The term "gesso" does not refer to an actual painting technique, but rather to a primer that can be used for many techniques. The primary properties of gesso not only allow it to seal off the paint surface from the paint medium, but also give the artist flexibility and control over texture.
Definition and Uses
The simplest definition of gesso is a paint primer. This type of primer is desirable for two reasons. First, it forms a barrier between your surface and your paint, keeping paint from being absorbed. It is most often used on wood and canvas, but is suitable for many surfaces. The second primary use of gesso is to add texture. When applied, it will form a thick, hard surface, but the exact properties depend on which form of gesso you choose. You will find it in two forms; its traditional form and a newer acrylic form. If you want to use gesso to add texture, be selective about the gesso used, as they will vary in thickness and hardness. Some artists prefer a very thick, hard layer of gesso which they can carve, while others prefer a thinner softer layer that adds a more subtle texture. The quality of the gesso can also affect texture, as higher quality, more expensive gessos are usually very thick, and can be thinned by adding water to create the wanted consistency.
Acrylic gesso was developed in the 1950s and is now the most readily available form of the primer. Most acrylic gessos are a mixture of calcium carbonate, an acrylic medium and a pigment. The pigment is usually titanium dioxide or titanium white. Acrylic gessos come in a variety of colors, but by slowly adding small amounts of watercolor or acrylic paint to white gesso, you can control the color. Acrylic gessos are water-based, and therefore can be painted over with other water-based mediums. The main advantage of an acrylic gesso over a traditional gesso is that it is more flexible, making it more suited to canvas and other flexible materials. This same quality makes it less suitable if you want to carve into the gesso for a precise texture. There is some debate in the art world, however, about the archival qualities of acrylic gesso. Because it is a relatively new product, it has not stood the test of time like traditional gesso.
Although its exact origins are debated, we know that traditional gesso has been around for thousands of years and was heavily used during the Renaissance. It was made from animal glue, titanium dioxide, calcium (normally either chalk or marble powder) and water.
Traditional gessos can also be painted on by both water and oil-based mediums. Oil-based mediums are where the traditional versus acrylic gesso debate gets heated: some artists believe that oil-based mediums should only go over traditional gesso, while others believe that acrylic gesso is acceptable for oil paints. Regardless of the paint medium, traditional gesso is better suited to wood or other hard surfaces because it dries much harder and can crack when on canvas.