Bronze is a metal alloy of copper and tin, and for a long time it was the hardest, most durable material available to human civilization. Just about every major global civilization went through a significant period of time where the mechanical properties of bronze enabled the creation of better tools, sharper weapons, and stronger structures – a Bronze Age.
What is Bronze?
Bronze is a metal with a deep brown color and a golden sheen. You’ve probably heard someone with an especially deep tan referred to as “bronzed” before.
In its most basic form, it’s made up of copper and tin, with copper making up somewhere between 60 to 90 percent of the mixture. The process of making it is straightforward: heat both metals until they melt, stir them together, then pour the mixture out to cool and solidify. Voilà, bronze!
The specific proportions of copper and tin vary significantly, however, and other metals and non-metals may be added to imbue the resulting bronze with useful properties. Confusingly, tin is sometimes replaced entirely with another metal but the resulting alloy is still called bronze. For example, aluminum bronze is copper alloyed with aluminum instead of tin.
Bronze is also closely related to brass, an alloy of copper and zinc. Because of the overlap in their properties and the imprecision associated with the terms bronze and brass, it’s often simpler to refer to “copper-based alloys” as a group.
A Better Metal
All versions of bronze are harder and more durable than either pure copper or tin alone. Copper and tin are both soft metals that are easy to shape – great for making wires or foil, but less ideal if you want an axe that will hold its edge.
In fact, bronze is harder even than pure iron – and much more resistant to corrosion. In the history of civilization, the Bronze Age eventually gave way to the Iron Age as iron became the primary metal used throughout civilization, but this had more to do with iron’s relative abundance than its relative strength – as bronze has an extremely high strength compared to many materials.
Today, stronger metals like steel and tungsten abound, but bronze still finds wide use because of several other helpful characteristics:
- It glides smoothly against other metals with low friction, making it great for use in industrial components like ball bearings or bushings.
- It is naturally resistant to corrosion, making it a good metal to use in shipbuilding and other situations where exposure to seawater is a concern.
- It is also resistant to oxidation beyond just a superficial level, so the internal metal is kept strong and intact. When bronze does oxidize, it forms a patina that actually works against further corrosion.
- Copper-based alloys don’t generate sparks when they strike hard surfaces, making them safer than steel hammers and tools when working near extremely combustible materials like fireworks.
- Burnished bronze metal has a unique and appealing color that makes it popular in artwork and home furnishings.
- Bronze is used as bell metal because bronze bells are supposed to have the best sound and resonance.
- Bronze is a high degree of machinability, meaning that it can be easily milled with other metal tools for industrial and prototyping uses.
- It is also less brittle than many other common alloys like cast iron and stainless steel.
Specialized Bronzes and Uses of Bronze
There are almost as many types of bronze as there are uses of bronze. Even within a given type, formulations vary, as do the specific properties. As an alloy of copper, bronze is a combination of alloying elements that can vary depending on the requirements of the material. Some of the most common bronze alloys are:
Phosphor Bronze (aka Tin Bronze)
Copper with tin (0.5 percent to 1.0 percent) and phosphorus (0.01 percent to 0.35 percent). Phosphor bronze has increased resistance to wear and improved stiffness, making it particularly useful for springs and washers.
Copper with aluminum (6 percent to 12 percent), iron (6 percent maximum) and nickel (6 percent maximum). An extremely tough alloy with great corrosion resistance, it’s often used in marine hardware or components that may come into contact with corrosive fluids
Copper Nickel (aka Cupronickel)
Copper with nickel (2 percent to 30 percent). Notable for its thermal stability, copper nickel improves on the melting point of bronze and can endure high heat without softening. This makes it particularly good for making electrical resistors and heating wires, as it also has suitable electrical conductivity.
Nickel Brass (aka Nickel Silver)
Copper with nickel and zinc. Not as strong as other copper alloys, the nickel gives it a silvery coloration that makes it well-suited for applications where appearance is important, such as musical instruments.
A small percentage of manganese mixed with bronze. It is highly resilient and protective against saltwater corrosion. Manganese bronze has good ductility – meaning it is not brittle – so it deforms on impact instead of breaking. Components like ship propellers and other aquatic machinery are often made using manganese bronze.
Bronze with around 6% silicon. It is used for a variety of parts because of high durability and ease of manufacturing.
It also only has about 6% bismuth, but even these small changes help to increase malleability, thermal conductivity, and it is commonly used for optical application because it can be polished for high reflectivity.
This is not an alloy of bronze, instead a chemical compound of copper, but it is a versatile chemical that can be used for bronze plating or pigmenting.
About the Author
Kenrick Vezina is a writer, editor, and educator whose career highlights include writing for National Geographic, co-developing an online science-writing course for MIT, and pulling a kid out of chest-deep mud in a salt marsh. He loves animals and alliteration.