Why was the green "patina" on copper
once called a poison?
The roof of a temple or shrine, the Statue of Liberty, an old 10-yen coin — you may have seen copper turn green over the years. This colour is called "patina" (ryokusho, or verdigris). There was a time when people said "patina is poisonous," but that claim has since been revisited.
A brand-new 10-yen coin is a shiny, orange-tinged red. But a coin that has spent years at the bottom of a wallet or piggy bank can turn brownish, and sometimes greenish.
The copper roofs of temples and shrines, or the Statue of Liberty towering across the sea, are far more clearly green. Copper that started out reddish-brown ends up, over many years, an entirely different colour.
There's a clear reason for this change.
Only two things make it turn green
Copper slowly undergoes chemical reactions with oxygen, water, carbon dioxide and other substances in the air. But this happens much more slowly than iron rusting.
The new compound that forms on copper's surface through this reaction (the main component of patina) is a blue-green colour. This is a common colour trend among copper-containing compounds.
Let's look at each in turn.
Reason 1: Copper reacts very slowly
Among metals, some react as soon as they touch air, while others barely react at all. Copper sits somewhere in between. It's harder to react than iron, but not as unreactive as gold or silver.
On copper's surface, oxygen, moisture, and the small amount of carbon dioxide in the air slowly drive the reaction forward. This reaction is thought to be so slow because these substances are already present in only tiny amounts in the air, and because the film the reaction produces then slows down any further reaction. That's why the colour change takes years, even decades.
Near the coast, salt-laden air plays a role; in cities, differences in air composition play a role — either way, how patina forms and its exact shade are thought to vary slightly by location. Even under the name "patina," some coatings lean more blue, others mix in blackish tones, giving a different look depending on the environment.
Reason 2: The resulting compound just happens to be green
When iron rusts it turns reddish-brown, but when copper reacts it turns blue-green. This difference comes down to which compound forms. Iron compounds tend to look reddish-brown, while copper compounds tend to look blue or green.
This is thought to be related to how a metal atom's electrons absorb and reflect particular colours of light. Even though "a metal reacting with substances in the air" is the same underlying phenomenon, the colour that results depends entirely on which metal is involved.
In Japan, patina was once believed to be highly toxic, and there was a period when school textbooks said as much. Since then, research examining patina's toxicity in detail has confirmed that its toxicity is about as low as that of other common copper compounds found in everyday life. Following this finding, textbook descriptions were revised. It's one example of "old common knowledge" being corrected later by careful investigation.
Summary
Copper turns green because of two things working together: ① it slowly undergoes chemical reactions with substances in the air, and ② the new compound that forms happens to be green. And the "patina is poison" claim is itself an example of a scientific misunderstanding later corrected by research.
Patina isn't a sign that copper is breaking down.
It's evidence that copper is slowly, continuously changing.
- Soak a dull 10-yen coin for a few minutes in a mixture of a little vinegar and salt
- Take it out, rinse it with water, and gently wipe it with a dry cloth — watch the surface regain its shiny reddish colour
The acid in the vinegar dissolves the very thin compound film on the surface, revealing the original copper colour underneath. Left alone, though, it will slowly start to oxidize again. Don't pour the used vinegar straight down the drain — dispose of it according to local guidelines.
Want to know more? ― Terms, formulas, and textbook linksClearly marked, from junior-high science to university-level specialist subjects
- JHSCovered in junior-high science
- HSCovered in high-school "Basic Chemistry"
- HS+High-school "Chemistry," or advanced/column content in textbooks
- UnivNot taught in high school — university-level specialist content (inorganic chemistry, conservation science)
- ResearchNot yet settled "fact" even at university — something researchers are actively studying
JHSTerms: vocabulary around patina
- Oxidation: a chemical change in which a substance combines with oxygen or similar.
- Patina (ryokusho): the general name for the green compounds that form on copper over long periods.
- Basic copper carbonate: one of the main compounds thought to make up patina. Its exact composition varies with environment.
HSChecking with formulas: how much copper actually changed?
