Art Nouveau (fl. 1890-1910) is a European architectural style characterized by its elaborate naturalistic ornamentation and usage of modern materials. A reaction against revival styles that were popular at the time, such as Beaux-Arts and Gothic Revival architecture, Art Nouveau buildings were meant to be more original and modern. Unlike Arts and Crafts, which sought to reject the technology of the Industrial Revolution, Art Nouveau used it to create its stylish designs. It was only briefly popular and was so outside the United States, so no true Art Nouveau architecture exists here.
"Art nouveau" was a term used in the 1880s to describe the work of Les Vingt and popularized by the art gallery Maison de l'Art Nouveau, which opened in 1895. However, the style known holistically as Art Nouveau today had various names across different countries, such as Style Moderne in France, Jugendstil in Germany, and Modern Style (not to be confused with Modern architecture) in English-speaking countries.
Art Nouveau traces its origins to the Arts and Crafts movement in Britain and the writings of Viollet-le-Duc in France. It was first seen in the decorative arts; but in architecture, the earliest Art Nouveau building is considered to be Gaudi's Guell Pavilions, completed in Barcelona in 1887.
Early Art Nouveau architecture was developed in Brussels. Two notable examples are Victor Horta's Hotel Tassel and Paul Hankar's Hankar House, which were completed in the same year. Horta and Hankar, along with Henry van de Velde, are considered pioneers of the style. The 1897 Brussels International Exposition exposed Art Nouveau architecture and art to a worldwide audience.
Art Nouveau architecture was practiced in other countries by the mid-1890s. Hector Guimard designed Castel Beranger in France, which was influenced by Hotel Tassel, and beginning in 1900 his metro entrances were installed along Paris' train stations. Paris was considered the epicenter of Art Nouveau architecture after 1900. However, buildings were also being designed in other countries, such as Germany, Great Britain, and Spain. In the United States, some people consider Louis Sullivan's architecture to be in the Art Nouveau style (which I don't, although his ornament would probably fit the description), and Louis Comfort Tiffany's glasswork could is an example of Art Nouveau artwork.
Art Nouveau is mainly characterized by its form and ornamentation. Buildings are often asymmetrical and employ modern materials, such as steel, glass, and concrete. Ornament is inspired by nature and may either take the exact form of stems, flowers, and trunks, or it may abstract these forms. Their curvy, undulating shapes create a sense of motion. It also had an innovative relationship with light, as many of Horta's buildings included skylights and copious windows. Art Nouveau's extravagant expression created very busy, eclectic spaces.
Art Nouveau was a rather short-lived style, as it declined in the 1910s amidst criticism and the beginning of World War I. It ultimately influenced Art Deco architecture, which followed it along with early Modern architecture.