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About Copper Patinas
Copper is an extremely durable metal that withstands corrosion and retains its functional properties for up to 90 years. And once it’s reached the end of its useful life, at least 90 percent of it can be recycled for reuse.
Whatever stage of its life, copper’s appearance changes when it is exposed to the elements. A colored film called a patina is formed on the copper surface. The different patinas are the reaction of copper with moisture oxygen and sulfur-based compounds. The patina acts as a barrier protecting the copper underneath against corrosion.
In this blog, we look at various colors and shades of patinas, the timelines for the color changes and why, over enough time, copper becomes bright green.
Why Copper Changes Its Color
In the first few weeks of exposure, especially in a humid climate or in places with frequent rainfall, the surface of copper can dramatically change colors with the metal displaying streaks of pink, orange, red interwoven with yellow, blue, green and purple. With continued exposure to moisture and other forces, the initial interference colors will give way a solid russet brown that is often called an oxidized or statuary finish.
In some copper mills, due to variable fabrication techniques, the surface of the flat or coiled sheet stock may be coated with a thin film of anti-stain oil. This film may produce a black or dark purple surface coloration shortly after it is installed and exposed to weather. However, this is just a temporary color change that will be wash off from rain after a short time and the natural weathering process of the copper will continue.
In coastal regions or heavy-industrial areas, the natural patina typically forms within five to seven years. In the country and rural areas, where the level of sulfur dioxide in the atmosphere is relatively low, the patina formation takes 10 to 14 years to attain a dominant stage. In dry regions, the basic patina may never be formed because of the extremely low atmospheric moisture.
Meanwhile, exposed horizontal copper surfaces develop their patinas faster than sloping surfaces. And patinas form faster on sloping surfaces compared with those on vertical surfaces. In each instance, the key factor affecting the patina is the amount of time that moisture and sulfur compounds interact with the exposed surface.
Three kinds of films form on exposed copper surfaces: sulfate, sulfide and progressive oxide. The sulfide and oxide films don’t provide much resistance to corrosion while the sulfate film provides strong resistance to all types of atmospheric corrosion as soon as it is fully formed. Therefore, it enhances the durability and lifespan of copper roofing tiles and sheets.
Timelines for Color changes
According to the Copper Development Association, the following timelines exist for the color changes that occur to natural weathering. The natural cycle of copper weathering is depicted by the 12 sets of sequential colors. These colors represent a typical sequence.
However, the weathering of any internal or exterior copper structure depends on the residual lubricants, environmental factors, and the orientation after installation. Every copper surface will attain a final patina color within its local environment and thereafter no more weathering or color changes will occur.
Unexposed: Natural salmon pink
4 months: Brown
8 months: Rosette brown
1 Year: Deep rosette brown
2 Years: Chocolate brown
3 Years: Dark chocolate brown
4 Years: Rusty brown
5 Years: Green with some tint of brown
7 Years: Dark green
10 Years: Leaf green
15 Years: Blue-green
25-30 Years: Bright green
Using Chemicals to Creating Different Copper Patinas
Due to the time it takes for copper to develop different patinas through weathering; architects, engineers and home designers know how to use chemicals to speed up this process and obtain the type of copper patina they desire.
Treating copper with chemicals is an art and the outcome depends on surface preparation, temperature, time, humidity and other factors. Virtually all natural patinas and several other colors can be produced on pure and copper based alloys with the aid of chemical conversion coatings. The conversion coatings speed up the natural weathering effect that occurs from exposure to the elements. Some coatings are also added to preserve the original color of the copper surface and prevent it from developing other patinas through natural weathering
Chemical coloring is basically achieved through the use of acid sulfate treatments or acid chloride treatments. However, due to several factors, patinas produced by chemical induction are usually prone to undesirable conditions such as poor adhesion, uneven coloration over a large surface area and excessive staining of neighboring materials. Therefore, these drawbacks should be carefully considered before applying chemicals.
Coating for Preservation of Original Tones
Meanwhile, preserving the original salmon pink color or the polished gold tones on the copper and bronze or brass surfaces can be achieved by using clear protective coatings that act as a barrier to natural weathering and prevent the oxidation of the copper surface. However, please note that virtually all coatings will wear out and get degraded over time. So these coatings will need to be maintained through stripping and replacement.
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