Plaster is calcium sulfate used for casting sculpture and making molds since ancient Egypt. From death masks to Renaissance reliefs to Rodin's models, plaster is sculpture's most versatile intermediate.
In 2007, archaeologists excavating a Roman bath complex at Baiae, near Naples, found something unexpected buried in the ruins: over 400 plaster casts of Greek sculptures, stacked in a workshop dump. These were not finished artworks. They were working models, the kind a Roman sculptor would use to copy Greek bronzes and marbles for wealthy clients. The Baiae find is the physical evidence for a process that scholars had only known from written sources: Roman sculptors made plaster casts of Greek originals, then used those casts as templates for marble copies. Plaster was the medium that transmitted Greek sculpture to the Roman world, and by extension, to us. Without plaster, the classical tradition might not have survived.
Plaster is a material made from calcium sulfate (gypsum), which is heated to drive off water and then rehydrated to form a hard, white solid. In sculpture, plaster is used for casting copies of sculptures, making molds, creating original models, and preparing surfaces for painting. The most common form, plaster of Paris, is made from gypsum mined primarily in the Paris basin, though the term is now used generically for any gypsum plaster. Plaster has been used in art since ancient Egypt and remains a standard material in sculpture studios and conservation labs today.
This entry covers what plaster is, how it is produced and used, its history from ancient Egypt to modern sculpture, and its role as both an intermediate material and a finished medium.
Plaster is made from gypsum, a mineral composed of calcium sulfate dihydrate (CaSO4-2H2O). When gypsum is heated to about 150 degrees Celsius, it loses three-quarters of its water content and becomes calcium sulfate hemihydrate (CaSO4-0.5H2O), known as plaster of Paris or bassanite. When this powder is mixed with water, it rehydrates and recrystallizes, forming a hard, white solid that replicates the shape of the mold or form in which it is cast. The setting process is exothermic, meaning it releases heat as it hardens.
The working properties of plaster can be modified by the way it is processed. A 2022 study published in PNAS replicated two historical gypsum plaster formulations: Gach-e Koshteh from fourteenth-century Iran and Gesso Sottile from fifteenth-century Italy. The researchers found that the Iranian method produced a more hydrophilic plaster, while the Italian method produced a more hydrophobic one, despite both being made from the same raw material. The differences were caused by changes in the crystallographic texture of the material, revealing what the authors called "an astonishing technical achievement in a historical context." Read the study at PNAS.
Plaster is distinct from lime plaster, which is made from limestone (calcium carbonate) rather than gypsum. Lime plaster is harder, slower-setting, and more water-resistant than gypsum plaster. Both materials have been used in art, but the term "plaster" in sculptural contexts usually refers to gypsum plaster.
Plaster is the primary material for making molds and casts in sculpture. A mold is made by pouring liquid plaster over an original sculpture (in clay, wax, or another material). Once the plaster hardens, the original is removed, leaving a negative impression. Liquid plaster is then poured into the mold to create a positive cast. This process can be repeated to produce multiple copies. Plaster molds capture extremely fine detail, including fingerprints and tool marks, making them ideal for reproducing sculpture.
Sculptors use plaster to create intermediate models between the initial clay sketch and the final work in marble or bronze. A clay model might be cast in plaster to preserve it while the final sculpture is being carved or cast. Plaster models can also be measured with pointing machines to transfer proportions to a marble block, or used as patterns for bronze casting.
Plaster is sometimes used as a finished sculptural medium, not just an intermediate. Donatello and other Renaissance sculptors made devotional reliefs in plaster. In the eighteenth and nineteenth centuries, plaster casts of classical sculptures were collected by museums and academies as teaching tools. The Victoria and Albert Museum in London, the Cast Courts of the Metropolitan Museum of Art, and other major institutions hold extensive plaster cast collections that reproduce famous sculptures from antiquity and the Renaissance.
Plaster is used as a ground layer for panel painting and gilding. Gesso sottile, a fine gypsum plaster mixed with animal glue, was the standard preparation for Italian Renaissance panel paintings. This gesso layer provided a smooth, white surface for gesso painting and gilding. A 2020 review published in Studies in Conservation documented historical techniques and materials used to produce and coat plaster casts, drawing on manuals, treatises, and recipe books from the nineteenth century. Read the study at Taylor & Francis.
