Metals are the most abundant material on the planet. The Earth’s core, itself, is a gigantic ball of iron. You can find them in almost every building, vehicle, or machine, as well as most of the everyday items you use. They comprise 77 percent of the Periodic Table of Elements—and these are just the pure metals. The alloys produced by combining two or more of them are not yet included. Simply put, metals play a critical role in how the world runs, and yet a lot of people know very little about them.
There’s more to the metals that make up your doorknob and window frame than you probably know. If you take a closer look at them, you’ll find some fascinating facts that can make you understand how vital metals are to human life. One of the most interesting things about metals is their susceptibility to corrosion, a process that results from electrochemical reactions between materials and substances in their environment.
When a metal is placed in an unstable environment, its chemical composition changes, particularly when it makes contact with a reactant, a substance it reacts to. Different metals react to different substances and how they react varies according to their classification. Ferrous metals, for example, slowly transform into rust when exposed to oxygen.
Non-ferrous metals or those metals that do not contain iron, on the other hand, react by forming a protective outer layer called patina. The best example of a metal that undergoes this type of corrosion is copper and all its alloys. When exposed to oxygen, copper’s surface turns green over time.
Have you ever wondered why the Statue of Liberty has a green color despite lack of coating? That’s mainly because its cladding is made entirely of copper. After over 140 years of exposure to the elements, its copper covering has long reached the final stage of its transformation, which is colored green.
When You Don’t Always Want Green
While the green color of corroded copper looks appealing to some, it does not to others. If anything, they think the color doesn’t fit to certain decorative applications and they prefer dark brown, black, or anything in between. In that case, they prefer an alloy of copper where copper’s properties are not that prominent, so they choose bronze.
Bronze has been used for making statues, ornaments, and all kinds of accessories for thousands of years. Hundreds of bronze sculptures have been unearthed and displaced in many museums all over the world. It’s their thick layer of patina that preserved them for so long, some of them are still intact and need little polishing to look their best again.
Unfortunately, it might cost you a fortune to have any of these artifacts displayed in your own house. The good news is that these are not the only bronze sculptures available out there. There are bronze sculptures that, despite being created more recently, have almost the same distinct color and shade as those of the ancient artifacts. You may wonder how those who made them were able to achieve that. Well, the secret is to speed up corrosion.
How to Speed Up Bronze’s Corrosion
It’s actually no secret that bronze’s beautiful patina can be expedited. In fact, the technique called “intentional patination” became so popular that it has created a whole new industry. It involves exposing bronze to a variety of chemicals until the desired color and shade are achieved. The process is quite simple; anyone can try it at home if they don’t want to spend a lot on purchasing an already patinated bronze furniture or sculpture.
All you have to do is prepare your bronze sculpture by cleaning its surface and making sure it is free from any foreign elements that might get in the reactant’s way. Ferric nitrate is the safest and most cost-efficient choice for a reactant. However, direct application on bronze can cause rapid reaction, which is why it is advisable to apply an undercoat of bismuth nitrate.
The corrosion is usually aided by change in temperature, and so it is important to heat the bronze while applying the ferric nitrate. After covering the entire surface, start applying cupric nitrate to add complexity. Let the coating sit overnight and then wax the surface for finishing. The wax serves as a shield to preserve the color of the patina you’ve forced to come out.
Copper Supplies from the Right Supplier
If you are planning on a project that involves the use of copper alloys, particularly bronze bars, make sure you will get your materials from a trusted supplier, such as Rotax Metals. This is the only way to ensure that you will be able to achieve the right patina for your bronze. Whether it’s a simple bronze or a special one like Muntz metal, it helps you get it from the right source.
The Artificial Patination of Bronze Sculpture, vam.ac.uk
What Is Patina?, thebalance.com
Patina Formulas for Brass, Bronze and Copper, sciencecompany.com