Calcite Pseudomorphs in Ash-Containing Grounds

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Author: José Luis Lazarte Luna

Editors: Christine Gostowski, Anne Schaffer

Related Terms


English calcite pseudomorphs, ash-containing ground
Spanish pseudomorfos de calcita, aparejo de cernada, aparejo de ceniza
Italian pseudomorfi di calcite
French pseudomorphes de calcite
Portuguese pseudomorfos de calcita


The technique of preparing canvases with wood ash mixed with animal glue in Madrid was first mentioned in the art treatise by Francisco Pacheco (1564 – 1644), El Arte de la Pintura: Su Antiguedad y Grandezas. Ash is a heterogeneous material with variable chemical and mineral characteristics dictated by the nutrients in the soil, plant species, plant parts burned, fire temperature, and weathering after combustion, among other factors. Wood ash is mostly composed of calcium carbonate as well as sodium and potassium hydroxide and carbonate soluble compounds with minor inorganic constituents containing silicon, manganese, aluminum, iron, phosphorus, sulfur, chloride and zinc. Most of the calcium carbonate in ash comes from insoluble recurrent crystals that are present as calcite pseudomorphs after calcium oxalates. The unique morphology of these crystals has been identified as a reliable marker to determine if calcite particles found in paintings originate from a plant source (Carò, Centeno, and Mahon 2018).

Calcite pseudomorphs have been studied widely in geological and environmental contexts, among others (Canti 2003; Naylor and Schmidt 1986), but their identification in the ground layers of paintings is a relatively new development. Calcite pseudomorphs have eluded morphological and chemical identification in cross-sections until recently (Jover de Celis and Gayo 2014; Carò, Centeno, and Mahon 2018; Lazarte el al. 2019; Mahon, Centeno, and Smieska 2019; Centeno et al. 2020). The first scientific evidence of the use of ash-based grounds described in Spanish historic sources came from a study of 39 paintings created in Madrid throughout the 17th century (Jover de Celis and Gayo 2014). Later, an investigation of calcite pseudomorphs established the correlation between the composition and the morphology of laboratory ash with cross-sections taken from Baroque paintings (Carò, Centeno, and Mahon 2018). Most recently, ash grounds have been reported on paintings from the 17th and 18th centuries by artists working in Madrid and Latin America, specifically in Mexico City, Puebla, Cuzco, and Potosí (Centeno et al. 2020). In these cities, the practice shows regional idiosyncrasies that deserve more attention. Even though the use of ashes seems to be a Hispanic tradition, the practice is mentioned in a French art treatise from the late 16th century, suggesting it was in place in other European countries as well (Stols-Witlox 2014; 2017).

Ash-Containing Grounds in Historic Treatises

Two Spanish treatises mention the use of sieved wood ash in preparation layers, El Arte de la Pintura by Francisco Pacheco (1564 –1644), published in 1649, and La Escala Optica by Antonio Palomino (1655 – 1726), published in 1715 and 1724. Both authors describe double ground preparations, referring to the first layer as aparejo and the second one as imprimación (Bassegoda 1990; Palomino 1797; Veliz 1986). These preparation layers have been translated to ground and priming, respectively (Jover de Celis and Gayo 2010; 2014). Pacheco, writing from Seville, reports that in Madrid, canvases were first prepared with a ground of animal skin glue and sifted washed ashes, applied with a brush, and then leveled with a knife. Once dried, the ground was sanded with pumice and a priming of red earth and linseed oil was applied on top (Bassegoda 1990).

