Short answer
Copper is one of the few coloured metals: it absorbs blue-green light, so it reflects red-orange. Alloying with zinc (brass) or tin (bronze) makes it paler and yellower. In air all three quickly grow a thin oxide that darkens them to brown; outdoors, over years to decades, that turns into a green or blue-green patina of basic copper sulphates, or chlorides near the sea. How fast and which colour depend on the alloy, the pollution and climate, and whether the surface is lacquered, waxed or left alone.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Oxide darkening | Bright salmon to brown to near-black | Cuprite and sulphide films forming in air | Lacquer, wax or regular polishing |
| Green patina outdoors | Blue-green surface over years to decades | Basic copper sulphates (brochantite, antlerite) or chlorides (atacamite) near the sea | Pre-patinated sheet if the green is wanted from day one |
| Run-off staining | Green streaks on stone and render below | Soluble copper corrosion products carried by rain | Drip details and gutters that keep run-off off porous surfaces |
Most metals reflect all visible wavelengths about equally and look grey or white. Copper absorbs more strongly in the blue and green, so its reflection is salmon to red-orange. Adding zinc makes brass: the more zinc, the paler and more yellow the alloy. Gilding metal, around 95% copper and 5% zinc, is almost red; common yellow brasses at 30–40% zinc are golden yellow; brasses with more than half zinc look nearly silver. Bronze is copper with tin, typically around 85–90% copper; it is browner and less yellow than brass of similar copper content. Architectural 'bronze' is often actually a brass alloy chosen for its bronze-like colour, so specification by name alone is unreliable.
Freshly polished copper and brass begin to change within hours. A thin film of cuprite (copper(I) oxide) grows, darkening copper through pink and salmon to brown; sulphur compounds in air add darker sulphide films. Brass dulls to a warm, slightly greenish gold and then brown; handled areas stay brighter because they are rubbed. This is why unlacquered 'living' brass taps and handles develop a varied patina, bright where hands touch and dark elsewhere. Clear lacquer or wax slows the reaction but eventually fails unevenly, leaving blotchy patches where tarnish creeps under a broken film; the usual remedy is to strip the lacquer and either relacquer or let the whole piece age evenly.
Exposed to rain, copper roofs and cladding go from bright to brown to dark brown-black within the first years, then gradually develop green. Studies of historic roof copper find a thin inner layer of cuprite under a thicker outer layer of mostly brochantite, a basic copper sulphate that gives the familiar blue-green; in more sulphur-rich air antlerite forms instead, and in marine settings atacamite, a copper chloride, joins them. The green takes years to decades, faster in polluted or coastal air and slower in dry, clean climates; it forms first where water lingers, so patinas are streaky before they are even. Inclusions in the metal disrupt the inner layer locally and make the patina patchier. Pre-patinated copper is sold for designers who want the green from day one.
Copper corrosion products are partly soluble, and rain carries them down onto whatever lies below. Stone plinths under bronze statues, render under copper flashings and paving under copper gutters take on green streaks that can be hard to remove. Bronze sculpture and plaques are usually finished with deliberate chemical patinas — browns, greens, blacks — and then waxed or lacquered; in polluted air, uncontrolled corrosion turns these into streaky pale green with lighter run-off tracks, and conservation work aims to stabilise and re-wax rather than strip. For designers, the practical choices are to keep a bright metal by lacquering and maintaining it, to let it darken evenly by leaving it unlacquered, or to accept and design around green and its run-off.
| Alloy | Approximate composition | Colour when bright | Ageing tendency |
|---|---|---|---|
| Copper | ≥ 99% Cu | Salmon to red-orange | Brown, then green outdoors |
| Gilding metal | ≈ 95% Cu, 5% Zn | Almost red | Browns like copper |
| Yellow brass (cartridge / common) | ≈ 60–70% Cu, 30–40% Zn | Golden yellow | Dulls to brown-gold; greens slowly outdoors |
| White brass | > 50% Zn | Nearly silver | Greys |
| Bronze | ≈ 85–90% Cu, 10–15% Sn | Warm brown-gold | Brown to green; streaky outdoors |
Why: Tarnish spreading under breaks in the lacquer.
Fix: Strip the lacquer, polish, then relacquer or leave unlacquered to age evenly.
Why: Soluble copper corrosion products carried by rain.
Fix: Redirect run-off with drips and gutters; specialist cleaning of the stone.
Why: Patina forms first where water lingers and where the metal has inclusions.
Fix: Allow time; pre-patinated sheet if an even green is required at handover.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Brasses with more copper are more golden and those with more zinc are paler and more silvery; gilding metal, about 95% copper and 5% zinc, is almost red, and brasses above about 50% zinc have virtually no yellow.
Source: Brass
FactStrong evidence
Historic copper roof patinas consist of a thin inner cuprite layer and a thicker outer layer mostly of brochantite, with antlerite in more sulphur-rich atmospheres and atacamite where chlorides dominate.
FactModerate evidence
Inclusions in copper disrupt the inner cuprite layer locally, making the patina less uniform.
FactStrong evidence
Bronze contains about 85% copper and 10–15% tin; historic bronze markers were given chemical patinas for colour and clear coatings for protection, and pollutant gases turn exposed bronze into soluble minerals that stain the masonry below.
Source: Preservation Brief 48: Preserving Grave Markers in Historic Cemeteries
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.