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The Bronzed Patina: The curious case of patina. How to get it, and how not to.

by Sharon Tan on August 23, 2018

Patina to bronze is like rust to iron and tarnish to silver, only more desirable and beautiful. Chemically speaking, they are oxide layers that form when the reactive metals come into contact with anions (such as oxygen and chlroride ions). As it is usually a slow process, hence people see these as a sign of age and heritage. But like most chemical reactions, we can accelerate it by increasing concentration of the reactants, introducing catalysts or adding some heat. Today, let’s geek out and make some reactions.

 

*WARNING: Replicate these experiments at home at your own risk! We’re not chemists, and results may include other ingredients such as luck which are not listed*

 

 

Before we begin proper, let’s set straight the common confusion between copper, brass and bronze. Copper is the metal element while brass and bronze are metal alloys containing mostly copper. Brass is a mixture of mostly copper and zinc. Bronze is a mixture of mostly  copper and tin. Because of the different composition of these alloys, the patina that forms will also differ. For today, we focus on the bronze of the Bell & Ross Diver Bronze BR 03-92. For all other aspects of the watch, read at the full review here.

 

Cleaning Agents

We wore the watch for about 2 weeks, and it developed a natural dark brown type of patina. As with most patina, this doesn’t come off with water and soap. That’s because oxides generally are more reactive with acids, and soaps are neutral or slightly alkali.

 

Lemonade

With that knowledge, we can understand why lemon juice is a highly recommended cleaning agent for bronze patina. Lemon juice contains citric acid, and doesn’t hurt that it smells great too! To remove the patina in the photo below, it took approximately 30mins of soaking in lemonade (approximately 1 to 1 lemon juice and tap water). Healthy for our bodies, and apparently very cleansing for our bronze watches too! Check out the rosy tint that’s brought out with 30 minutes in lemonade!

 

Bell and Ross infused lemonade, anyone?

 

Freshly cleaned bronze on the right side of the dial.

 

Cold brew black coffee

If you didn’t already know, coffee is acidic and stains pretty much everything from countertops to teeth. How about bronze? Piping hot espresso would have been damaging to the watch, so we used home made cold brew coffee. We covered up half the watch, and gave it a good coffee bath. Surprise, surprise! No staining action here! The left half shows the patina colour prior to coffee, and the right half shows fresh and clean orange-pink bronze.

 

Only right side of watch exposed to coffee bath (1 hour)

 

Half the watch got cleaned

 

Comparison of before and after cleaning by black coffee

 

Liquid Vinegar

Another commonly found household acid is acetic acid, found in vinegar. Any type of vinegar including white vinegar, apple cider vinegar, black vinegar etc. We happened to have some good old white vinegar at home, so we dropped the watch into some undiluted vinegar after an episode of forced patina. Comparing this to lemonade and coffee, it definitely worked much faster! What took the others up to an hour to achieve, the same was achieved with less than a minute by vinegar. It happened so fast, you could actually watch the colour change.

Dipped the right side into vinegar for less than a minute.

 

Fully cleaned watch, ready for her next attempt of forced patina!

 

Acetone

I happened to have some industrial grade acetone on hand. Acetone is a pH neutral solvent, with low boiling point and highly flammable. Similar chemicals can be found around the house, such as in nail polish remover or paint thinners. Given the strength and corrosivity of this chemical, I didn’t dip the watch into acetone immediately. I tested it out by putting the bronze buckle into the chemical for a few minutes. Nothing apparent happened, which kind of put my mind at ease. I then wiped the watch with a cotton pad soaked with acetone- kind of like how I would remove nail polish. The good news is that the bronze was unharmed. The shocking result was that the paint on the bezel markers got wiped off! A serious warning here to our readers and watch lovers: Keep all solvents away from your watches!

 

After wiping with acetone

 

Subsequent exposures to vinegar continued to peel off or dissolve the markers.

