Bronze is copper alloyed with tin, and 10-12% tin gives the best balance of hardness, toughness and castability, while above about 14% tin it turns brittle and bell bronze at 20-22% tin shatters if struck with a hammer. Specialty bronzes replace or add elements, such as phosphorus for springs and bearings, aluminium for strength and sea-water resistance, and lead for bearings. Brass is copper with zinc and is not a bronze.
Part of Advanced Alloys
Bronze alloys are copper-based metals, traditionally copper-tin, whose hardness and casting behaviour depend on the alloying element and its amount.
Tin Hardens Copper By Jamming Its Crystal
Pure copper is soft and tin atoms are a different size from copper atoms. Dissolved in the copper lattice they distort it and block the slip of atomic planes, which is solid-solution strengthening (see Advanced Alloys). Past a certain tin content the lattice cannot hold more, and a separate hard, brittle phase appears. That is the structural reason behind the corpus rule that more tin is harder but only up to a point. Bronze Making gives the working table:
| Tin | Character | Use |
|---|---|---|
| 5% | Soft, ductile, poor casting | Wire, sheet |
| 8-10% | Medium, good casting | General tools, hardware |
| 10-12% | Hard, tough, excellent casting | Heavy tools, bearings |
| 14-16% | Very hard, less ductile | Mirrors, bells |
| 20-22% and above | Extremely hard, brittle | Bells only |
Bronze melts below copper, and the same article puts copper at 1,085 °C and tin at 232 °C. Phase Diagrams shows why: tin lowers the liquidus. Copper Properties notes that copper work-hardens quickly, and bronze does too, so cold-worked parts need annealing.
Specialty Bronzes Change The Mix For A Reason
These standard alloy families are named for their added element:
- Phosphor bronze. A little phosphorus acts as a deoxidiser in the melt and improves strength and fatigue life. Used for springs and bearings.
- Aluminium bronze. Aluminium in place of tin gives high strength and strong resistance to sea water.
- Silicon bronze. Good corrosion resistance and weldability.
- Gunmetal. Copper-tin with zinc added, for castings in marine and pressure service.
- Leaded bronze. Lead dispersed as soft droplets lubricates a bearing surface. Bearings and Bushings covers bronze bearings in use.
Brass is copper with zinc and is a separate family, easier to machine and cheaper in tin.
Where Bronze Beats Iron
Bronze wins on bearings (it runs against steel without seizing), on bells and musical instruments, on ship hardware because it resists salt water, and on castings, which fill thin sections and fine detail. The tradeoff is that bronze lacks the quench-hardening of steel and costs more in tin.
Worked Example: Fixing The Tin In Scrap
You want 10 kg of bronze at 12% tin, which needs 1.2 kg of tin. You have scrap bronze at 5% tin. Let x be the scrap mass and y the added tin, with x + y = 10 and 0.05x + y = 1.2. Subtract: 0.95x = 8.8, so x = 9.26 kg and y = 0.74 kg. Check: 0.05 × 9.26 = 0.463 kg, plus 0.74 = 1.203 kg. Add a little extra tin, because some oxidises and goes to the slag, and cast a test bar to confirm before pouring the part.
Casting Notes
- Segregation. Slowly cooled castings form tin-rich and tin-poor regions. A thin, fast-cooled casting is more uniform.
- Slag and gas. Skim before pouring, as the bronze-making article advises, and pour at a steady rate.
- Add tin to molten copper, not the reverse, as the same article instructs.
Fume and splash hazards in melting
Molten bronze at well over 1,000 °C throws metal if a wet tool or mould touches it, so dry and preheat everything. Zinc in brass or gunmetal burns off as a white fume that causes metal fume fever, and lead in leaded bronze, or in salvaged bearings, is toxic as fume and dust. Cadmium in some salvaged alloys is worse. Melt outdoors or with strong ventilation and do not breathe the fume. Unknown scrap should be melted separately and never used for food or drinking vessels.
Common Mistakes
- Using bell bronze for tools — it shatters.
- Calling every yellow alloy bronze — brass is a different alloy with different hazards.
- Pairing bronze with steel in wet joints — the steel corrodes. Isolate the joint.
What’s Next
- Learn next: Bronze Making — melting and pouring practice
- Combine with: Phase Diagrams — reading the copper-tin system