How to care for your bronze watch, from forcing patina to citric acid baths

Buffy Acacia

Bronze watch cases really hit their peak of popularity in 2020 and 2021, with seemingly every major brand putting their stamp on the rustic golden alloy. The hype may have died down since then, however I’m sure plenty of bronze watch owners have now had enough time to form their own personalised patina. Whether you want to preserve the luxurious shine of the polished metal or embrace the grimiest oxidisation possible, there are specific ways to go about caring for your bronze watch.

What kind of bronze are you working with?

The Oris Big Crown Bronze Pointer Date, made of CuSn6 phosphor bronze.

First, you need to know exactly what kind of bronze your case is made from. Every alloy is going to be slightly different depending on the manufacturer, but there are some generally accepted recipes out there. The two main groups of bronze are phosphor bronze and aluminium bronze. Both are mostly made from copper, then alloyed with other materials to increase its hardness, strength, workability, and corrosion resistance. Phosphor bronze gets mixed with tin, as well as a small percentage of phosphorous. You can recognise phosphor bronze by the metallurgical designations of copper (Cu), tin (Sn), and finally the percentage of tin used. For example, CuSn8 is a popular alloy containing 8% tin and ~92% copper, allowing up to 0.35% phosphorus. Plenty of microbrands such as Zelos use CuSn8, as well as bigger names like Oris using CuSn6.

The Baltic Aquascaphe Bronze Brown made of CuAl8. This one is currently available from the Time+Tide shop.

Aluminium bronze is comparably more modern than the historically significant alloy of bronze and tin, namely using aluminium instead of the latter. This results not only in a greater hardness than phosphor bronze which is quite a lot softer than stainless steel, but also better corrosion resistance and a brighter lustre when cleaned. It is also found with either iron, nickel, or manganese added in <0.5% quantities. Similarly to CuSn8, you can gauge the approximate aluminium content by the composition name, such as CuAl8. This is the alloy that the Baltic Aquascaphe Bronze uses, and aluminium bronze is also used by luxury Swiss brands such as IWC and the ever-loved Tudor Black Bay 58 Bronze.

What is bronze patina, and how does it form?

Bronze Oris Pointer Date Patina — The Oris Big Crown Pointer Date with 1.5 years of patina. Image courtesy of _El_Marc on Reddit.

Unless you’re a fan of rusty old cars or bikes, chances are that you haven’t encountered much actual corrosion. Modern materials science has been amazing for preventing metals from breaking down, then crumbling to dust. That’s why it’s important to realise the difference between corrosion and oxidation. Oxidation can be the first step of corrosion, for example iron reacting with oxygen in air to form iron oxide, also known as rust. Progressively, that spreads through all of the material. A metal resistant to corrosion such as bronze merely oxidises on the surface layer, then the rest is essentially sealed in and protected from further harm. That’s why high-purity gold, which doesn’t tarnish, has always been considered a precious metal, and why stainless steel is underrated as a modern miracle.

Bronze statue hand — The hand of a bronze sculpture, worn from constant touching. Image courtesy of Jonathan Nightingale.

Because bronze in any form is mostly made from copper, the patina that naturally forms is generally copper oxide, or CuO. Over time, it turns bronze’s golden surface to a darker brown hue, as well as dulling its shine with a matte appearance. If left completely alone, bronze will blacken completely – kind of like a statue – but because we wear watches on our wrist, bronze watch cases receive natural polishing from friction, just like statues of dogs which have bright golden spots from being stroked. Phosphor bronze oxidises faster than aluminium bronze, which can really be either a benefit or a drawback depending on your preferences. The patina of a phosphor bronze watch will typically be more unique to the wearer, as your body’s chemistry can react with the bronze in multiple areas differently, whereas an aluminium bronze alloy will typically darken in a more uniform and linear way.

Verdigris bronze patina — Extreme patination inducing verdigris. Image courtesy of r_g_d on Reddit.

In addition to the dark brown CuO, there’s another kind of patina called verdigris. This is the incredibly bright blue-green substance you’d recognise from the Statue of Liberty, and it’s the thing which makes bronze watches look like they’ve been submerged at the bottom of the ocean for decades. Verdigris isn’t an oxide, but rather the amalgamation of copper salts formed from a different chemical reactions involving acids, chlorides, sulphates, sulphides, or carbonates. Copper(II) acetate made by exposing copper to acetic acid was once a popular method of creating pigments for green paint known as verdigris, but the spots on your bronze watch can be any number of copper-coloured compounds. Copper salts are mildly poisonous, so you may want to be careful that this verdigris isn’t being ingested. Contact with skin shouldn’t cause too much concern, unless you have copper allergies or Wilson’s Disease which accumulates copper in vital organs.