Palomino, 66 years later, mentions the same method but associates sifted ashes with cernada, an archaic laundering term (Palomino 1797). Cernada was the residual ash left on the cernadero, a cloth strainer that was draped over a large cylindrical container, known as a coladero. This container could be made out of a variety of materials, such as stone, wood, straw or clay depending on the region, and large enough to hold several pieces of clothing prior to whitening. Ashes were placed over the cernadero and hot water was poured over them to leach the potassium and sodium salts. This potash solution would be cycled repeatedly through the clothes thanks to a spout or drain at the bottom edge of the container. Once drained, the liquid was reheated and poured back onto the ashes so that it could percolate through the laundry over and over again (Sarasúa 2003). Due to its direct relation to the byproduct of this process, ash-grounds are called aparejo de cernada or aparejo de ceniza. After the extraction of soluble salts, the clumps of leached ash were collected and likely sold to professional primers and painters working in Madrid (Jover de Celis and Gayo 2014). The process of circulating lye from ashes through clothes was known as hacer la colada (Sarasúa 2003), a phrase that remains in use in Spain to refer to the act of washing clothes in general.

As a cleaning agent, lye had many preparations and uses. A caustic solution could be made by simply heating a mixture of water and ashes in a container, which upon settling would allow the floating of charcoal particles within the ash to refine the solution. In addition to whitening, lye was used alone or mixed with urine, white wine and/or soap to clean oil paintings by artists engaging in restorations, who may have used ash to prepare their canvases (Bassegoda 1990; Montón 1734; Köster 1827; Poleró y Toledo 1853; Forni 1866). Once dried, clumped leached ash was pulverized to a soft and light material, which makes it suitable to prepare large canvases without adding unnecessary weight (Jover de Celis and Gayo 2014).

In addition to the Spanish historic sources, an anonymous manuscript dated to c.1580-1600, written near Toulouse, France mentions the use of ashes mixed with oil and chalk for the first layer in a two-layer preparation for panels. Evidence of this practice has not been found in paintings by artists working in France (Stols-Witlox 2014; 2019).
Alt text
Rocío Bruquetas Galán reconstructing the ash-based preparation mentioned in Francisco Pacheco’s El Arte de la Pintura, 1649.

Mechanism of Formation and Characteristics

Calcite pseudomorphs originate from the thermal decomposition of calcium oxalates, whewellite and weddellite, which line the tissues and organs of many plants. Calcium oxalate salts have distinct shapes, dominated by prismatic crystals that are sometimes twinned, round druses, elongated formations known as raphides and styloids, and crystallite dust known as crystal sand (Frey-Wyssling 1981; Canti 2003; Carò, Centeno, and Mahon 2018; Centeno et al. 2020).

Upon combustion, hydrated calcium oxalates (CaC2O4.nH2O) may form calcite (CaCO3) around 420-510°C and/or lime (CaO) after reaching above 600°C and completing decomposition at 850°C (fig 1). Thermal decomposition of oxalates first, and of calcite later, produces aggregates of small lime (CaO) nanocrystals that retain the shape of the original calcium oxalates. The lime crystals are very unstable, reacting with water vapor and carbon dioxide in the air to form calcite (CaCO3). The complete transformation of lime to calcite occurs when ash is hydrated with water to prepare lye. This process occurs without an apparent change in the morphology of initial calcium oxalates after calcite crystals are formed as pseudomorphs of the oxalates. After the potassium and sodium salts are dissolved from ash to prepare lye, the main constituents that remain are the calcite pseudomorphs (Kloprogge, Boström, and Weier 2004). Calcite pseudomorphs have polygonal shapes of calcium oxalates with four or five sides and typically range from 10 to 30 μm in size. In a cross-section, these crystals have skeletal or porous surfaces that are evident even when they are fractured (Carò, Centeno, and Mahon 2018).
Fig1 Mechanism of formation of calcite psedomorphs.jpg
Figure 1. Mechanism of formation of calcite pseudomorphs from calcium oxalates. PLM image of calcium oxalates from Arizona sycamore courtesy of Microlab Northwest. BSE images of calcite pseudomorphs courtesy of Federico Carò, Conservation Scientist, The Metropolitan Museum of Art. BSE images 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.