The use of plaster dates to the Neolithic period in regions including Iran, Cyprus, and Egypt. In ancient Egypt, gypsum plaster was used as mortar, for stucco decoration, as a repair material for ceramics and statues, and as a molding material. Plaster casts were made in ancient Egypt at Amarna during the reign of Akhenaten (c. 1340s BCE). The Egyptian Museum in Berlin holds plaster casts of faces from Amarna, including a head of Nefertiti, that were taken from clay or wax models and used as intermediate models for stone sculpture.
In ancient Greece and Rome, plaster was used for death masks, stucco sculpture, and copies of sculptures. According to Pliny, the sculptor Lysistratus of Sicyon (fourth century BCE) invented the technique of making plaster casts from living faces, which he then used to create portraits. The Baiae find, with its 400-plus plaster casts of Greek sculptures, demonstrates that Roman sculptors used plaster casts as a "library of form" from which they could produce marble copies on demand.
In Renaissance Italy, plaster was used for molding and casting devotional reliefs. Donatello and Lorenzo Ghiberti produced polychrome plaster reliefs in their workshops, likely by molding gypsum from original models in bronze or marble. The gesso sottile technique, developed in fifteenth-century Italy, used slowly hydrated plaster of Paris to create a fine, smooth surface for painting and gilding on panel paintings.
In the sixteenth century, Raphael and Giovanni da Udine experimented with stucco duro, a hard plaster formulation that could be carved and polished. This technique spread from Italy to France, England, and the rest of Europe, where it was used for architectural decoration.
The eighteenth and nineteenth centuries were the golden age of plaster cast collecting. Museums, academies, and universities assembled collections of plaster casts of classical and Renaissance sculptures as teaching tools. The casts were produced by specialized firms that traveled to museums and archaeological sites to make molds of famous works. Johann Joachim Winckelmann, the eighteenth-century art historian, praised plaster casts for their whiteness and their ability to transmit the forms of ancient sculpture to those who could not travel to Rome or Athens.
A research paper from the Warburg Institute notes that in the eighteenth century, plaster casts were seen as "propagators of good taste" and that the material itself, gypsum plaster, was sometimes valued for its own qualities. The sculptor John Flaxman praised plaster casts of Ghiberti's Baptistery doors, comparing them favorably with the bronze originals in terms of legibility of design. Read more at Warburg Institute.
According to Pliny, Lysistratus was the first sculptor to make plaster casts of living faces. He created plaster molds of a person's face, poured wax into them, and made final corrections to the wax cast. This technique, which Pliny describes as the origin of portrait casting, established plaster as a tool for reproducing the human face with accuracy.
Donatello used plaster for devotional reliefs and as an intermediate material in his workshop. His plaster reliefs, often polychromed, were produced by molding from original models and were sold as affordable devotional objects for churches and private homes.
Rodin used plaster extensively as an intermediate material. His plaster models, many of which survive, show the direct impression of his hands and tools. Rodin's plaster casts were the basis for bronze casts produced by his foundries, and he sometimes exhibited plaster versions of his sculptures, treating the material not just as a step toward bronze but as a valid finished medium. The Musee Rodin in Paris holds hundreds of Rodin's plaster models.
Plaster is closely related to gesso, the fine plaster used as a ground for painting and gilding. It is used in modeling as both a material for original sculpture and a casting medium for reproducing works in clay or wax. Plaster casts are used in the lost-wax casting process as intermediate forms. The material connects to the broader history of carving, since plaster models were traditionally used as templates for stone sculpture. For more on sculpture materials, read our entries on bronze and marble, or explore our post on how to read a painting.
The Cast Courts at the Victoria and Albert Museum in London hold one of the world's largest plaster cast collections, including full-size reproductions of Trajan's Column, the Portico de la Gloria from Santiago de Compostela, and numerous Renaissance sculptures. The Metropolitan Museum of Art in New York has a plaster cast hall. The Musee Rodin in Paris holds Rodin's plaster models, displayed alongside his finished bronzes.
The Agyptisches Museum in Berlin holds the Amarna plaster casts, including the head of Nefertiti, which are among the oldest surviving plaster casts in the world. For more on sculpture and casting techniques, read our entries on lost-wax casting and modeling, or explore our guide on how to read a painting.
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