How do you keep bronze shiny?

Tudor Black Bay Bronze 58 Clean — The Tudor Black Bay 58 Bronze as new, using an undisclosed alloy of aluminium bronze.

As with all watches, a bronze case is going to collect dead skin, oils, and all kinds of bio-gunk from your wrist unless you clean it. In addition to the odd wipe with a microfibre cloth, using an old toothbrush with warm water and a bit of dish soap will ensure that your watch stays clean. However, because bronze oxidises naturally, you’ll probably begin to see the signs of tarnishing again within weeks to months depending on the alloy. You can extend the life of bronze’s shine with the application of mineral oil or wax, which helps to prevent oxidation.

If you actually want to remove tarnish or patina that has already formed, it’s pretty easy. You’ll need that old toothbrush again, but this time use some low-strength acid such as lemon juice or white vinegar instead of the water and dish soap. The oxides and salts will be dissolved quickly, leaving the sparkling solid material fresh. Just be sure to wash it with water and soap afterwards, otherwise leftover acids will actually accelerate the patina process again once dry.

How can you force patina?

oris divers bronze sixty five cotton candy pink wrist — Oris Divers Bronze Sixty-Five Cotton Candy Pink

Whether you have a phosphor bronze or an aluminium bronze case, forcing reactions with the alloy’s copper works in the same way. The only difference is that aluminium bronze will react slower, so your methods will need to be slightly more extreme depending on the desired result. If you want your bronze to take on that dull brown look with golden undertones coming through in natural light, the best method is really just to wear it every day. Even an aluminium bronze case should be thoroughly oxidised after about six months of solid wear, and the natural polishing of wear will keep the rosiness visible. Of course the chemistry of your own skin and sweat will have different effects, so if you’re particularly acidic, the reaction will happen faster.

Bronze ketchup bath — Bronze watches in a ketchup solution. This image is demonstrating a cleaning technique, but anything that cleans patina will also cause it if you don’t scrub it then wash it. Image courtesy of Serious Watches.

A fan-favourite method of speeding up the patina process is to cover the watch case in ketchup and leave it in the sun for a day or two. Considering that vinegar, salt, and sugar are the main ingredients in ketchup aside from the already acidic tomatoes, it makes a lot of sense why bronze is particularly reactive to it. The sunlight just provides warmth, which speeds up all chemical reactions. If it’s still not having the desired effect, you can also add more salt.

Extreme verdigris bronze case — Extreme formation of verdigris crystallisation achieved by long-term salt water and vinegar vapour baths. Image courtesy of CaptMeow857 on Reddit.

Sulphur tends to get some pretty extreme results on bronze, so if you don’t mind cleaning up the smelly aftermath, you can mash up some boiled eggs in a sandwich bag, drop your watch in, add a bunch of salt for kicks, and leave it for as long as you dare. Although the egg trick can be fun, whatever results happen tend to be fairly delicate and can rub off easily. You can also purchase sulphur gels which seem to yield better finishes. If you’ve got a watch with significant water resistance, you can also mix up some salt water at home (or collect some from a nearby beach) and leave your watch submerged. This is a good technique because you can actually see the patina forming and take it out whenever you’re happy with the results, rather than needing to wash anything off for the final reveal.

Tudor Black Bay 58 Bronze Eroded Crown — The crown of a Tudor Black Bay 58 Bronze after 3 years of wear. The logo has eroded and the shape has unfortunately deformed due to particularly acidic skin. Image courtesy of man_childs on Reddit.

A final thing to remember is that bronze can be quite a soft metal, and that constant wear can actually cause some extreme erosion in unfortunate cases. Metal hardnesses can vary greatly, but phosphor bronze is approximately half the hardness of 316L stainless steel, while aluminium bronze is just a little bit softer than stainless steel. Experimenting with patina is a lot of fun, but purposefully oxidising the surface and then removing that layer is removing material which may end up altering the shape of the watch, and could even compromise its water resistance after enough time. You also need to be wary of bronze disease, which is when chlorides actually end up corroding the material beyond repair. If you notice deep pitting as a result of tarnishing, remove as much patina as you can and try to prevent any further chemical reactions.