Bronze is a metal alloy, traditionally composed of copper and tin, that has been used for thousands of years to create a variety of objects from weapons and coins, to tools and artwork. One of the reasons for its prevalence over the years is the durability of the substance. However, even though it is a strong material, it is prone to corrosion over the course of time— especially if it comes into contact with certain types of chemicals. This corrosion can result in the shiny, metallic exterior turning a green color. Here, we will breakdown the information behind why bronze corrodes, as well as ways to clean and maintain your bronze items in order to last a lifetime.
The origins of bronze date back to before 3,000 BCE. Although traditionally composed of copper and tin, bronze in modern times might not actually have any tin at all, but in addition to the copper may contain zinc, aluminum, or manganese. Because bronze is an alloy that combines copper with another metal, it is actually harder and more fusible than just the copper itself. In fact, “copper coins” are actually bronze, not pure copper, and contain about 2.5% copper with the rest being zinc.
There are different names for the variety of bronze types available
1. Bell Metal– This is bronze with a relatively high tin content of 20-25 percent.
2. Statutory Bronze– This has a tin content of less than 10 percent and contains zinc and lead as well. This is actually technically brass.
3. Phosphor Bronze– Phosphor bronze has an increase in strength and hardness over regular bronze. This is often used for pump bushings, valves, and plungers.
4. Manganese Bronze– There is only a small amount of tin, or none at all, present in this type of bronze, but it has up to 4.5% manganese, as well as some zinc. This type is often helpful for Mechanical Engineering purposes.
5. Aluminum Bronze– This type of bronze contains up to 16% of aluminum and might contain smaller amounts of other metals such as nickel or iron. This combination makes it very resistant to corrosion and usually makes up objects such as turbine blades, ship propellers, pumps, gears, and pipe fittings.
As a result of bronze being an alloy, composed of multiple metals instead of pure metal itself, it oxidizes when exposed to air, thus developing a patina coating in the process. In addition to the green coating you think of when say a penny gets corroded, it can actually turn other colors as well. These varieties include: red, blue, black, or brown. These colors are a sign of very typical corrosion where the patina actually serves to protect the inner layers of bronze.
While corrosion itself is quite normal for bronze, there is a specific type which is not, and is a sign of a larger issue. If you see brown or green “growth” on your bronze items, this is known as “Bronze Disease”. This means that the corrosion that is occurring is not protective, but rather active, and will continually corrode the piece which will lead to the damage of your piece of bronze. Bronze disease can be a result of a number of issues: the wrong type of cleaning or handling, a very salty environment from being near a beach, high humidity, exposure to ammonia, or even different rain patterns.
Cleaning a Normal Patina- Even though a standard patina does not do harm to the integrity of the bronze underneath, you may prefer the normal bronze color itself. In this case, you can do a simple at home cleaning to remove the patina that has formed. You will need the following: distilled water, a toothbrush you don’t mind designating for metal cleaning purposes, and finally a paste made from either baking soda mixed with lemon juice, or, from flour, salt, and white vinegar mixed together.
Start by rinsing the bronze in the distilled, warm water. Once it has had a chance to soak for a little bit, take the toothbrush and start to gently rub the bronze with it. Next, take the bronze out of the water, dry it off a bit, and then rub the paste all over the piece and leave it to set in for roughly 20 minutes. Once the 20 minutes are up, rinse it again with the warm distilled water. If there are any patches remaining, repeat the process focusing on those areas.
If an item is particularly old or valuable, you can alternatively try creating a paste out of water and chalk, and rubbing the bronze with this mixture along with a polishing cloth. Please note that for very antique or high value items, removing the patina might harm its resale value.
Cleaning Bronze Disease– Meanwhile, the removal of Bronze Disease is a lot more involved and time consuming than that of just a regular patina. The tools you will need to clean your piece include a toothbrush you don’t mind designating for metal cleaning purposes, sodium sesquicarbonate in a solution of 5-percent or less, and distilled water. Start by gently brushing off the flakes of the Bronze Disease with the toothbrush. Next, soak the piece in the sodium sesquicarbonate. This needs to be done for an extended length of time, so be sure to change the solution on a weekly basis. Once the solution has a neutral pH, the Bronze Disease will be officially treated. When rinsing off your bronze from the solution, be sure to only use distilled water.
If your bronze item has Bronze Disease, as opposed to regular patina, and is considered to be extremely valuable or an antique, it might be best to skip the at-home remedies and instead consult with a professional to be sure no lasting damage occurs.
In below video you can see alternate method of cleaning bronze.
Unfortunately, even if you have gone through the long process of cleaning your bronze, due to the nature of its being an alloy, it will immediately begin the process of oxidation once the cleaning process is complete. However, there are techniques to help maintain your bronze so that it stays in better condition, longer. The main strategy behind this is to protect the piece from the air itself as exposure to the elements is what can cause the oxidation.
One such technique is with a clear lacquer. The idea is to thoroughly coat the entire piece so that the air does not reach the top layer of bronze.
Another strategy is to coat your bronze piece with wax in the same manner. However, not just any wax will do— you will need microcrystalline wax thinned out with either lacquer thinner or mineral spirits. After you brush the wax over the entirety of the bronze, let the thinner or mineral spirits evaporate, at which point you should run the bronze down with a clean cloth.
For both methods, be sure that your bronze piece is grease and oil-free for the best results. Additionally, as is noted with cleaning bronze with Bronze Disease, if your piece is especially valuable, you will want to consult a professional beforehand as a lacquer may potentially devalue the bronze.
This should help provide a solid overview of bronze as a material itself, why bronze may turn green (or a host of other colors!) due to corrosion or Bronze Disease, as well as how best to clean your bronze and maintain it thereafter. Together, these tips will help your beautiful bronze pieces last a lifetime (and then some) in great condition.