Iridescence

Iridescence in Nature

Iridescence comes from the Greek word for rainbow, iris, and occurs when an object appears to be different colors when viewed from different directions. The color shifts of iridescence can be dramatic, as in precious opals, or subtle, as in a beetle wing, a kingfisher feather, or the corrosion on a glass. Several different phenomena can cause iridescence, and regardless of how it is created, it always remains captivating.

The beautiful emerald color of beetle wings changes depending on the angle from which they are viewed. This iridescent quality protects them from predators. In the small Hindu kingdom of Basohli, located in the Himalayan foothills in the modern state of Himachal Pradesh in northwest India, court artists in the seventeenth century applied shimmering beetle wings to the surface of paintings on paper. Meticulously cut to shape, they were used to simulate emerald jewelry. When viewers lifted the paintings to appreciate their details, the “emeralds” flashed and glittered.

Iridescence in bird feathers usually occurs due to their intricate patterns and the ways in which their structures interact with light. Different colors can be seen from different angles, leading to the amazing array of hues we see in peacock feathers, for example.

In shells, iridescence is caused by thin, alternating layers of different materials and the ways in which they interact with light. The iridescent inner surface of some seashells is called nacre or mother-of-pearl and is made from platelets of the same material that forms the bulk of seashells and coral. Layers of this material, comprised of calcium carbonate and an organic glue, accumulate to trap intrusive debris and to protect the animal’s soft tissue. These thin layers have almost the same thickness as the wavelength of visible light, which is why we see amazing rainbows in them.

Iridescence occurs in stones and minerals as well. In opals, iridescence is caused by small spheres of silica packed closely together. At the right size, the spheres diffract different colors of visible light, causing iridescence.

Frequently found both in nature and in works of art is mica. The mineral mica has a glittery appearance when used in painting due to the plate-like quality of the particles of ground mineral. Mica is a general term to describe minerals made of thin sheets of silicates that reflect light and retain their luster when ground to use in paintings or prints. It can exhibit a range of colors and can also be white or colorless. Mica is perhaps more shimmery than iridescent but still has wondrous interactions with light that have made it a valuable material to use in works of art.

Get closer to a collection of Japanese books decorated with mica with librarian Reiko Yoshimura:

Whether the structures in bird feathers, the thin layers of shell in nacre, or the thin layers of silicates in mica, when the thickness of many materials is close to that of the wavelengths of light, they interfere with the different colors of light and cause iridescence. This can also happen as a material ages. For example, when silver begins to tarnish, the thin layer of tarnish can interact with light and cause rainbow-like iridescence. This layer can also be produced artificially using chemicals to give the surface of a metal object an intentional iridescent patination. When glass or ceramic glazes are wet for long periods of time, whether because they are underwater or they have been buried, the water interacts with the material, forming thin surface layers. The visual appearance changes from transparent colors to an opaque iridescence depending on the surface of the glass or ceramic itself.

Achieving Iridescence

While some artists incorporated iridescent materials found in nature, others sought to emulate this dazzling visual effect artificially.

The technique of luster-painted ceramic, which originated in ninth-century Iraq and spread to the rest of the Islamic world and to Europe, is considered one of the most important developments in the history of ceramics. One of the most difficult techniques to control, luster had already been used on seventh-century Egyptian glass before Iraqi potters adapted it to earthenware to lend their low-fired clay surfaces the shimmering effect of precious metals. Vessels or tiles were first fired with an opaque white (tin) glaze, and designs were then painted on the cold surface with a mixture of metal oxides, sulfur, and other materials. During a second firing, oxygen was drawn out of the metallic oxides, leaving behind a shiny film. This complex ceramic tradition spread from Iraq, Egypt, and Syria to Iran in the late eleventh century, where it reached new levels of technical and artistic sophistication during the Seljuq dynasty (1038–1194). The principal city associated with this technique was Kashan in central Iran, which also gave its name to the Persian term for tile, kashi. Adapted to utilitarian vessels, such as plates, bowls, and jugs, as well as tiles destined for both religious and secular structures, Persian lusterware often combined spontaneous cursive writing, usually poetry, with lively figural scenes.

In China and Japan, this artistic interest in creating iridescence is especially apparent in ceramics that were used for drinking tea. For example, Jian ware bowls from the Jian kilns in Fujian, China, are coated in iron oxide–rich feldspathic glazes that create a microcrystalline structure when fired in the kiln. The resulting lustrous glazes exhibit a variety of patterns whose appearance has been likened to oil spots or hare’s fur. The kilns produced tea bowls almost exclusively in the eleventh and twelfth centuries. Several factors contributed to the rise in popularity of Jian ware bowls in the middle of the Song dynasty (960–1279). The most important of these factors was the practice of tea drinking. In the Song period, tea was made from finely ground tea powder. Powdered tea was placed in a bowl, which was filled with hot water and whisked to a white froth with a bamboo whisk. The foamy tea shows best against black glaze. While these tea bowls eventually fell out of fashion in China, they traveled to Japan where they became prized vessels for preparing powdered green tea, or matcha.

Beginning in the sixteenth century, a new style of hand-carved, low-fired ceramics developed in Japan and became popular as tea bowls. Called Raku ware, these bowls were finished with the addition of combustible materials such as straw or leaves, which create a reduction atmosphere that causes the glaze to interact with carbon in the atmosphere and form metallic oxides. The resulting bowls have a deep and lustrous sheen.

The now sadly destroyed city of Raqqa, located on the Euphrates River in Syria, was once home to the powerful caliph Harun al-Rashid (reigned 786–809), who was also featured in the popular A Thousand and One Nights. Stories about Syria’s fabled past, in part, inspired Western collectors to travel to the region. Charles Lang Freer (1954–1919) visited Syria in the early 1900s and amassed a noteworthy collection of ceramics that had been recently unearthed in Raqqa. In the first half of the thirteenth century, Syrian potters developed their own distinctive styles in response to luster-painted wares from Egypt and vibrant blue glazes from Iran. These wares, made both for local and regional markets, were used throughout Syria and beyond. In the 1260s, invading Mongols destroyed the city of Raqqa and put an end to this rich tradition. Fortunately, the ceramics were rediscovered in the late nineteenth century. Because these vessels had been buried for centuries, the glaze had become almost iridescent due to their exposure to moisture in the earth. Charles Lang Freer was particularly attracted to this surface quality of Raqqa ware.

Freer was also an early patron of the Pewabic Pottery, a ceramics workshop in Detroit founded by Mary Chase Perry Stratton (1867–1961) at the turn of the twentieth century, and he acquired thirty-five pieces between 1908 and 1918. An avid collector of ceramics, Freer was keenly interested in aesthetic similarities between pottery glazes and the rich surfaces of the paintings he acquired. For her part, Stratton was inspired by the iridescent glazes of the antique Near Eastern ceramics in Freer’s collection. In exchange for making his paintings available to Stratton for study, Freer challenged her to create a ruby red glaze. In 1909, she succeeded and went on to develop six iridescent hues for which Pewabic became famous.

Freer’s abiding interest in iridescence informed how he displayed his collection. His choice to display his Raqqa ware directly across from the windows in the Peacock Room was likely a deliberate decision to showcase the ceramics’ shimmering glazes when the shutters were open to let in sunlight. You can see the Peacock Room illuminated in person every third Thursday of the month. You can also watch the opening of the shutters in the video below: