Bronze is an alloy of two metals: copper and tin. The quick answer to 'bronze is made of what two metals' is that simple, but the practical answer is more useful: the ratio of copper to tin, and the small extra elements alloyers add, determine whether the bronze bar you order will work as a bearing, a gear, or a decorative plate. Get the composition wrong and you may pay for wear resistance you don't need, or get premature failure where you do. Throughout this guide, I'll separate the copper-tin core from the alloy families built on it, and show you how to read a bronze spec before you order.
The Two Metals in Bronze: Copper + Tin
The standard definition of bronze is a copper-base alloy in which tin is the principal added element. That definition comes from suppliers and metallurgical references alike, and it is the first thing to verify when you are comparing materials. Bronze is not a single composition; it is a family of alloys built on that copper-tin pair. The copper provides the base metal, typically around 88 percent by weight in a classic casting bronze, while tin, even at 10 to 12 percent, sharply changes how the material behaves. The presence of tin is what separates bronze from brass, because brass uses zinc as its main addition. When a data sheet says 'C90700 phosphor bronze,' the 'phosphor' refers to a small phosphorus addition that deoxidizes the melt and improves the alloy's wear characteristics, but the defining pair remains copper and tin. So when someone asks which two metals define bronze, the accurate answer is copper and tin, with the caveat that real-world bronze often carries trace additions.
Bronze proportions vary by application, but the classic casting bronze is about 88% copper and 12% tin, a ratio that shows up in the quick-comparison table used by suppliers like Baosheng. That 88\/12 mix is a starting point, not a universal recipe. Bearing bronzes, for instance, often replace some tin with lead to improve machinability and embed dirt, while phosphor bronzes add a small amount of phosphorus to the same copper-tin base. The price difference is real: with copper around $13.34 per kilogram and tin around $53.46 per kilogram, the tin content pushes bronze to cost 40-60% more than brass, which uses zinc at about $3.31 per kilogram. Because tin is the expensive ingredient, alloy makers adjust the ratio to hit the right balance of strength, wear, and cost. This is why the phrase 'copper and tin' answers the composition question, but the exact percentages need to come from the alloy spec, not a general rule.
The most common mix-up is treating brass as a type of bronze, or assuming bronze is just copper with zinc. It's not. Brass is copper alloyed with zinc, typically in the range of 60-70% copper, while bronze is copper alloyed with tin. This distinction is not academic. In a CNC machining context, brass is prized for its high-speed cutting, bright finish, and lower tool wear, which makes it the default for valves, fittings, and decorative trim. Bronze, in contrast, earns its place in bearings, bushings, and load-bearing parts because tin gives it hardness and corrosion resistance that zinc-based alloys can't match. If you order a bronze bushing and receive brass instead, the tin-based wear surface you paid for is gone. That is why suppliers and machinists treat the copper-tin vs copper-zinc line as the first filter in material selection, before any brand-name alloy gets involved. Even the color difference, though often subtle, traces back to that zinc-versus-tin split.
Tin's Role: Hardness, Wear, and Corrosion Resistance
What tin actually does inside the copper lattice explains why bronze is specified for heavy-duty parts. Tin atoms are larger than copper atoms, so they distort the crystal structure when dissolved in the solid solution, making it harder for dislocations to move. That solid-solution strengthening raises the alloy's yield strength and hardness compared to pure copper. The same mechanism is why bronze gets its wear resistance: a harder surface resists adhesive and abrasive wear better under sliding contact. The HMaking machining guide makes the practical point directly: copper wins on electrical conductivity (C11000 at 101% IACS), but for anti-friction and wear resistance, bronze C93200 is the pick. That distinction explains why a bearing bronze with 10-12% tin can outlast a copper bushing by a wide margin in a dirty or un-lubricated environment. The corrosion resistance also improves because tin promotes a stable oxide film, so marine fittings and pump components stay intact where dezincification would attack a brass part. The takeaway is that tin is not a minor additive; it is the element that turns soft copper into a wear-resistant engineering material.
You can see the composition-to-performance link in the alloy tables used by foundries. According to the Fraser Alloys reference table for typical bronze and brass grades, ASTM B505 is the specification assigned to the copper-based casting alloys listed there. That table identifies C83600 and C84400 as leaded gunmetals with excellent machinability and good pressure tightness. C86300 is a manganese bronze with exceptional strength and good wear properties, though its machinability rating drops to 25. C90700 is a phosphor bronze with a machinability rating of 30 but high strength and good corrosion resistance, making it suitable for heavy-duty gears and bearings with medium to high loads. These numbers are not decorative. The machinability rating tells you how much tool wear and cycle time to expect, and the ASTM B505 designation tells you the spec you are buying. When a supplier lists C90700 next to 'phosphor bronze,' they are signaling that the alloy has a specific tin-phosphorus balance tuned for gear and bearing service, not a generic copper-tin mix. The C84400 alloy, for example, is widely used in the pump industry for bowl and lineshaft bearings, a fact that shows how the same copper-tin base gets tailored for a specific service.
The tin content is the lever that connects composition to your purchasing decision. Because tin costs roughly $53 per kilogram against copper at about $13, every point of tin raises the alloy price. That is why the Baosheng comparison shows bronze typically running 40-60% more expensive than brass. If your part lives in a clean, low-load environment, the extra wear resistance from high tin may be wasted money. Conversely, a bearing running under heavy load and marginal lubrication will justify the premium because the alternative is downtime and part replacement. The practical move is to specify the minimum tin content that meets your performance need, rather than assuming all bronzes are alike. That is also why suppliers publish the exact composition ranges for each alloy. A C90700 bar at 10% tin and a bearing bronze at 7% tin are both 'copper-tin' alloys, but they are not interchangeable in a gear application.
