Font Size
Return to Previous Page

What is Patination?

Patination is the process by which a metal object develops a thin layer of oxidized metal on its surface. This thin layer is called the “patina,” and it can be formed intentionally (by using oxidizing chemicals) or unintentionally (by exposure to an oxidizing environment, such as polluted air). Patinas usually appear in colors different from the color of the substrate metal, with variations depending upon the composition of the substrate and on the chemicals introduced. For example, bronze, which is a yellow metal, can form many colors as it undergoes patination, including black, brown, red, and green.

Intentional patinas are created by an artist, or under an artist’s direction, through the use of a variety of different chemicals to achieve a precise color. Picasso’s Man with a Lamb (shown below) is an example of an intentional patina. This sculpture’s lustrous black-brown surface covering a bronze substrate is partially due to the formation of copper sulfide (Cu2S) during the patination process. An example of an artist-applied patina that produced an intense red surface on a copper substrate is the desk set made by La Pierre Manufacturing Company in 1895–1910 (a blotter corner from the set is pictured below). The red color is from the mineral cuprite (Cu2O).

Man with a Lamb
Man with a Lamb, 1943-44
Pablo Ruiz y Picasso, Spanish
Bronze
79 1/2 x 28 inches (201.9 x 71.1 cm) Base: 30 x 13 inches (76.2 x 33 cm)
Gift of R. Sturgis and Marion B. F. Ingersoll, 1958
1958-155-1
[ More Details ]
Blotter Corners, from a Desk Set
Blotter Corners, from a Desk Set, 1895-1910
Made by La Pierre Manufacturing Company, New York, 1885-1893, and Newark, New Jersey, 1893-1929,
Silver, patinated copper
each: 3 1/2 x 3 9/16 x 11/16 x 4 15/16 inches (8.9 x 9 x 1.7 x 12.5 cm)
Purchased with the Richardson Fund, 2000
2000-123-1a--d
[ More Details ]

Unintentional, or “natural,” patinas occur as a result of an object’s exposure to chemicals in its environment. The two masks from India shown below exemplify how local environments affect individual objects differently. Notice the distinct patina of each mask. The Buffalo-Headed Bhuta on the right shows more extensive corrosion, as can be seen by the dull blue-green nodules, than does the Ferocious Male Bhuta on the left, having a smooth and more reflective metal surface. The blue-green areas of the right-hand mask are produced by copper (Cu0) initially oxidizing to a brownish color (Cu+) and then undergoing further oxidization (Cu2+).

Detail of 2004-169-1

Ferocious Male Bhuta
Ferocious Male Bhuta, c. 19th - 20th century
India
Copper alloy
15 1/2 x 14 1/8 x 4 3/4 inches (39.4 x 35.9 x 12.1 cm)
Purchased with the Stella Kramrisch Fund, 2004
2004-169-1
[ More Details ]
Detail of 2004-169-3

Buffalo-Headed Bhuta
Buffalo-Headed Bhuta, c. 18th - 19th century
India
Copper alloy
18 1/16 x 14 x 7 3/8 inches (45.9 x 35.6 x 18.7 cm)
Purchased with the Stella Kramrisch Fund, 2004
2004-169-3
[ More Details ]

Cavalry Helmet of Etruscan-Roman type
Cavalry Helmet of Etruscan-Roman type, 2nd century BCE
Etruscan or Roman
Bronze
Width: 10 1/16 inches (25.5 cm) Height (approximate): 9 3/16 inches (23.3 cm) Weight: 4.7 lb., 4 pounds 11.3 ounces (2135.5 x 2135.5g)
Bequest of Carl Otto Kretzschmar von Kienbusch, 1977
1977-167-51
[ More Details ]
The Roman helmet pictured left displays a very thick and uneven patina. Mollusks actually grew in its interior and their shells remain attached to the corroded surface. These shells indicate that the helmet was submerged in seawater at some time during its 2,500-year history. The combination of water and salts catalyzed the corrosion, or patination, process and influenced the coloration of the helmet’s surface. Usually, a patina formed through exposure obliterates any intentional patina that the original surface possessed. However, by examining a patina that developed over time it is often possible to determine the circumstances of its earlier existence; whether, for example, the object was kept indoors or out, buried in the ground, or displayed in an urban environment.

In the case of an outdoor sculpture, the metal surface is exposed to the weather and to pollutants carried in rainwater and air. The resulting patina bears witness to the various environmental chemicals that created it. The bronze sculpture Social Consciousness (pictured below) that sits on the West Terrace of the Philadelphia Museum of Art demonstrates how rain etches a path as it runs down the metal (see detail). Notice also the black patches on this work of art. Beginning with the Industrial Revolution in the late eighteenth and early nineteenth centuries, increased air pollution hastened the rate of unintentional patination of outdoor bronzes such as this one. Many developed uneven blue-green patinas and unsightly black crusts, which raised awareness of the need for bronze conservation. The City of Philadelphia has over five hundred outdoor bronze sculptures (more than any other city in the United States); adequately conserving them presents a very great challenge. Conservators often protect intentional patinas by applying protective coatings.

To learn more about conservation treatments of outdoor bronzes, view the Conservation Department's 1992 work on Rodin's The Thinker.

Above: Social Consciousness, bronze
Detail of Social Consciousness showing how rain has etched a path on the metal.
Cross-section of an unintentional patina that developed on a bronze sculpture due to outdoor environmental exposure. Magnified here 200 times, the blue-green copper corrosion products can be seen forming an uneven layer on the bronze surface.
 

Top >>

Return to Previous Page