cleaning bronze/copper

[billj pump]

when copper/bronze has been brazed can anyone tell me how to get the charred black burnt colour off the metal please.

billj

[GeorgeRay]

If you mean the flux residue, the normal way is to put the item into an acid pickle. Various solutions are used diluted sulphuric acid or citric acid are a couple. If using the former remember this is highly corrosive and dangerous if not diluted correctly i.e. add it to water and not the other way round. I cant remember what the dilution is I'm sure the more knowledgeable will come up with the answer tho.

[alanstepney]

I assume that you havent been pickling the job after silver soldering.
The traditional pickle is dilute suphuric acid, which I prefer, but some people use acetic acid, which is easier to obtain, albeit slower in action.

NB, for some silver solder fluxes acid is no good, you need sodium hydroxide.
Check on Johnson Matthey metal joining for full details.

[the_viffer]

As Alan says but not a lot of people know silver solder flux residue comes off best in alkaline solution.

You can use acetic acid ie cheap vinegaras pickle. It works better with some salt in it. Oddly enough the Frau Doktor who has worked as a research chemist for more than half her life didn't know why. (The explanation is long, technical and mathematical but she should have known it).

Personally I favour citric acid which you can get cheaply from home brew shops: much more expensive at the chemist or cookery shop. It is used in the conversion of cocaine to crack and so if you buy lots you might well have to explain what you are up to.

[Tel]

I have a preference for phosphoric acid - mainly because I aquired about 300 litres of the stuff several years ago

[Ansty]

I used to use sulphuric acid many years ago but with the current Health & Safety implications of disposal would now use citric acid. Much easier to dispose of if taking slightly longer to do it's job. As the_viffer says it's cheaper from a home brew shop.
Hope that the boiler turns out a success billj. Please let us all know.
Happy boiler making.

Brian

[steammadman]

citric acid is available from WILCO (wilkinsons)
if you need afew small tubs it's best to order

[billj pump]

i tried using that stuff advertised on tv "cillit bang" you know the stuff,when he puts the penny in a bit and it comes out gleaming!!, it did actually bring most of the black stuff off but maybe needed an overnight soak + its expensive stuff, ill go to the home brew shop for some cocain stuff and ill get high at the same time,ha! whats the best stuff for polishing bronze back to its proper colour? ive tried duroglit but that doesnt work very well and tends to polish bronze to a brassy colour.

[houstonceng]

billj

In my experience, the "proper colour" of new Bronze is a sort of "brassy colour".

You're not thinking of the brownish colour of bronze statues, I hope, which is chemically generated as part of the finishing process (either applied cold or with a flame). Polishing that colour off an artifact will definitely result in a "brassy colour" accompanied by a considerable loss in intrinsic value.

The actual colour of a sample of Brass or Bronze (both of which are "brassy" coloured), will depend on the proportions of Copper, Tin and/or Zinc in the alloy modified by other metals added to it. There can also be an effect caused by the hardness of the metal and the way it is produced - cast, rolled, etc.

We are also a bit "sloppy" in naming Bronzes and Brasses - for example, "Naval Brass" is a term used for Bronze and "German Silver" is a name used for a silver coloured Brass (usually caused by adding Nickel). Then there's Phospher Bronze - - - -

Thinks ! Why do writers - eg Ian Fleming refer to a Greyish clour as "Gun-metal" when Gun-metal was another name for brass/bronze (See James Bond's gun-metal lighter).

Eggcuse me if I've just brought my Grandmother and a few ovoids, sucking for the use of, to the list.

Andy

[alanstepney]

The other thing that will clean bronze / copper if Coke. (The liquid, not the powder!)
In fact, left long enough it will eat most things away to nothing.
and people actually drink it.......

[the_viffer]

ooooooooohhhhhhhhhhhh more chemistry.

