True Gesso versus Acrylic Gesso
What are the benefits /drawbacks of true gesso versus an acrylic gesso for egg tempera painting? What is the viability of tempera painting executed on an acrylic ground?
A reply by Ross Merrill, head of the conservation department at the National Gallery of Art in Washington.
Let's take the three materials in question, an egg tempera paint film, a tradition glue gesso, and an acrylic "gesso" and look briefly at their properties.
Acrylic "gesso" is a misnomer since "gesso" is gypsum in Italian and there is no gypsum in most acrylic "gesso" formulations. Unfortunately, the term "gesso" was adopted by the acrylic paint industry to promote sales of the acrylic emulsion grounds rather than to indicate a relationship to traditional glue gesso. One must think of these acrylic "gesso" grounds as acrylic emulsion paints rather than glue gesso. In my experience if you discuss grounds with oil or tempera painters and use the term acrylic emulsion ground, rather than acrylic "gesso", they are much more skeptical of using it below their oil or tempera paintings, probably because gesso has such a reassuring tradition.
There are literally hundreds of formulations for acrylic emulsion grounds; the cheapest being virtually repackaged latex house paint. These acrylic emulsion grounds have an "open" surface, similar to glue gesso, but there the similarity stops. The open surface of the acrylic emulsion ground is intended to provide a mechanical lock to the next layer of acrylic emulsion paint. The resulting acrylic emulsion ground is exceptionally flexible (unlike traditional gesso which is very brittle) and will elongate up to 200+ %. Acrylic emulsion grounds are also dimensionally responsive to changes in temperature and humidity, especially temperature.
Egg tempera is well documented as producing a very brittle paint film upon aging, much more brittle than an aged oil paint film. For this reason, it is not recommended for execution on flexible supports such as canvas. For example, a typical dried oil paint film can be elongated (in analytical tests) up to about 12 - 14 %. A dried egg tempera paint film will elongate much less. While egg tempera paint may be diluted with water, upon drying it becomes insoluble in water.
A glue gesso ground on a wooden panel is as brittle or more so than egg tempera and is not very responsive to temperature or relative humidity changes. Glue gesso has a very porous surface, (known in the paint industry as an "open" surface) and acts much like a sponge in accepting water based or water diluted paint. Water (as in an egg tempera paint) applied to a dried glue gesso ground may have a slight softening effect on the glue binder in the gesso, creating a very dose bond between the egg tempera paint layer and gesso ground.
There is a very long (and successful) history of egg tempera painting on glue gesso grounds, as in Italian panel paintings.
In my opinion, the flexibility of the acrylic emulsion ground below the very brittle egg tempera presents a basic incompatibility between the two layers that can only lead to adhesion problems in the future. There is growing skepticism of using oil over acrylic emulsion grounds due to basic differences in the two materials.
The short answers to your questions -What are the benefits/drawbacks of true gesso versus an acrylic gesso for egg tempera painting? Traditional glue gesso works without any doubts. The long-term durability of acrylic emulsion grounds below egg tempera paint is still unknown.
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