 

Patina Accelerators

I’m sure we have heard plenty about various ways to accelerate patina on bronze. Some apply these techniques to furnitures and ornaments to give it the recovered-from-Titanic look, some prefer the deeper brown or green hues (we get into the colour chemistry soon) that patina produces, and then there are those, such as ourselves, who do it out of curiosity. We also do it so that you don’t have to go through the same trial and error to get the desired look on your bronze diver. Let’s look at some methods that are commonly recommended online, and then some that we tried out of curiosity.

 

Baking Soda with Vinegar

This is a popular cleaning formula around the house. Whenever you have a greasy sink, stubborn floor stains, you have probably tried sprinkling some baking soda on it, followed by pouring vinegar over it. That fizzy action you see is actually the resulting carbonic acid decomposing into carbon dioxide and water. During this process, the carbonic acid also helps dissolve away these stains. We didn’t want to risk fizzing the watch into a mess, so we tried to use the settled down solution as a cleaning agent. Instead of cleaning up the watch, it unexpectedly made the watch darker all round.

 

After exposing the watch to cooked eggs (more on eggs later)

 

 

 

Eventually we added in more vinegar to the solution to see if the extra acidity will help with the cleaning. Here’s where it gets interesting- It did get the watch cleaner, but the bezel got significantly cleaner than other parts of the watch.

 

After soaking it in a concoction with more vinegar

 

See sideways close-up, where patina is not removed from sides of the bezel and case.

 

Eggs

Likely the top recommended method when we did our initial rounds of research. Boil an egg, mash it to expose more egg, and then place it in a sealed bag with the watch. What’s happening here is that egg releases hydrogen sulphide gas, at an accelerated rate when you boil (preferably overcook) it. The sulphur in the hydrogen sulphide reacts with the copper in bronze to create copper sulphide, which is a black compound. In our experiment, I started the experiment while the egg was warm, so the heat probably accelerate the whole process. Within 15 minutes, the colour was similar to that after 2 weeks of wear. After another hour, it was such a deep hue that it looked like it has gone through a month long adventure in the wild.

 

Steamy eggs and the watch in a sealed bag.

 

After 15 mins exposure

 

Detail: after 15 minutes of exposure

 

After 1 hour exposure

 

Detail: after 1 hour exposure.

 

 

Vinegar

Yes, you read this correctly. We have already confirmed that vinegar is an efficient cleaning solution to remove patina. Little did we know that vinegar can also be an accelerator by keeping the vinegar near but not touching the watch. Confused? What we mean is that we expose the watch to the fumes of vinegar, instead of the liquid itself. It’s not that hard to set up. In a bowl or glass, prop the watch high enough as if you were to steam it. We recommend covering the glass, because vinegar doesn’t really smell that great. The results? Green! The most iconic patina of bronze, as seen on Statue of Liberty. The copper in bronze is reacting with water, oxygen, carbon dioxide and hydrogen ions in different yet simultaneous ways, in a unpredictable manner to create different shades of blue and green. The key chemical present in vinegar fuming, and absent in vinegar liquid, is oxygen. As mentioned earlier, patina is the surface of copper metals oxidising. There cannot be oxidation in the absence of oxygen, which is why vinegar liquid cleans while vinegar fumes when mixed with air will accelerate patina.

 

Cup-in-cup set up to prop the watch up.

 

Within an hour, what appeared to be droplets of green residue is dry to the touch, but could be mechanically scrubbed off, to leave behind a more yellow-ish hue of patina, as observed on most parts of the case. Leaving it longer would simply intensify the green deposits, eventually to a point where it becomes completely green like the Statue of Liberty.

 

30 minutes later, the watch case develops a yellowish tinge on the front and…

 

Underside of case that is facing the fumes has green deposits

 

The experiment continues with a little more vinegar, and the watch placed facing down.

 

After an additional hour of fuming

 

Deposits taking shape of my fingerprint. Likely to have left behind a thin film of oil that protected the bronze.

