Vinegar can cause copper to turn green over time due to a chemical reaction. The acetic acid in vinegar reacts with the copper, forming a green patina layer on the surface. This patina layer is made up of different copper acetate compounds. While not harmful, it can be unsightly. There are ways to clean or prevent the green patina from forming if desired.
What causes copper to turn green with vinegar?
Copper reacts slowly with oxygen in the air to form a copper oxide layer which protects the underlying metal. This layer is typically brown or black in color. However, when acetic acid (vinegar contains 3-9% acetic acid) comes into contact with copper, a chemical reaction occurs that causes different green-colored copper acetate compounds to form.
The main reactions are:
- Copper + Oxygen → Copper (I) oxide (black/brown layer)
- Copper + Acetic acid → Copper (II) acetate (blue-green)
- Copper (II) acetate + Acetic acid → Copper (II) acetate hydroxide (green)
As these green copper acetate products build up on the surface they create the characteristic green patina layer. So in essence, the acetic acid in vinegar etches the copper, stripping away the protective oxide layer and allowing the patina to form.
Does the vinegar need to be acidic?
Yes, the acetic acid in vinegar is required for this reaction. Vinegar contains 3-9% acetic acid which gives it a pH of around 2-3, making it quite acidic. If vinegar is diluted with water to become less acidic, it will be less effective at causing copper to turn green.
Pure acetic acid would cause copper to turn green even faster. Other acids like citric acid or hydrochloric acid can also react with copper in this way.
On the other hand, if vinegar is neutralized with a base like baking soda so it is no longer acidic, it will not cause the patina formation. So the acidic component of vinegar is key.
How long does it take for vinegar to turn copper green?
The amount of time for vinegar to turn copper green depends on several factors:
- Vinegar strength – Undiluted vinegar works faster than diluted vinegar.
- Temperature – Warmer temperatures accelerate the reaction rate.
- Copper surface – Smooth/polished copper reacts faster than textured surfaces.
- Exposure method – Submerging copper in vinegar causes a faster reaction than just wiping on vinegar.
- Oxygen access – Good airflow to the copper speeds up the patina formation.
Under typical conditions with full-strength vinegar at room temperature, copper usually starts to show light green patina spots within 30 minutes to a few hours. Over 24-48 hours the patina fully develops with a thick green layer.
One simple way to demonstrate this is to soak a copper penny in vinegar. The penny will begin turning green within an hour and be fully green by the next day.
Does all vinegar turn copper green?
All types of vinegar can induce patina formation on copper surfaces since they contain acetic acid. However, the strength of color change depends on:
- Vinegar type – Stronger vinegars like white vinegar (5% acetic acid) work better than mild rice vinegar (4%) or balsamic vinegar (6%).
- Dilution – Diluting vinegar with water makes it less effective.
- Age – Older vinegar may be less acidic.
So while all vinegars can potentially turn copper green, high strength, undiluted white vinegar generally produces the most rapid and prominent patina development.
Does salt and vinegar turn copper green faster?
Yes, adding salt to vinegar can accelerate the patina formation on copper. The sodium chloride in salt acts as an electrolyte which facilitates electrochemical reactions between the copper metal and acetic acid.
This effect is more pronounced when using an vinegar-salt solution soak compared to simply wiping copper with a salt-vinegar mixture.
Salt concentrations of around 5-10% in vinegar are typically used. Too much salt can be counterproductive by reducing the accessibility of acetic acid to the copper surface.
Can you remove the green patina from copper?
Yes, there are several ways to remove or reduce the green patina on copper:
- Baking soda scrub – Make a paste of baking soda and water and scrub the copper. Baking soda is alkaline and reacts with the patina.
- Vinegar soak – Soak the copper in vinegar for 5-10 minutes before rinsing and scrubbing.
- Lemon juice – Contains citric acid which dissolves the patina.
- Salt and flour – Make a paste with salt and flour. The abrasives help mechanically remove the patina.
