A ground is an even coating applied over a substrate to provide a uniform surface for painting. Grounds may be white or colored, smooth or textured, depending upon medium, era, and artist’s intent. Ground preparation was and is a time-consuming, but necessary process that greatly affects the stability and final appearance of the painting. Although the methods and materials have varied by region and time period, the basic idea has remained the same. The ground evens out the surface of the substrate, creates a uniform color, and controls the absorbency of the surface, facilitating subsequent paint manipulation. Panels generally received layers of gesso or chalk-glue ground on top of a sizing layer. A thick gesso layer was necessary for paintings that received gold leaf, punch-work, or incising. The thick gesso layer provided room to carve out these designs without cutting into the wood. The gesso ground was scraped into a perfectly smooth, flat surface before painting or gilding began.
The common ground for panel paintings created north of the Alps was a mixture of animal glue and natural chalk (calcium carbonate). Gypsum (calcium sulfate) was the more common white filler for grounds created in the south. Some Early Italian and Spanish paintings are executed on a double ground system composed of a lower coarse ground called gesso grosso and an upper, smooth layer called gesso sottile . Most northern painters used a single chalk-glue ground layer. The grounds were generally smoothed by first darkening with charcoal and then scraping smooth with a metal scraper. Paintings to be executed in oil paint usually required an additional layer to reduce the absorbency of the ground and facilitate subsequent paint manipulation. This was most often accomplished by the application of a thin oil-bound layer that might be tinted. This layer was called an imprimatura in Italy.
The author Cennino Cennini described the use of one-layer grounds for small, simple works. Cennini also suggests that the two-part ground system was commonly used for Renaissance paintings with extensive gilding and decorative work. The coarser lower layer, or gesso grosso, was made from combining plaster (ground gypsum that had been heated to drive off the waters of hydration to create a semi-hydrate) with animal glue to create a thick paste. We call this processed gypsum “plaster of Paris” today. It readily sets into a solid mass in the presence of water. The addition of glue slowed down the setting of the plaster and created a very thick, hard ground layer. A flat, wooden slice or spatula was used to spread the gesso grosso over a prepared panel.
Once the layer was dry, powdered charcoal was rubbed onto the surface. This was smoothed using a metal scraper. This was continued until all the charcoal was removed, indicating that the surface was perfectly smooth and flat. This thick layer was necessary for subsequent gilding and punchwork, but the surface would be too coarse to be used without further layers. The fine smooth surface was provided by the next layer called gesso sottile. This was composed of very fine calcium sulfate created by slaking plaster in an excess of water and stirred for long periods of time to prevent the whole setting into a mass. After slaking, the water was poured off and the sottile was mulled on a mulling slab. The mulled material was then wrapped in fine cloth and squeezed to remove excess water. Finally, the processed material was shaped into loaves and allowed to dry. When the panel was ready for gesso sottile, the dried loaves were placed into a basin of water and allowed to soak. The softened loaves were cut into slices placed into a pot, and animal glue was added. This mixture was slowly stirred on indirect heat until it became smooth and homogeneous. Excess stirring and heat were avoided in order not to cause bubbles in the sottile ground. The prepared gesso sottile was applied with a brush. Cennini recommends at least eight coats of the sottile for flat areas of the panel, allowing drying time between each layer. The sottile was scraped, using the same methods as for the grosso, after the ground had dried for at least two days.
With the start of the 15th century, glue-ground systems gradually simplified. Single-layer grounds became more prominent than double-layer ground systems. Although gypsum-glue grounds remained popular in Southern Europe, analyses reveal a decline in ground quality. More pinholes, resulting from air bubbles, and scratches have been found in these later grounds. Mixtures of different hydrations of calcium sulfate (gypsum) have also been found.
Early oil paintings on canvas were generally executed on fabric prepared with a thin layer of gesso or other aqueous ground scraped into the interstices of the sized fabric. This was then usually coated with an oil-containing layer to reduce absorbency, to provide moisture resistance, and later to provide a preliminary color. The initial aqueous layer was abandoned for the most part by the mid-16th century when artists began applying oil grounds directly onto the sized canvases. Oil grounds are far more resilient to atmospheric moisture and damp climates typified by Venice. They are also far less absorbent which makes oil paint manipulation easier. Oil grounds do require much longer drying time greatly lengthening the preparation time. Drying can be hastened by the use of pigments and other materials that catalyze the oxidation of the oil film such as white or red lead and umbers. Initial oil grounds were lightly colored. Colored grounds on canvas became popular in the mid-16th century.
Some artists such as Tintoretto and El Greco apparently used palette scrapings as an ingredient for their mid-toned and dark grounds. Artists working on colored grounds often used the ground as a mid-tone or left portions exposed. Double grounds consisting of a lower earth-colored layer and an upper light toned layer became popular in the 17th century. This may have been done for economy as the earth pigment was cheap and would fill up most of the canvas texture requiring only a thin layer of the more expensive lead white. There may have been desired optical effects as well. Neo-classical painters of the 18th century returned to the use of white or pale-colored grounds, influenced by a belief that these lighter ground colors were used by ancient Greek and Roman painters. Industrial manufacturers in the 19th century produced new types of supports, such as canvas and Academy boards, often with pre-applied off-white or other neutral ground colors. Because of the high amount of sulfur pollution during the industrial revolution, lead white paint was susceptible to darkening (caused by the formation of black lead sulfide). Because of this, zinc white increased in popularity and was often layered over or mixed with lead white grounds.
Period manuscripts suggest that oil-containing imprimaturas and oil grounds were applied using tools such as a wooden stick or “sword,” a knife, the palm of the hand, or a brush. Smooth applications of oil layers are facilitated by using a knife-like shape rather than a brush (the brush hairs tend to leave evidence of the brush strokes).
Cennini, Cennino, and Daniel V. Thompson. The Craftsman's Handbook; "Il Libro dell'Arte." New York: Dover Pubications, 1960.
Dunkerton, Jill, Susan Foister, Dillian Gordon, and Nicholas Penny. "Techniques." In Giotto to Dürer: Early Renaissance Painting in the National Gallery: 161-164 New Haven: Yale University Press, 1991.
Dunkerton, Jill., Susan Foister, and Nicholas Penny. "Original Developments" In Dürer to Veronese: Sixteenth-Century Painting in the National Gallery. 265-271. Yale University Press and National Gallery Publications.1999.
Maartje Stols-Witlox."Grounds." In The conservation of easel paintings. 161-188 Abingdon, Oxon England: Routledge, 2012.
Townsend, Joyce, Tiarna Doherty, Gunnar Heydenreich, and Jacqueline Ridge, eds. Preparation for Painting: The Artist’s Choice and Its Consequences, Archetype Publications. 2008.