Bronze is made of two metals: copper as the base and tin as the principal addition. A common tin bronze sits near 88% copper and 12% tin by weight, an alloy family that spans leaded versions and high-tin grades. That pairing is the entire difference between bronze and brass, because brass replaces tin with zinc. The consequences are practical: bronze takes on copper's resistance to corrosion and formability while tin adds the hardness, strength, and wear resistance that suit bushings, marine fittings, and heavy-load gears. So if someone asks what two metals make bronze, the answer is copper and tin; the alloy number then tells you exactly how they are blended.
The Definitive Answer: Copper Plus Tin
Every bronze designation in the supplier catalog starts with the same base: copper mixed with tin. Material comparison guides we use for procurement list a standard tin bronze at roughly 88% copper and 12% tin, while the same guides place brass at about 60–70% copper with the balance being zinc. That one substitution, putting zinc where tin belongs, separates two industries. Bronze's higher hardness and sliding-wear resistance, as one manufacturing comparison puts it, make it the go-to material for bearings, marine fittings, and parts that face harsh environments; brass wins when a part must have a bright-gold appearance, excellent machinability, or low electrical resistance. Naming the two metals is therefore not a chemistry exercise; it is a warning label that tells you which failure modes the material can survive. When you say 'I need bronze,' you are saying you need the alloy whose base is copper-tin, not a zinc-bearing brass.
Metallurgists use the word 'alloy' whenever a base metal is intentionally blended with other elements. In bronze, copper is the base (the metal that gives the material its color, electrical behavior, and natural resistance to atmosphere and seawater), and tin is the addition that transforms it. The phrase 'two metals' is the starting point rather than the complete specification: real bronze families add small amounts of lead, phosphorus, aluminum, silicon, or nickel to hit a target machinability, strength, or casting behavior. That is why a designer who only says 'bronze' has not yet given the supplier enough information; the exact alloy number or at least the expected application is needed. The composition is doing real work: a small shift in the addition changes whether the alloy is sold as a pump grade or a gear grade. In practice, staff buyers should know the base answer because it anchors every downstream conversation about cost, machining, and corrosion.
The composition choice becomes a budget problem on the shop floor. Brass is a copper-zinc alloy that machines quickly and costs less, so high-volume precision parts and decorative trimming naturally default to it. Bronze, a copper-tin alloy, costs more partly because tin is dramatically more expensive than zinc, yet it is the material named for bearings, saltwater fittings, and components in harsh environments. A manufacturing guide we checked frames the decision this way: choose brass for valves and decorative components, choose bronze for parts that must carry load, face salt, or resist constant rubbing. That guide, written for engineers rather than for marketing, says the look-alike metals differ in elemental composition more than in color. The two-metal answer is therefore the point where a purchase decision forks: the expected service life and failure risk, not just the raw material invoice, should decide which alloy you buy.
Why Tin Is the Hardener Copper Needs
Copper is the element that makes bronze feel like a 'red metal' and keeps it from rusting like steel. As the base, copper forms the continuous matrix that holds the entire alloy together, and it is responsible for the material's workability before and after casting. A copper-rich matrix can be bent, forged, or machined without cracking, which matters when a part must be formed into a complex shape. Copper's natural resistance to atmospheric corrosion and seawater is what allows bronze fittings to be bolted to boat trailers and dock hardware. Pure copper, however, lacks the body for sliding or loaded contact; under pressure against a steel shaft, it deforms plastically, wears quickly, and can even gall or seize. The base metal, in other words, supplies the alloy's personality—formability, electrical and thermal conductivity, and corrosion resistance—but it cannot deliver the stiffness and wear life that bearings and gears demand. So when a designer chooses bronze instead of pure copper, the intention is to keep those copper advantages while adding a strengthening agent. That is also why bronze is rarely specified for structural beams or electrical wire; it is specified where metal rubs against metal or against saltwater.
Tin is that hardener. When tin atoms dissolve into the copper crystal structure, they strain the lattice and block dislocation movement, which is the metallurgical reason bronze becomes harder and stronger than pure copper. The effect is visible in the alloy tables we use: C83600, a leaded gunmetal rated for excellent machinability and good pressure tightness, and C90700, a phosphor bronze described as having high strength and good corrosion resistance for heavy-duty gears and bearings. Tin atoms are larger than copper atoms, so the distortion they create makes it harder for the alloy to yield under load. This explains why a high-tin bearing grade like C90700 is chosen when the job involves medium to high loads, while a lower-tin version would be used for lighter general bushing service. The element doing that strengthening work is tin, usually present in the range of 5–12% depending on the bronze family. So when a bearing spec calls for bronze rather than copper, tin is the reason: it converts a soft, gummy base metal into a surface that can carry load for years.
