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Two images side by side. At left, a close-up of a large 18th-century Cuzco School painting depicting the Virgin of Valvanera. She holds the Holy Child with her right hand and a bouquet of flowers in her left. At right, a magnified paint sample in black and white shows the different pigments and particles in the paint and ground layers of the artwork. A 10-microns scale bar denotes the magnification in the corner.

Plant Ash in Ground Preparations: Morphological Identification Uncovers Novel Artistic Patterns in Baroque Paintings from Spain, North and South America

Silvia A. Centeno, Research Scientist, and Federico Carò, Research Scientist, Department of Scientific Research; José Luis Lazarte Luna, Conservator, and Dorothy Mahon, Conservator, Department of Paintings Conservation

The identification of the materials used for the ground preparations of paintings is of critical interest to scholars as certain materials can point to specific regions and schools of painting. Historically, a variety of inorganic materials, such as calcite and gypsum, bound with natural glues, have been used by artists to seal and protect the canvas support prior to the application of subsequent priming and paint layers containing drying oils.

This is the case of a particular ash-based, calcite-rich material obtained as a byproduct of lye production. In a previous work, we provided unequivocal tools to identify these calcite particles from ashes in paint cross sections. We compared the morphology, texture and composition of calcite crystals, called pseudomorphs, found in laboratory ash to the crystals observed in the ground preparations of paintings. (Read more about these discoveries in Heritage Science.)

The unique polygonal shapes and skeletal morphology of the pseudomorphs, as well as their abundance, make them ideal markers to recognize ash in ground layers, even if present in a small quantity (Figure 1).

A three-by-three grid view of nine black and white scientific images of microscopic particle samples

Figure 1. BSE images showing calcite pseudomorph particles, most of them with skeletal structure, found in the ground layer of Velazquez's Portrait of a man (a–c) and Juan de Pareja (d–f), and of Villalpando's Adoration of the Magi (g–i). Reproduced from 'Painting with Recycled Materials: on the morphology of calcite pseudomorphs as evidence of the use of wood ash residues in Baroque paintings,' Federico Carò, Silvia A. Centeno, and Dorothy Mahon. Heritage Science, 2018, 6:3.

The study focused on paintings by Diego Velázquez, completed in Madrid and Rome; by Luca Giordano, painted in Madrid and Naples; and by José Sánchez and Cristόbal de Villalpando, possibly painted in Mexico City and Puebla. We discovered that the practice of using ash in grounds, described in the Spanish treatises of Francesco Pacheco (1564–1638/1644) and Antonio Palomino (1655–1726), also occurred in Colonial Latin America. Our results indicate that ashes were used in different ways by Spanish and Latin American artists. Indeed, the application of a ground can be as personal as the finished product and variations within Spain and Mexico exist. We are now investigating local preparatory practices in a study of a larger number of works, with more diverse geographical origins and executed across a broader span of time.

Paintings from Colonial South America

To answer the questions posed regarding the geographical extent of the use of ash in preparation layers, we analyzed the grounds of six paintings from colonial South America. The selection was from an outstanding donation of ten paintings by James Kung Wei Li and his family in celebration of the museum's 150th anniversary. These late 17th and 18th century paintings were created in the Viceroyalty of Peru and as a group, they are representative of the artistic production of the Cuzco School. Due to the limited presence of South American colonial paintings in the collection, these artworks are truly foundational to the museum. As is frequently the case with paintings from the region, they were mostly created by unknown artists. Therefore, the investigation of the materials and techniques employed in these works is pivotal to understanding artistic methods in the region.

A photograph of two paintings perched on easels within a conservation studio with large windows in the background; the first smaller, portrait-oriented painting on the right depicts a large male figure holding a cross and encircled by a group of five ornately dressed smaller figures; the landscape view painting on the right features a female figure holding white lilies and a baby; both the female figure and child wear ornate crowns and robes; they are set agains a symmetrical background of a tree with tropical-like red birds and a mountaineous landscape; there is a male figure kneeling at the female figure's feet on her right-hand side.

Figure 2. Two of the Cuzco School paintings, part of the recent gift to The Met, during preliminary examination in the paintings conservation studio. Left to right, Christ Carrying the Cross, called The Lord of the Fall, and Our Lady of Valvanera, both painted between 1770 and 1780 by unknown artists.

The grounds in the paintings vary in color from red and dark brown, to black. Five of these paintings have a heterogonous and complex composition, but only two of them have calcite particles with the typical morphology of plant ash origin. The characteristics of the grounds in these five paintings from South America are consistent with the use of unrefined ash rich in soil that may have been ground and sieved, but not leached as described in the Spanish treatises. As a consequence, calcite pseudomorphs are less abundant than in leached ash.

A four-by-three grid view of twelve scientific images of microscopic particle samples; the first column of four images on the left-hand side are in color and the two adjacent columns are in black and white.

Figure 3. Photomicrographs of sample cross-sections taken with visible illumination (a, d, g, and i) and backscattered electron images (b, c, e, f, g, i, k, and i): The Soul of the Virgin Mary (a, 400x, b and c); Christ Carrying the Cross (d, 200x, e and f); Our Lady of Mercy (g, 100x, h and i); Saint Christopher (j, 400x, k and l). The red rectangles in b, e, h, and k indicate the areas where, respectively, the details c, f, i, and l were taken. Red arrows indicate calcite pseudomorphs, blue arrows glassy particles, and the green arrow the location of amorphous Si-rich particles.

This is a novel contribution to the field of technical art history that provides analytical evidence about connections in artistic practice in the Baroque world. As an increasing number of paintings produced in Latin America, Spain, and other European countries are investigated, we hope that more patterns will begin to emerge to help us better understand how the use of ashes spread beyond Spain.


'Painting with Recycled Materials: on the morphology of calcite pseudomorphs as evidence of the use of wood ash residues in Baroque paintings.' Federico Carò, Silvia A. Centeno and Dorothy Mahon. Heritage Science, 2018, 6:3.

'Cristóbal de Villalpando’s Adoration of the Magi: A Discussion of Artist Technique.' Dorothy Mahon, Silvia A. Centeno and Louisa Smieska. Latin American and Latinx Visual Culture. 2019, 1(2): 113–121.

'Old World, New World: Painting Practices in the Reformed 1686 Painter’s Guild of Mexico City.' José Luis Lazarte Luna, Dorothy Mahon, Silvia A. Centeno, Federico Carò and Louisa Smieska. AIC Paintings Specialty Group Postprints, 2018, 31, pp. 69–74.

'New Light on the Use of Ash in the Ground Preparations of Baroque Paintings from Spain, North and South America.' Silvia A. Centeno, Dorothy Mahon, Federico Carò and José Luis Lazarte Luna. Mobility Creates Masters: Discovering Artists’ Grounds 1550–1700. Archetype Publications, forthcoming, 2020

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The Metropolitan Museum houses a world-renowned complex of scientific research and conservation facilities, each of which serves as a training ground for conservators from around the world. This project list provides a small sample of their research activities.