From Bearing Bronze to Decorative Bronze
Bronze families are defined by the additional elements alongside the copper-tin core. Bearing bronzes (like C93200) typically add lead to improve machinability and provide embedded lubrication; they are the workhorse of bushings and sleeve bearings. Phosphor bronzes (like C90700) add phosphorus, which deoxidizes the melt and improves fatigue strength, making them the choice for gears and heavy-duty bearings. Manganese bronzes (like C86300 and C86500) add manganese and sometimes aluminum; they deliver exceptional strength and are used for high-load structural parts, though machinability suffers. Gunmetals (C83600 and C84400) add zinc and lead, giving excellent pressure tightness for pump and valve bodies. The HMaking guide's quick summary captures the functional split: for wear resistance, the bearing bronze C93200 stands out; for electrical purposes, copper C11000; for high-speed machining, brass C36000. C86500, also a manganese bronze, offers good strength and reasonable machinability, making it a candidate for general machinery parts that need to absorb shock. When you categorize by the added element, you can predict which bronze will work for your application without memorizing every number.
Reading an alloy spec is about finding the right columns in the supplier's table. Start with the alloy number, such as C90700, which identifies the composition family. Then look at the standard it conforms to, most commonly ASTM B505 for bronze castings. The machinability rating (often expressed relative to free-cutting brass at 100) tells you how aggressively the material can be cut; C83600 rates 85, while C86300 rates 25, so expect longer cycle times and more tool wear with the manganese bronze. The tensile or yield strength values, and any corrosion notes, come next. For example, the Fraser Alloys table lists C90700 with good corrosion resistance, which is why it appears in marine and chemical pump applications. Finally, check the tin and phosphorus ranges in the composition column. These small print numbers determine whether the material can handle the load you are putting on it. If a supplier only gives you the name 'bronze' without the alloy number, that is a red flag—ask for the C-number and the ASTM spec before ordering.
Once you know which bronze family you need, ordering becomes a matter of finding the right category. Online Metals organizes its catalog with a Bronze section alongside Brass, Copper, and other materials, so you can quickly narrow to the alloy number and form you need. The site's navigation also flags related product groups like Bearings, Shafts and Gears and Sample Packs & Protoboxes, which is useful when you want a small piece to test before committing to a full bar. If you need help locating the right spec, the company's customer service line at 888-527-3331 and live chat are direct routes to a person who can confirm inventory and sizing. For buyers in the Chicago area, the will-call pickup option with $0 shipping can save both time and freight cost on an urgent bronze order. The key is knowing your alloy number before you start browsing—that lets the site's filters do the work.
Bronze or Brass? A Simple Decision Rule
Putting bronze and brass side by side on practical axes settles most sourcing questions quickly. Machinability: brass is the easier cut, with lower tool wear and faster cycles, which makes it the default for high-volume precision parts and any component with thin walls or fine cosmetic finishes. Bronze cuts more slowly and wears tools more, but the trade-off is a tougher, more wear-resistant finished part. Corrosion: bronze handles saltwater and chemical exposure far better than most brasses, especially zinc-rich alloys that suffer dezincification; marine hardware failures are still traced to high-zinc alloys in saltwater. Cost: bronze runs 40-60% higher than brass because of the tin content. The CNC machining guide from uneed sums it up: brass shines in high-volume precision parts, bronze earns its place in bearings, bushings, and load-bearing or corrosion-prone components. In practice, the machinability gap shows up as cycle time: brass parts can come off the machine in a fraction of the time a bronze part takes, which is why high-volume producers often choose brass despite its lower strength. Neither is universally 'better'—the axis that matters is the one your application actually stresses.
A simple decision rule can replace the alloy anxiety. Choose bronze when the part carries a load, slides against another surface, or faces moisture, salt, or chemicals. That covers bearings, bushings, gears, pump impellers, and marine fittings. Choose brass when the part is primarily cosmetic, requires intricate machining, or sees light, clean service—think valve stems, decorative trim, electrical terminals, and fasteners. If cost is the deciding factor, calculate the price gap: with tin at $53.46 per kilogram and zinc at $3.31, a bronze part will routinely cost 40-60% more than an equivalent brass one. Ask whether the extra wear and corrosion resistance pays for itself over the part's life. In a high-wear environment, it almost always does. In a decorative application, it rarely does. For a one-off prototype, the bronze price gap may be irrelevant; for a production run of thousands, it is a decisive factor. That is the boundary: invest in bronze where failure is expensive, and use brass where appearance and machinability matter.
The verdict, then, is that bronze consists of copper and tin—but the more useful answer is that the alloy you choose has to fit the job. The copper-tin pair is non-negotiable for any material sold as true bronze, and the tin content is what gives it the hardness and corrosion resistance that brass cannot match. But no single bronze composition serves every purpose. The boundary is practical: if you are ordering a bearing or a gear, confirm the C-number and tin content; if you are ordering trim or a low-load bracket, brass may be the smarter buy. The two metals answer the trivia question, but the decision rule answers the purchasing question. The same logic applies whether you are sourcing a single bar from Online Metals or a truckload from a foundry: the name tells you the family, the alloy number tells you the grade, and the application tells you which one to pick. Keep that distinction in mind when you are on a supplier's site, and you will avoid the most common mistake—confusing the simple composition with the complex selection.
The two-metal answer is a door, not a destination. Bronzes that all trace back to copper and tin still vary enough to change a project's outcome, so the final judgment is always the same: match the alloy to the load, the environment, and the budget. That is what separates a correct answer from a correct purchase.
