Well I seem to have stumbled on some mysterious metallurgy.
I have been messing around with patinas and trying methods. I haven’t got any ammonia, so a bit of old urine had to do the job……
So I put the copper part in some white vinegar, sprinkled some salt on top and put a little pot of my wee wee in the corner of a sealed tub.
I left it a few days and there was plenty of blue forming so I chucked and old brass fitting in to see what would happen.
Left it a few days and to my surprise the brass is somehow being plated by the copper! crazy!
It seems to be even more coated today, those photos were a couple of days ago, I’ll update with some new ones as time goes on if it changes.
That is white vinegar in the tub….not pee lol! Sadly the patina bit of the blue looks better than it actually is - it isn’t in the copper, it’s on it and just rinses off…… any ideas?
Hmm, brass is Cu and Zn, so it might not be that the Cu gets plated onto the brass, but that the (sacrificial) Zn gets eaten out of the brass.
Electrum was a naturally existing alloy of Au and Ag. Make coins or whatnot, soak ’em in fruit juice to eat away the Ag, and the leftover pure Au on the surface could be smoothed down to make the coin look like pure Au.
You could always add a battery into the mix to speed up the process and maybe make it stick.
See How electroplating works - Explain that Stuff and scroll down a bit to the copper/brass picture…
What is your goal?
Chemical methods work well only if you have pure components and know exactly alloy composition.
I have some positive experience with adding patina to non-pure bronze (cast parts usually have wide composition) with ammonia spirit. Not sure if it is widely available, but I could get some for a little money in the first pharmacy I entered.
You need some can that can be closed tightly, and the same time you need to organize a place near the top of this can for part (using copper wire or bent nail etc.)
For degreasing liquid, you need another container with hot and salt (hardly salt) water.
Put part in the saltwater, rub clean brush around.
After put this part inside the can with ammonia spirit. You don’t need much of it, 10-20g is more than enough for 1L can. Close the cap.
For faster results, use a heat gun to heat up this can with ammonia spirit vapours.
After several minutes, you can clean the part in regular water and check how it looks like. This process can be repeated several times (I prefer a little scotch-brite sanding between, not sure if this is necessary).
Not have photos, but freshly machined, sanded and polished part received a look of 40-50yo coin.
My goal was to get the blue patina on copper - my problem is it only seems to be forming on the surface and just rinses off - it doesn’t seem to be sticking.
It seems to basically forming blue copper sulphide? crystals which presumably dried out could be used to copper plate nickel etc in a solution.
Sadly I can’t get ammonia easily here, which is why the urine, eventually it will produce a little ammonia.
I suppose I could have used some sort of hair dye/treatment as my wife’s always smell of ammonia, probably a better more strong source than a bit of urine lol!
I have some copper pipe that has been used (offcuts) that has been there years, it has a lovely blue patina inside, and more importantly it is dry and you can’t just scrape it off - this is what I want to achieve, but quicker than 3 years in shed, and maybe 6 years in use in the house with water running through.
Could be that, but also I notice under that copper part (the blued one) the blue crystals have formed down into the salty vinegar liquid and you can see (just about) in the 1st photo the river of blue flowing down to that brass part.
I think you are probably correct though, and it may explain why the blue isn’t sticking - maybe the surface copper is being sacrificed constantly due to the brass being in there, hence giving the blue no surface to cling to since it’s being eroded albeit very slowly?
Time for a fresh set up I think with various conditions in each box.
I’m not trying to plate it with copper, was just an interesting side effect - I’m trying to make copper go blue (done) but STAY blue, not just fall off lol!
So that’s what they were alluding to! 
So, are you trying to do this?
Exactly this - I’m thinking my copper wasn’t clean enough.
If I get time tonight, I’ll try again.
Turns out you can get ammonia on ebay……
Hrm, used to be able to get it at the drug store too as a cleaning supply, at least here in the US. Havent looked lately (maybe for a decade or so). Perhaps it’s been pulled because it can be used for nefarious purposes or something.
Yup brass is an alloy of copper and zinc and you are eating away the zinc to leave just the copper, in the summer I found a few old empty 303 cartridges down the beach which had been there since about 1944 and they were just pure copper and very crumbly, the zinc had all gone leaving just the copper matrix, quite fascinating.
Unno, but here you can get ammonia as plain/clear, “cloudy”, and “sudsy’. No idea what the last two are, what kind of additives are used.
Plain ammonia is just NH4OH, or NH3•HOH. Like CO2 dissolved in water to make seltzer, it’ll eventually fizzle out.
If you can’t get straight ammonia, “smelling salts” works. Small amount of ammonia in ampoules, crack ’em to release the gas and wake up someone unconscious.
Heck, here’s a site with instructions on doing it with Miracle-Gro. You can get that almost anywhere, including amazon.co.uk
Copper Patina | How to Patina Copper | Patina Color (scroll almost to bottom)
I have some of that - I’ll give it a go 
Ok so as of 9.15pm we have 3 experiments on the go. 3 boxes are sealed with clip on lids, not completely airtight but should suffice.
In box 1 is freshly emery rubbed copper pipe, cleaned in isopropyl. Sprayed with a miracle gro solution. 1 part gro, 1 parts water.
In box 2 is again freshly cleaned pipe from the same length. This time dipped in regular water, then covered in salt with a small pot of…… (until ammonia arrives lol)
Same in box 3 but dipped in white vinegar, then salt - with my trade mark pot in the corner, until ammonia arrives. The pinkness is because it’s pink rock salt.
