At left is a 3/16" thick birch ply panel from Pandora, and on the right is a 1/4" thick masonite panel from Chester Gilder
Certain types of surfaces, or supports, are important to use with the medium of egg tempera. Egg tempera requires a painting surface that is both rigid and very absorbent. Some of the popular miniature painting surfaces such as drafting film, mylar, polymin, and ivorine would not ideally meet either of these requirements. The surface most universally preferred by egg tempera painters, which oil painters also still enjoy is traditional gesso, sometimes also called true gesso, which is the gesso that was produced prior to the advent of acrylic gesso. Traditional gesso recipes vary, but are usually composed primarily of rabbitskin glue and some form of whiting.
There is a fundamental incompatibility between the plastic content in acrylic gesso and egg tempera which can be risky for long-term adhesion. The greater absorbency of true gesso compared to acrylic formulas is an additional reason why it is so nice for egg tempera painting. It's piano-key smooth surface also makes it ideal for the refined details in miniature painting. Egg tempera painting superimposes many thin translucent layers with glazing and scumbling as the primary method of paint application, and traditional gesso can absorb these thin layers.
The labor involved in producing a true gesso panel, which involves cooking the gesso, painting at least four to eight layers on a wood or masonite panel, avoiding air bubbles and sanding it smooth, (combined with the difficulty of doing smaller cuts in the case of miniature panels) accounts for the cost of purchasing pre-made gesso panels. In fact, some artists actually prefer to make their own, sometimes doing "panel-making week" once or twice a year to produce the supply they will need.
I prefer to buy mine pre-made so that I can spend more of my time painting, and there are a number of panel makers out on the market producing quality panels. It's easy to see how True Gesso got it's name, and their panels come highly recommended, so I look forward to trying them too. My suppliers so far have been Pandora, which also produces icon panels, and Alexandra Hadik of Chester Gilder (*click on 'custom framing') who airbrushes the gesso on. Pandora will cut to custom sizes, and Alexandra has been willing in the past to cut me a batch of small mini panels to approximate sizes for my workshops, or for my own use, which for a flat fee gives me an ongoing supply.