You can easily buy commercial patinating solutions to turn brass and bronze green from horological/jewellers suppliers eg Walsh. I think Axminster Power Tools sell it as Fluid Tourmaline.

Of course you can diy. Liver of sulphur aka potassium sulphide is good for it but not for enhancing your chances romantically. Available from Walsh and the like and one of the more disgusting smelling materials (tho' not as bad as some of the organic sulphides with which I used to work). Exposing the job to the fumes of either 880 ammonia or vinegar will do a similiar job over the course of time.

[jackrae]

If you want to get any alloy back to its "natural" clour then avoid at all costs the use of anything chemical. These invariably contain acids which preferentially etch away the various constituents of the alloy and in so doing so change its colour to that of the slower etching constituent.

By way of example, "Twinkle" an Amercian copper/brass etc polish based on nitric acid is absolutely fantastic at polishing these metals but tends to leave brass rather "white" since it preferentially etches the copper and leaves the finished surface zinc rich.

I would suggest the only way to get a true finish is to use abrasives such as rouge dust and elbow grease or the old polishing wheel and the appropriate grade of "soap"

By the way I really liked your web-site especially the reworked lathe/tool grinder

Jack

[the_viffer]

jackrae said:

If you want to get any alloy back to its "natural" clour then avoid at all costs the use of anything chemical. These invariably contain acids which preferentially etch away the various constituents of the alloy and in so doing so change its colour to that of the slower etching constituent.

By way of example, "Twinkle" an Amercian copper/brass etc polish based on nitric acid is absolutely fantastic at polishing these metals but tends to leave brass rather "white" since it preferentially etches the copper and leaves the finished surface zinc rich.

I would suggest the only way to get a true finish is to use abrasives such as rouge dust and elbow grease or the old polishing wheel and the appropriate grade of "soap"

By the way I really liked your web-site especially the reworked lathe/tool grinder

Jack

Jack, I am sorry but you are in error in part here.

Zinc is more soluble in acid than copper. You can check that is so by putting a piece of copper and a piece of zinc in an acid pickling bath. If you have some fresh pickle you can put a piece of brass in it. It will come out not white from the unreacted zinc but pink from the unreacted copper. (If you use used pickle it will also come out pink but that might be due to plating out of the dissolved copper salts on the brass.) You can make bakers fluid by dissolving zinc in spirits of salts ie hydrochloric acid but copper is pretty well inert to it.

Copper barely reacts with most acids. That is why acid pickling baths are used: so that the acid dissolves the copper oxides which are soluble in acid leaving the copper unchanged.

Copper does react with very strong nitric acid. However the reaction is rather complex involving not only oxidation of the copper but reduction of the the nitrate. It is a reaction I have only done once. It is truly spectacular in that not only does the almost colourless solution turn blue but great clouds of red-brown and highly toxic nitrogen oxide fumes are evolved.

"Twinkle" does not contain any nitric acid. You can see the list of ingredients here www.malcopro.com/cgi-win/malmsds.exe/5252 . It works in a combination of two ways. First the ammonium chloride reacts with copper oxidation product and solubilises it. Secondly the silicates act as abrasives providing the action you recommend.

Ammonia based cleaning of brass is widely used in the cleaning of antique clocks and other old items typically using "Horolene". It has the advantage over a wheel in not removing anywhere near as much material (and tooling and other marks on the surface of the metal) and not risking rounding sharp edges on the item.

I am a little wary of using ammonia based compositions on cleaning copper alloys as in some cases it can induce stress cracking. This is a particular problem with 1930's spun brassware.

[jgb7573]

Way up the thread was a comment on the strength of diluted sulphuric acid for pickling. 5 to 10 percent is what is usually recommended. And if you're diluting it yourself, add the acid to the water, slowly, and stirring as you go. A fair bit of heat will be generated If you add water to acid, you can end up with a fine mist of sulphuric acid throughout the workshop, in your eyes, lungs etc. - not what the doktor ordered at all!