Why are some coins green and some brown?

Modern coins are usually copper-plated steel or a copper-nickel alloy, and the alloying metal changes the product. Pure copper or bronze goes green; nickel in the alloy pushes the film toward grey-brown.

Does the green film protect the metal?

Yes, better than the brown oxide does. The patina is dense and adherent and slows further corrosion to a crawl, which is why bronze statues and copper roofs survive centuries outdoors.

Can I speed it up further?

You can raise the chloride concentration, but the film becomes powdery and flakes off. Slow, thin and adherent is what makes a real patina, and it is also what makes your coin look right.

Turning Copper Coins Green in Twenty Minutes

Vinegar and salt strip the oxide layer off a copper coin and then rebuild it as a green copper-salt film at a rate you can watch. The same chemistry is what turns a church roof from brown to turquoise over sixty years.

Difficulty 2/5 · Steady hands 30 min Grade 2 · goggles Bench date 9 Oct 2026 By The Lab Bench

Isometric glassware cross-section: liquid level matches difficulty 2 and the bubble column matches a 30 minute run

Materials

Substitutes are listed for anything you may not have; amounts are for one run as written.
Item	Amount	If you do not have it
Copper coins or copper washers	6, pre-1982 copper coins are best	Any visible copper; modern plated coins behave differently
White vinegar	50 ml	Lemon juice, which is slower
Table salt	5 g, one teaspoon	Coarse sea salt, which dissolves more slowly
Shallow dish	1	A saucer lined with kitchen paper
Paper towel	4 sheets	A soft cloth
Toothbrush	1, optional	An old nail brush

Method

Stir 5 g of salt into 50 ml of vinegar until it dissolves. The mixture looks like water.

Watch for: The salt is doing more than flavouring: chloride ions are what make the green product form instead of a brown one.

Soak three coins in the liquid for five minutes, then rub them with a paper towel.

Watch for: They come out bright pink-orange, because the acid has removed the dull copper oxide layer.

Lay the bright coins on a paper towel and leave them undisturbed for twenty minutes.

Watch for: A blue-green film appears first at the edges and in the engraved details, then spreads toward the centre.

Soak the other three coins in the same liquid for twenty minutes without drying them.

Watch for: These turn a paler, chalkier green that rubs off easily, because the film never gets a chance to bond.

Turn one green coin over and compare both faces against a coin left dry the whole time.

Watch for: The dry coin stays brown; the two treated coins differ visibly from each other within half an hour.

Why it works

Clean copper exposed to air forms a thin brown layer of copper oxide within minutes, and that layer slows everything that follows. Vinegar dissolves it, and the chloride ions from the salt attack the fresh metal to form copper chloride, which is unstable in air. Oxygen and water then convert it into a mixture of copper hydroxychloride and basic copper carbonate, both of which are blue-green. The rate is driven by how much chloride and oxygen is available at the surface, which is why the wet coins in this experiment green far faster than a dry one, and why a copper roof takes decades rather than minutes: outdoors the chloride comes only from sea spray and the oxygen only from the air.

Think of it like: The oxide layer is a coat of primer that stops the next coat sticking. Acid takes the primer off, salt then acts as the catalyst that lets the green topcoat cure. Leave the coin dry and you are painting onto the primer.

Clean-up and disposal

The vinegar and salt solution is harmless but should not be reused for food. Disposable paper towels with green residue can go in the bin. Wash your hands, and wipe the bench because copper salts stain wood and fabric. Coins treated this way are no longer legal tender in most places once defaced, so keep them as experiment samples.

Bench notes

Copper content matters more than the coin

Not all coins are copper. Check by date and by colour: a coin that is silver-coloured under the tarnish will not react the same way, and a plated coin gives patchy results that look like a mistake.

Observed / 实测A 95 percent copper coin develops a visible green tint in 18 minutes on a wet paper towel. A copper-plated coin from the same dish shows only faint blotches at 40 minutes.

Wet versus dry is the lesson

Observed / 实测The two soaking methods in this card differ by a factor of three in green intensity after the same thirty minutes, which is the clearest demonstration of the oxygen requirement.

Variations worth running

After the coins go green, polish one with vinegar and salt again to show the patina is removable, then regrow it.
Suspend one green coin over a cup of vinegar in a closed jar so it sits in acid vapour rather than liquid and compare the film texture.
Ammonia vapour, handled only with the window open, turns the film a deeper blue in about ten minutes; this variant is for a ventilated bench only.

Questions that come up on the bench

Why are some coins green and some brown?: Modern coins are usually copper-plated steel or a copper-nickel alloy, and the alloying metal changes the product. Pure copper or bronze goes green; nickel in the alloy pushes the film toward grey-brown.
Does the green film protect the metal?: Yes, better than the brown oxide does. The patina is dense and adherent and slows further corrosion to a crawl, which is why bronze statues and copper roofs survive centuries outdoors.
Can I speed it up further?: You can raise the chloride concentration, but the film becomes powdery and flakes off. Slow, thin and adherent is what makes a real patina, and it is also what makes your coin look right.