- Commercial cleaners – Patina removal solutions are available. Contains acid cleaners.
This can return the copper surface to its natural shiny appearance. However, the patina will begin to form again if exposed to vinegar or air over time.
Will coating or sealing copper prevent patina?
Yes, you can apply coatings and sealants to copper surfaces to prevent patina formation:
- Paint/lacquer – Applying a protective paint or lacquer coating prevents air/acid contact.
- Oil rub – Rubbing oil on copper creates a barrier.
- Wax – Applying a wax coating protects the copper surface.
- Varnish/sealant – Clear varnishes or sealants prevent patina formation.
This is commonly done for copper cookware, architectural copper features, jewelry, and other decorative copper items you want to keep shiny instead of turning green.
Is the green patina on copper harmful?
No, the green patina formed on copper from vinegar exposure is harmless and poses no risks to human health. The main compounds in the patina layer are copper acetates which are not toxic.
The green patina is simply a surface effect and does not deteriorate the actual copper material itself. It can be removed by cleaning the copper if desired.
However, if the patina is allowed to build up substantially over years, the acetic acid can eventually start to eat away at the copper metal beneath it. But this effect is minimal for most practical purposes.
Can you eat/drink from copper with a patina layer?
Yes, copper cookware or containers with a patina layer are generally safe to use for food and beverages. The green patina is a stable, inert coating that separates the reactive copper metal from food.
If the patina layer is intact, it prevents the copper from leaching significantly into acidic foods and liquids during cooking or storage. Over time very small amounts of copper may dissolve into foods but this is typically nutritionally insignificant.
However, if the patina layer is damaged by scouring or cleaning, then exposed copper surfaces could potentially leach higher amounts of copper ions. So it is best to leave the patina intact.
Does temperature affect patina formation?
Yes, temperature has a significant effect on the copper-vinegar reaction rate. Higher temperatures accelerate the chemical reactions leading to faster patina formation.
This is why heating copper in vinegar results in a darker green patina layer compared to room temperature exposure. Even small temperature increases can make a noticeable difference.
Here is an approximate guide to the effects of temperature on patina formation time:
|Temperature||Estimated Time for Patina Development|
|Room temperature (68-77°F / 20-25°C)||24-48 hours|
|Warm (100°F / 38°C)||12-24 hours|
|Hot (150°F / 65°C)||2-4 hours|
|Boiling (212°F / 100°C)||15-30 minutes|
The accelerated reaction rate allows patinas to be formed quickly with heat, such as when intentionally antiquing copper by boiling it in salted vinegar.
Does the type of copper affect patina formation?
The most common types of copper are high-purity copper, brass, and bronze. All of these develop green patinas with vinegar but some differences include:
- High-purity copper – Reacts the quickest and forms the most uniform green patina.
- Brass – May develop blotchy patina due to the zinc content.
- Bronze – Can form green patina but slower due to higher tin-zinc content.
So high-copper content alloys like copper and brass tend to patinate the fastest and most evenly. Bronze takes longer to patina due to it’s higher alloying element composition.
Does patina change the copper color permanently?
The green patina formed on copper from vinegar exposure is permanent if the copper remains exposed to air.
Even if patina is removed by cleaning, it will re-form over time again when the bare copper is exposed to oxygen, moisture, and/or acids.
To permanently prevent patina formation, the copper surface needs to be protected by coatings (paint, oil), sealants (varnish, wax) or kept away from air exposure. This preserves the natural shiny copper color.
So in summary:
- Bare exposed copper – Develops permanent green patina over time
- Sealed/coated copper – Retains its natural color
Vinegar readily reacts with copper to produce green copper acetate compounds known as a patina layer. This is due to vinegar’s acetic acid content. Salted, warm, or heated vinegar solutions accelerate the patina reaction. While not harmful, patinas can be removed by cleaning if desired. Proper sealing or coatings prevents copper from turning green when exposed to vinegar or air.