The Hidden Layers of Oil Painting

When we see a work of art, we don't always think about what lies underneath and how it can impact what we see. But how a painting is built can greatly affect the paint, as well as the longevity of the piece. And we're talking specifically about the construction of an oil painting. Underneath all of the paint, oil paintings begin with a support structure, very often a layer of You can think of a painting support just like you would think of painting a wall in your home. In this scenario, the support would be the sheets of drywall. It's the hard surface that will hold the upcoming layers. In oil painting, the support can be in the form of a wood panel, canvas, paper, metal such as copper, board, and sometimes even stone such as marble. When oil painting was first catching on in the early 1400s, wood panel was the primary painting support. And the type of wood that was used depended on the wood that was available in the particular region where the artist worked. For example, poplar was common in Italy, and oak was common in northern Europe. Panel remained the standard until around the early 1500s when canvas began to increase in popularity, first in Venice and eventually throughout Europe. Canvas fabric is usually made of either linen, which is from flax, or cotton. And within 200 years, canvas became the preferred support material due to the fact that it could be easily rolled for shipping, it was affordable, and it made painting in a large format much easier. Panel then became the second most popular surface, with a small number painted on board or metal. Today, the most common supports are stretched canvas, boards, and cradled panel. No matter what support material you choose, it's going to need to be prepared, usually with sizing and then layers of gesso. Sizing is a thin layer of binder or glue that once dry, is impermeable to oil, creating basically a barrier between the support and the remaining layers of the painting. The intention is to seal the surface so that the support material, whether wood or cotton, doesn't come into direct contact with the oil in the paint. Without this separation, the oil can cause the support material to eventually rot. It also prevents any acids or chemicals in the support material from leaching into the layers of paint. Because, in a sense, the sizing isolates the support from the rest of the painting layers, it functions kind of like an isolation layer. Sizing is sometimes used as a true isolation layer later in the painting process, typically after the ground in order to reduce the absorption rate of the gesso. Which leads us to the third layer in an oil painting, the ground layer. I would define a ground as a plaster-like foundational layer that enables the surface to take the paint. A ground consists of a form of gesso, which, as I mentioned, is a form of plaster. It's made up of a liquid binder to hold the material together and seal it onto the surface, a bulking agent to give it form, like chalk or ground marble, and pigment for color. Gesso creates a gentle surface texture that the paint can grab onto, it has the proper absorption level, and it provides an additional barrier between the support material and the paints. Now there are different types of gesso. The very first gesso, the one the early masters used, was what we call traditional gesso. And that consisted of chalk, animal skin glue, and usually white pigment. Pigment is simply ground plant, mineral, or synthetic material that gives paint its color. Chalk and gypsum do provide an element of color, but it's weak. Pigment provides the dominant hue. Lead white was the pigment most often used by early Flemish artists, such as Jan van Eyck, because the lead in the pigment helped speed up drying time, and because they needed the reflective quality of a white surface. The choice of chalk versus gypsum was, like the choice of wood for the support, a matter of what was available locally. Chalk was readily available in northern Europe. Gypsum was more common in Italy and Spain. In fact, this difference in filler material is one of the ways experts can narrow down the origin of historic paintings. If a painting uses gypsum in the ground, they know it was likely produced by an artist living in Spain or Italy. The concept of applying gesso to panel actually goes back very far in history, as far back as ancient Egyptians, who would prepare surfaces for scribes to practice their writing on, or to take down first-hand accounts prior to transcribing onto papyrus. Gessoed panels were also used by the Egyptians to create painted funerary objects. When it comes to oil painting, historically the most common binder used for ground was animal skin glue. As time passed, we began to see the inclusion of other binders, including drying oil. Gesso is normally named after the type of binder used in the mix. So glue gesso uses glue, and oil gesso uses drying oil. The layer created with the gesso is called the ground. Sometimes you'll also hear the oil ground referred to as priming. When artists first started using canvas, it was logical that they would initially use the same methods for applying gesso to canvas as they had with panels. This meant thinner grounds, and primarily glue grounds. But by the time Titian came on the scene, he and other artists of his time began using both a glue ground and an oil ground in the same painting. This was referred to as a double ground. And it came about because artists discovered that if they replaced the glue binder with drying oil, the ground layer became less absorbent and resulted in more vibrant colors. This is because the oil in the paint didn't sink into the ground. Now sometimes masters would drop the glue ground altogether, using only an oil ground instead. But let's stop here for a minute. I need to explain the difference between a single and double ground. A single ground consists of one or more layers of identical ground. Each layer referred to as a coat or an application. No matter how many coats you apply, if it's all the same kind of ground, you refer to the entire grouping as a single ground. If you use two different types of grounds, whether they're two different binders or two different colors, it's referred to as a double ground. You can also have a triple ground and so on. For example, Rembrandt often used one umber colored ground and then a light gray ground. These layers would be referred to as a double ground because it involved two different colors. The color of the ground can come from the chalk alone or from the pigment added to the mix. As mentioned, although chalk conveys as white, it has a very low reflective quality. So the additional pigment color will easily dominate. White and off-white grounds remained the most popular color options until around 1500, when the use of medium and dark colored grounds became increasingly popular. Typically umber or ochre. Unlike a white ground where you can play off the reflecting light of a bright surface, a dark ground means that you're starting with a dark value and have to build up the medium and light values to establish your forms. It worked well for the chiaroscuro lighting effects that became popular around that time. This was a trend in ground color that remained popular for approximately 300 years, until about the mid-1800s, when light and even white grounds began to come back into fashion, as artists once again wanted to take advantage of the benefits of a white or light surface, enhancing and illuminating the paint colors. Light grounds were especially important to the Impressionists, who valued the more vibrant color schemes, and whose goal was to capture light in nature. Now one caution when it comes to using a darker ground is the fact that oil paint tends to become more transparent as it ages, meaning light areas with dark grounds underneath will become dingy over time, and dark areas with light grounds underneath will pale, becoming less dramatic. Let's look at an example. Because of the increased transparency of several of the paints in this Titian painting, experts feel that areas of the piece would have been brighter when originally painted. If we're talking about a chiaroscuro portrait, with the darks being conveyed in part by the dark ground, there's not much to worry about. But if that same dark tone sits beneath the pale skin of the figure, you could have a problem. With a light ground, those darks will increase in transparency and thus allow more light to gradually show through. However, keep in mind this is over a very long period of time. Now choice of ground color is often related to the subject being painted, and can be strategically selected. Many masters left areas of their ground unpainted as part of the final product. Perhaps a white area for a highlight, or a dark area for part of a shadow. They may also use the color of the ground as a contrast to the upper layers, such as a warm-toned ground that contrasts with a cooler tone in the upper layers. A colored ground can be used to unify a painting, even if the bulk of it's covered up, because hints of that color can sometimes show through and provide an all-over underlying tone. And of course, artists would use different colored grounds for different paintings. There's a lot that goes into the creation of an oil painting, and much of it lies beneath what the eye can see. The base layers and the planning can affect not only how the surface takes the paint, but also how long the piece will survive.