained and I ended up having to sand off the rain spots and start again. An oil film is better than a fake shine. Once you take the oxide film off ally, anything can happen.

S

[spamlet]

What's the practical likelihood of that happening on a door knocker?

Perhaps your household is better run than my own, but I'm afraid we have such terrible trouble in finding parlourmaids these days there's no likelihood they'll be able to polish the brassware and starch the antimacassars before tea.

Dealing with dirty, crazed, lacquer, which has caused pitting where the air and crud has gone down the cracks, is a lot harder than a regular once over with an oily rag. Your choice.

S

[Rob]

On 27/05/2010 00:40, spamlet wrote: snip

Excellent, thanks for that. Just did 6 door handles and the letter box surround. I've tried on a number of occasions over the past 5 or 6 years with Brasso/similar, and never got anywhere.

Nitromors didn't *seem* to do a great deal except degrease. I used one of those decorator's sanding sponge things for awkward corners. I'm going with your oily rag maintain sheen method.

Rob

[spamlet]

Never really looked at brass, but with the right etching solution on a properly polished and clean metal surface you can see the crystal structure. Most people don't realise that metals are crystalline. (I was jealous of a metallurgist I knew who could bring up the crystals in mag alloy beautifully, but I never could get the etchant right.) The point being that alloys are complex materials made even more complex by impurities, bubbles, and the like. Each of these, and the surfaces and edges of the crystals will have different reactivities. Then there is the electrochemical environment to consider: what other metals are coming into contact, and is there an electrochemical potential difference between them? Then there is the acid in sweat; differences between people and the things they put on their hands. And the air and rain quality and many times so on. The problem is made worse by attempts to preserve the surface with lacquers or varnish. Even if they do not crack, they are still too thin to keep the air out forever, and the metal underneath gradually dulls. Once they craze or chip the metal in the chip or craze, is preferentially attacked and pitted, and then the corrosion spreads under the lacquer. And that's before we think of all the damage we do with our keys, rings etc.

This is why you usually see museum staff handling things with disposable cotton gloves. And also why a lot of conservation effort goes into undoing the damage done by earlier attempts at conservation...

S

[spamlet]

Glad of the feedback. If you were able to polish it without too much swearing, the lacquer was probably mostly gone or wasn't there in the first place, but it is always worth doing as even with Nitromors, some of the finishes take some time to wrinkle up: and as you say, it degreases and brightens a bit even if there is no actual lacquer left.

Now you have it properly cleaned and polished you can give it an occasional Brasso to remove any slight dullness (You can do similar with white spirit and ammonia on cotton wool), but still go over it with the oily rag after.

I just keep a cotton cloth with 3 In One oil on it, in a plastic bag with the dusters.

Cheers,

S

[geoff]

So - solvosol, grease, hermatite and KY then ...

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Gib Bogle saying something like:

Carbonic acid in the rain, impurities in the brass, forming simple cells of reaction.

[Andy Dingley]

Not much. Brass is usually failrly resistant to such attacks. it will discolour readily, but not pit as such. If it does start to corrode to the level of pitting, it's usually (depending on alloy) by dezincification. This is a corrosion of the more reactive zinc part of the alloy, leaving excess copper behind. It forms large pink areas.

Pitting in "brass" is usually most serious because it's an indication that the "brass" wasn't brass in the first place, but was a relatively inert plating of brass over a reactive diecast zinc body. Pitting is caused when the plating is perforated and the zinc underneath starts to react (which it will do, with almost anything). The pitt can be hiding a more serious hole beneath, and polsihign it just strips away even more of the plating.