In order to identify calcite pseudomorphs, it is necessary to take a deep sample that comprises the complete stratigraphy, especially since the pseudomorphs may only be present in lower ground layers. Due to the hardness, cohesion, and angular morphology of calcite particles, the delicate ash-containing ground may break easily upon sampling. Calcite pseudomorphs have a whitish to translucent appearance, although the proteinaceous binder in which they are typically bound, as well as pigments added, may affect their perceived color. Regardless of how carefully the charcoal was floated and removed during washing of the ash, microscopic charcoal flakes are typically present as natural impurities in these grounds.

Because of its elemental composition, it is possible that in the past, calcite derived from ash was confused with geological calcium carbonate. At the same time, calcite pseudomorphs have been confused with coccoliths found in chalk (Arroyo 2017). Raman spectroscopy and SEM-EDS have been used to confirm the presence of calcite pseudomorphs; however, initial identification can be done under high magnification when the crystals are large enough. Calcite pseudomorphs may be hard to distinguish when fractured, but if they are large and complete, they may be seen at 500x magnification. The proteinaceous binder in which they have been observed, tends to fill the porous embayments in the crystals, enhancing their visibility in UV light under the microscope. Recognition through SEM-EDS and BSE images has been decisive to show their morphology as well as to determine potassium and sodium ratios, which determines if the ashes have been leached or not (fig 2).

Fig2 Ashes in Sanchez.jpg

Figure 2. Sample from Marriage of the Virgin ca. 1690 by José Sánchez (active from 1686 to 1695 in New Spain, Mexico), showing a double red ground in which, the first is an ash ground. (A) Cross-section in normal light (viewed at 200×) and (B) under UV light (viewed at 200×). (C) Backscattered electron image of the same sample (viewed at 250×), (D) showing a detail of calcite pseudomorphs in the first ground layer (viewed at 800×). Reproduced from ‘Old world, New World: Painting Practices in the Reformed 1686 Painter’s Guild of Mexico City.’ Jose Luis Lazarte Luna, Dorothy Mahon, Silvia Centeno, Federico Caró and Louisa Smieska. AIC Paintings Specialty Group Postprints 2018(31): 69–74.

Occurrences in Preparations


The application of leached ashes for the first ground of a double ground preparation has been found in several paintings by Spanish and foreign artists working in Madrid (Jover de Celis and Gayo 2010; 2014; Romero 2009; García-Máiquez and Gayo 2017, Carò et al. 2018; Portús et al. 2019; Centeno et al. 2020). In these paintings, the lower ground is composed of leached ash and animal glue alone, while the oil priming on top is found in a variety of colors such as pink, orange, red, off-white and grey. The earliest painting identified with an ash-containing ground dates to 1625 and is by the Spanish painter Juan van der Hamen y León (Jover de Celis and Gayo 2014). It appears that the Spanish painter Diego Rodríguez de Silva y Velázquez (1599 -1660) was particularly given to using ash in his grounds (Jover de Celis and Gayo 2010; 2014; García-Máiquez and Gayo 2017, Carò et al. 2018; Portús et al. 2019; Centeno et al. 2020). The presence of an ash-containing ground in his well-documented Portrait of Juan de Pareja, which he painted during his second trip to Rome between 1649 and 1651, suggests his preference for this preparation (fig. 3) (Carò, Centeno, and Mahon 2018). However, as Velázquez’s enslaved assistant, it is likely that Pareja himself prepared the canvas on which his portrait was painted as well as several other works by Velázquez.

Both Pacheco and Palomino, who worked in humid and warm conditions in Seville, claimed that the use of ashes and animal glue was detrimental to the preservation of paintings, as it would cause eventual flaking (Bassegoda 1990; Palomino 1797). But these purported issues have not been noted on paintings in which these preparations have been found. In Madrid, ash-containing grounds appear throughout the 17th century and seem to have fallen out of favor by the 18th century.