 

Green deposits in the nooks around the watch. Lack of index marker is from an earlier experiment with acetone (read above for advisory)

 

 

Milk and wine

A very unlikely pairing at the dinner table, but here we are, about to perform an experiment with them. To be honest, this combination occured by chance. Initially, the plan was to clean the watch with red wine after exposing it to milk as an patina accelerator. We assumed that wine would be a cleaning agent due to its mild acidity; and milk would release sulphates when left in the open. As such, I exposed half the watch to milk, and left the other half above the milk to be exposed to vapours that’s released. The results were certainly unexpected, and very unique. There was not much acceleration to be observed after almost 3 hours of soaking. But when I checked back after almost 9 hours of soaking, the end exposed to milk vapour showed negligible colour change, whereas the half soaked had a patina unlike the others. There were areas where a beautiful iridescent rainbow patina has formed. As reluctant as I was, I had to clean off the patina with experimental cleaning agents. Well, there was an opened bottle of red in the fridge, so it wouldn’t really be too much of a waste to use it as an acidic cleaner right? There begins the soaking of the rainbow patina into the red wine, and nothing happened! I left it for hours, and still, nothing happened. So, I guess we can safely conclude that wine is not going to be an effective cleaning agent. Well I had to clean off the patina in any case, so I used the preferred cleaning agent of vinegar. Surprisingly, this rainbow patina is very difficult to come off! It took hours of soaking in vinegar (in addition to a sloppy wipe of acetone) before the patina finally faded to the original reddish bronze.

 

Got milk?

 

Overnight watch in milk, your new healthy alternative to overnight oats.

 

Iridescent patina formed around edges

 

 

Red wine bath in an attempt to clean off the patina

 

Iridescent patina is still there, for a long time.

 

Side experiment to verify that both milk and wine soaks are required.

I repeated the experiment with the buckle, to see if the same “persistance” of the patina can be achieved without the wine. In short, nope. Appears to be that whatever makes red wine stain so difficult to remove has a similar effect on the patina of milk. After exposing the buckle to milk, I masked off half off if before dipping it in wine for another hour. There was some slight darkening on the half exposed to wine. Then I went on to soak it completely in lemonade and vinegar (sequentially). The half that was masked from wine came off much sooner than what it took to remove the patina that has been exposed to wine.

 

Concluding thoughts

What have we learnt? I think the experiments have been very interesting and I summarise the results in the handy table below:

 

Desired outcomeMaterialsSet UpComments
Sparkling fresh orange-pink bronzeLemonadeSoak it.

A brush to help brush off loose patina from grooves would be helpful. Rinse thoroughly after cleaning.

Slower than vinegar. 1:1 dilution seems to work faster than black coffee. Patina should fade or come off in about 30 minutes.
Black coffeeSoak it. Similar to using lemonade.Slower than 1:1 diluted lemonade. Patina should come off or fade in an hour or two.
VinegarSoak for a few seconds.

Wipe with vinegar soaked cloth, or with toothbrush. Rinse thoroughly after cleaning.

Very fast. Most patina came off in matter of seconds. Do not soak beyond neccessary as we have not verified if it causes any pitting or corrosion with extended exposure.
Deep brown patinaFreshly boiled eggs (sliced or mashed)Place in a dish, prop watch above eggs or place next to dish. Cover the set up, or place the set up into a sealed bag.Freshly boiled eggs speed up the process because of the heat introduced. Cool eggs would also work, but may require a few hours. Patina formed comes off easily with above cleaning methods.
Baking Soda, vinegar (add salt if required)Be sure the stable solution is not acidic by adding in more baking soda until fizzing is minimal. Soak the watch until desired darkness achieved.

We did not try this with a freshly cleaned watch, use this method with care.

Too much vinegar could result in a cleaning solution.
Blue/ green/ yellow patinaVinegar (vapour)“Steam” set up, at room temperature. Preferably flip the watch midway to encourage a more even patina across the front and back of the watch.Green deposits can come off with some dry rubbing. Eventually a yellow-green tint forms. Perhaps extended exposure with fresh vinegar fumes will increase green intensity.
Iridescent patinaMilk and red wineSoak in milk overnight, followed by soaking in red wine for an hour.Special Deployant formula. Besides creating a special iridescent patina, seems difficult to remove with conventional cleaning agents.

 

Armed with this knowledge, what will you try with your watch? Remember our disclaimer above…but if you do accept the risks and try, please do share with us your results in the comments.

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