The copper-tin pairing is the baseline, and the real specification work happens when a manufacturer adds a third element to tune a property. In C83600, lead is added specifically for excellent machinability while keeping medium strength and good pressure tightness, which is why pump suppliers keep choosing it for pressure-containing components. In C90700, phosphorus works with the tin to raise strength and improve the bearing surface, which is why the same alloy table lists it for heavy-duty gears. Add manganese and you move into C86300, a grade with exceptional strength and good wearing properties but only fair machinability; leave out the manganese and you have C86500, a high-tensile material that is easier to cut but susceptible to dezincification. Those trade-offs are the reason 'copper plus tin' is only the beginning of an answer. Each addition shifts the part toward one job and away from another, so buyers should match the alloy number to the load case rather than assume all bronzes behave alike.
Bronze vs. Brass: The Copper-Zinc Impostor
The most common mix-up is treating bronze and brass as two names for the same reddish metal. They are not. Brass is a copper alloy whose secondary metal is zinc rather than tin, and the failure consequences show up most clearly in seawater. The problem is not mere surface staining: in saltwater, zinc leaches out of the brass grain structure in a process called dezincification, leaving a porous, copper-rich skeleton that is weak and prone to cracking. For this reason, supplier guides report that over 30% of marine hardware failures still trace back to high-zinc alloys in saltwater. The loss happens below the surface, so a brass fitting can look intact until it cracks under load. Bronze, built on tin instead of zinc, does not fail that way; tin stays in the alloy, preserving the structure. Even if the two metals look similar under a scuffed surface, their primary alloying element tells you whether a boat trailer bushing is likely to survive a marina season.
On the machine tool, the difference appears as cycle time and tool wear. Brass machines so readily that CNC shops use it for high-volume, precision parts with thin walls and fine cosmetic finishes; bronze is harder on tools and slower to cut, but it earns that cost in service. A CNC comparison we keep near the quote desk puts it directly: brass offers easier cutting, faster cycles, and lower tool wear, while bronze earns its place in bearings, bushings, and load-bearing or corrosion-prone components because of superior wear resistance and durability. The practical implication is that a part designer who selects only for machinability will choose brass, then discover the part seizes or corrodes in a bush or underwater fitting. In design reviews, that is the moment we ask which metal is actually in contact with the load and the water, because the two-metal composition, not the color of the bar stock, determines whether the part fails.
Because tin is the expensive ingredient, bronze typically costs 40–60% more than brass in comparable shapes, and that premium tempts buyers to substitute brass and call the job done. The failure mode described above is exactly why engineers specify bronze for saltwater fittings, sliding wear surfaces, and heavily loaded gears: the cost difference buys decades of service instead of a corroded replacement. The myth to drop is not just that bronze and brass are interchangeable; it is also the idea that once you know copper and tin are present, any bronze behaves like any other bronze. A leaded bearing bronze and a phosphor bronze share the same two base metals, yet one is a pump alloy and the other a gear alloy. Color and common name cannot carry that distinction, so the label is no substitute for reading the alloy number on the supplier's page.
From Two Metals to a Part Number
Step one in a real purchase is confirming the material exists in the supplier's catalog, and that is where a supplier's own category listing is useful. Online Metals, for example, lists Bronze as its own material type beside Brass, Aluminum, and other metals, which tells a buyer that copper-tin alloys are stocked and cut to order rather than being a special request. That category is a signal of availability, not a technical certification: it does not say which alloy number to choose, nor does it guarantee a specific grade is in stock. What it does is confirm that the two-metal answer has a commercial home—when a catalog treats bronze and brass as separate categories, the procurement conversation can move directly to alloy families such as leaded gunmetal or phosphor bronze. The availability signal and the grade selection are different questions; the category list answers the first, and the alloy number answers the second.
A sound decision rule for a bronze order starts with environment and load. If the part will live in saltwater or carry a spinning or sliding load, specify a bronze designation from the bearing-bronze family: choose a leaded grade when the job needs pressure tightness and easy machining, and a high-tin phosphor grade when the job is a heavily loaded gear or bearing that needs strength and fatigue resistance. If the job is purely decorative, electrical, or high-volume precision machining, a free-machining brass is the cheaper, faster-cutting alternative. Then stop judging by color: confirm the exact specification with the supplier, ask what the alloy number means, and request a mill test report whenever the application is safety-critical. The shortcut that earns the right part is the alloy number, not the name 'bronze' alone and not a glance at the bar's hue.
So the full answer to 'what two metals make bronze' is copper and tin, and the full specification is the alloy number that tells you how they are blended. Copper resists corrosion, tin adds strength, lead and phosphorus tune machinability and toughness, and zinc should send you to the brass aisle instead. That rule matters on a boat trailer and on the shop floor alike, where the cost of guessing wrong is a seized bushing or a cracked valve body. For a marine fitting, a bushing, or a heavily loaded gear, start with that two-metal answer, confirm the designation with the supplier, and you have turned a ten-second fact into a procurement decision that survives contact with saltwater. Choose the designation, not just the color.
When someone next asks what bronze is made of, the answer begins with copper and tin—and it ends with a question back: which alloy number, under which load, in which environment? Let the two metals start the conversation, but let the designation finish it. That is the difference between memorizing a material fact and making a confident purchase.