Let's use the Statue of Liberty as an example and calculate how much copper has turned into the patina layer.
| In symbols | m = ρ × S × d, and the proportion is f = d ÷ D |
| In words | "mass = density × area × thickness," and "proportion changed = thickness of the changed layer ÷ thickness of the plate" |
| Where it comes from | Just a combination of the definition of density (mass ÷ volume) and the volume of a plate-like shape (area × thickness). |
| What the symbols mean | m: mass (grams) / ρ: density of copper (grams per cubic centimetre) / S: area (square centimetres) / d: thickness of the changed layer (centimetres) / D: thickness of the copper plate (centimetres) / f: proportion changed |
| Thickness of the copper plate (Statue of Liberty, approximate) | 2.4 mm (0.24 cm) |
| Thickness of the patina layer (assumed, after 100+ years) | 0.1 mm (0.01 cm) |
| Density of copper | 8.96 g/cm³ |
※ The patina layer thickness is a hypothetical figure used to illustrate the mechanism. Real values vary by location and age.
| Volume of the patina layer (per 1 m² = 10000 cm²) | 10000 × 0.01 = 100 cm³ |
| Mass of the patina layer | 8.96 × 100 = 896 g |
| Volume of the original copper plate (per 1 m²) | 10000 × 0.24 = 2400 cm³ |
| Mass of the original copper plate | 8.96 × 2400 = 21504 g |
| Proportion of copper that changed | (896 ÷ 21504) × 100 ≒ 4.2% |
Even after more than 100 years, the calculation shows that only around 4% of the copper has changed. Even though the surface looks entirely different in colour, only a very thin layer has actually undergone a chemical change — most of the copper underneath remains as it was.
HS+Why is copper slower to react than iron?
Metals differ in how readily they give up electrons and react — some do so easily, others resist it — and this ranking is called the "ionization tendency." Iron has a greater ionization tendency than copper, meaning it reacts more readily. Copper's smaller ionization tendency means that, even in the same air, its reaction proceeds more slowly. This difference is one reason iron and copper change colour at such different speeds.
UnivPatina isn't a single substance
The green film we call patina is not actually a single compound. Depending on the environment (air composition, humidity, distance from the sea, and so on), it's thought to be a mixture of several copper compounds, including basic copper carbonate and basic copper sulfate. In the field of cultural-heritage conservation science, researchers study how to handle this patina layer without simply stripping it away — weighing it as part of an object's historical value, and as a layer that protects the copper underneath. This kind of film on copper's surface is called a patina, and its main component is basic copper carbonate (corresponding to the minerals malachite and brochantite). A film that prevents further corrosion underneath is known, in materials science, as a passive film.
📖 For more on composition and formation: Ryokusho (Patina) — Japanese Wikipedia
ResearchWhat's still not fully understood
- The precise relationship between patina's composition and its toxicity is still being studied in detail. Because composition varies by environment, not every pattern has been thoroughly characterized.
- How far — and how — the patina on cultural artefacts should be preserved is a question experts still disagree on. Balancing visual beauty, historical value, and protecting the underlying metal remains a challenge in conservation science.
- How changes in air pollution affect the way patina forms is also an ongoing research topic. Air composition shifts over time, and patina's colour and rate of formation may well shift along with it.
Links to school curricula (by level)
| Level | Subject / Unit | Where in this article |
|---|---|---|
| JHS | Science — Oxidation and reduction | The basic fact that copper reacts with substances in the air |
| HS | Basic Chemistry — Amount of substance, density calculations | The full calculation in steps ①②③ |
| HS+ | Chemistry — Ionization tendency | Why copper and iron react at different speeds |
| Univ | Inorganic chemistry / cultural-heritage conservation science | The diversity of patina's composition, its treatment in conservation science |
| Research | Materials science / conservation science (ongoing) | The detailed link between composition and toxicity, preservation methods, effects of air pollution |
| ― | History of science / correcting misconceptions | How the "patina is poison" claim came to be revised |
- General descriptions of metal oxidation and ionization tendency found in chemistry textbooks.
- Survey materials from public institutions and experts on the composition and toxicity of patina.
- General descriptions of how patina on copper artefacts is treated, found in specialist cultural-heritage conservation science materials.
- Commonly known reference materials on the Statue of Liberty, including the thickness of its copper plates.
※ Values such as thickness and density are approximations and assumptions used to illustrate the underlying mechanism. Actual values vary by object and environment.
※This article is a general-audience science explainer. The figures given are approximations and assumptions meant to illustrate the underlying mechanism. For judgments about the toxicity or safety of substances, please consult information from specialist institutions.