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When mixed in small amounts and held on the surface of copper, these three compounds produce the pleasant luster of patina. Each of these three compounds can form beautiful crystalline structures on their own, like precious rocks. Darker greens indicate an abundance of malachite and brochantite, while blues suggest that azurite is abundant. The products that result from Equations 4-6 in the previous section each has their own shades of color:ĭepending on what combination of these three is present, the color of the patina can vary from shades of green to shades of blue. This section describes in broad terms why the presence of certain chemical compounds can affect the green and blue hues often seen in patina. The next section explains the physics and chemistry of why the colors of patina can vary so much from place to place and from time to time. Patina has different colors and patterns that depend on the unique experience that each copper object has had.

Equation 4: 2CuO + CO2 + H2O → Cu2CO3(OH)2.

The chemical reactions described above can be written in atomic symbols as follows: The result is a compound called brochantite, which varies in color from shades of dark green to emerald. The third reaction that influences the color of patina is when four molecules of copper oxide from Equation 2 reacts with one sulfate molecule and three water molecules (Equation 6). The result is azurite, which is a compound that varies in color from shades of blue to shades of purple. The second reaction that adds to patina’s color is when three molecules of copper oxide from Equation 2 can react with two molecules of carbon dioxide and one molecule of water (Equation 5). The result is a compound called malachite (Equation 4), which varies in color from shades of dark green to shades of blue. The first reaction that gives patina its color is when one molecule of carbon dioxide and one molecule of water, both in the air, react with two molecules of copper oxide on the surface of the copper. It contains carbon dioxide, which we breathe out, along with water vapor.

The greenish and blueish colors of copper’s patina come from the next set of reactions.Īir contains more than just the oxygen that we breathe. If sulfur is present on the surface of the copper, then the two can react to form copper sulfide, which is black (Equation 3). This copper oxide from reaction 2 is the main culprit that will later form the colors of the patina. The copper dioxide then reacts with more oxygen to form copper oxide (Equation 2). Both happen when oxygen in the air interacts with the metal atoms in the presence of water.Ĭopper reacts with oxygen that is in the air, resulting in copper dioxide (Equation 1).

The Chemical ReactionĮver ask yourself, “Why does copper turn green?” The formation of patina on copper is similar to the formation of rust on iron. This knowledge will also allow you to influence what color of patina forms, and how fast it will do so. Knowing how copper patina forms will give you a deeper appreciation for the unique patina that appears in your family's history. The shades of patina that form on the surface of a copper furnishing can store the memories of meals, spills, new homes and life changes over the course of centuries.Ĭopper is more than a shiny metal - it is one that can be adorned with beautiful carvings that tell a story. The natural changes that happen to copper objects make them a one-of-a-kind symbol of heritage that's hard to replicate. The beauty of copper is unique among decorative materials because the changes it undergoes are unique to each home and location. Copper home furnishings provide an elegant beauty because copper is a “living” metal, meaning it changes color over time.