Fig3 Velazquez Juan de Pareja.jpg

Figure 3. (A) Diego Rodríguez de Silva y Velázquez (1599-1660), Juan de Pareja (1606-1670),1650. Oil on canvas, 32 x 27 1⁄2 in. The Metropolitan Museum of Art (MMA 1971.86). Photo credit: Juan Trujillo (B) Cross-section from the lower left edge under UV light (viewed at 200x). Photo credit: Dorothy Mahon (C) Backscattered electron image of the lower layer in the sample (viewed at 1300×) Photo credit: Federico Caró.

Viceroyalty of New Spain (1521 – 1821)

Artists working in the central valley of Mexico during the viceregal period also employed ash in their preparations. The prevalence of white or red thick grounds followed by red oxide primings have been noted in several paintings from 18th century New Spain (Mues Orts 2017). New Spanish grounds appear to have more variation than those found in Madrid. However, only a limited number of studies confirming their use in Mexico have been published (Carò, Centeno, and Mahon 2018; Lazarte el al. 2018; Mahon, Centeno, and Smieska 2019; Vite Hernández 2019; Centeno et al. 2020). Ash grounds colored with red earth were found in two paintings, one each by Cristóbal de Villalpando (ca. 1649–1714) and Jose Sánchez (active from 1686 to 1695), two artists who became authorities in the reformed Guild of Painters in Mexico City in 1686 (Lazarte et al. 2018).

Recent studies show that ash-containing grounds were used before and after 1686. Ash was detected in a 1670 double portrait by an unknown Mexican painter of a Chichimeca indigenous woman and María Luisa de Toledo y Carreto, daughter of the Marquee de Mancena, Viceroy of New Spain (Bruquetas 2018). Calcite pseudomorphs have been found in the ground of a painting by Antonio de Torres from 1723 (Corbeil, Helwig, and Bégin 2019). The practice continued well into the 18th century in paintings by Miguel Cabrera (1695-1768) (Vite Hernández 2019; Sánchez Ledesma 2020), and most significantly, an ash-containing ground was detected in a 1797 painting by the Poblano painter Miguel Jerónimo Zendejas, suggesting that the tradition in Mexico may have continued into the 19th century (Vite Hernández 2019). It is not known if dedicated primers were active in Mexico, but the sheer number of large format paintings in New Spain suggest that priming could have been a viable profession. We do have examples in which painters hired other painters to provide primed canvases for commissions (Galí i Boadella 1996).

Viceroyalty of Peru (1542 – 1824)

The components in the ground of five paintings dating to the 18th century from the colonial Andes have been identified as having the characteristics of unprocessed ash-rich soil. These highly heterogeneous grounds were applied in a single layer and are mainly composed of silicate particles including quartz and feldspars in a matrix of finely divided, undetermined compounds containing silicon, calcium, potassium, aluminum, phosphorus, sulfur, iron, manganese, sodium, and chlorine. These particles have been interpreted as amorphous particles formed during mid- to high-temperature firing of wood in the presence of fine-grained silicates and alkaline salts that act as fluxes. Carbon particles in these layers supported this interpretation, however similar glassy particles may also have a volcanic origin. Pseudomorphic calcite was only found in two artworks in low concentrations and mixed with the amorphous particles, one depicting the Virgin of Mercy by an unknown Cuzco painter and the other attributed to Melchor Perez Holguín (ca.1660 - after 1732), an important painter working in Potosí in modern-day Bolivia. The presence of fewer calcite particles in a heterogeneous matrix indicates that the ash mixture was unrefined or was poorly refined. This impure, unrefined ash was intentionally added to the ground mixture. (Centeno et al. 2020).

Further Research

The use and identification of ash of different compositions and refinement in preparation layers is a subject that needs to be developed at a larger scale. For example, little attention has been given to the binders used to apply these grounds as well as their possible use in pictorial layers. To date, case studies and systematic investigations have been able to generate a rough schematic of this preparation tradition in the Hispanic world, but more studies are needed to fill the gaps. Finally, cross-sections in which calcium carbonate has been identified need to be revisited, especially those from Southern European paintings in which calcium sulfate would be expected.


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