Copper Green

                                           

You can see a statue or old age roof that looks a little green around that. 

That's because of patina. 

A patina finish is a surface change that has grown more beautiful with time. Scientifically speaking, patina is the green or brown film that forms naturally on the exterior of copper due to a series of chemical reactions. 

Copper forms a patina when exposed to oxygen and weathering long time. You'll see this change in colour on old roofing, copper statues. These natural changes to copper objects be over time as the metal oxidises, making copper a one- of-a-kind symbol of heritage that is hard to replicate. The chemical reaction The formation of the patina is similar to how rust forms on iron.

The chemical reaction:

Copper will react with the oxygen 

molecule in the air to form copper( I) oxide. 

• 4Cu + O2 → 2Cu2O (reddish) 

The copper(I) oxide will continue reacting to 

oxygen molecule long time to form copper(II) oxide. 

• 2Cu2O + O2 → 4CuO (black) 

As the copper( II) oxide continues to react with 

carbon dioxide and water molecule in the air it coats 

the exterior with that iconic blue-green patina colour. 

• 2CuO + CO2 + H2O → Cu2CO3(OH)2 (patina)

Depends  upon the availability of carbon dioxide and water molecule in the air, different chemical reactions occur to turn the copper oxide colours of blue, green, and purple. The city  where the copper present will impact its final colour. 

For example, larger cities may have more aerosol sulfates (produced from burning fossil fuel energies) which turns the copper oxide to a dark shade of green and emerald.

There is a reason that a patina surface has come to be associated with beauty. It is a visual representation of time.

A simple copper cleaner to vanish tarnish can be made by combining lemon juice with baking soda and stirring until mixed completely. Once mixed, apply to the copper surface and buff in a circular motion using a soft and clean cloth. Then Rinse and dry.

Reference:

1) https://education.seattlepi.com/causes-copper-tarnish-4611.html

2) https://en.wikipedia.org/wiki/Tarnish

3) https://www.copperh2o.com/blogs/blog/how-to-clean-copper-the-complete-guide