All the Fayyum portraits are either painted on wood or cloth, sized or unsized.
The thickness of the wood panels varies between 2 and 20 millimeters (.08 to .8"); it has been
noted that the thinner panels often take the form of steles, that is to say that the upper edge
is curved or the corners trimmed. Their dimensions vary between 40 and 44 centimeters (15.8 to 17.5")
in height and 21 to 24 centimeters (8.25 to 9.5") in width. The most common woods are native species,
such as sycamore or acacia; one of the Antinopolis portraits was even done on a simple board of fig
wood (ficus carica). But as the portraits were luxury items, imported species were also used, in
particular cedar and lime, the density and fine grain of which were highly valued.
With certain rare exceptions, and whatever the species chosen, the wood was always worked in such a way that the grain of the surface to be painted was vertical. One might imagine that this made it easier to place the portrait on the mummy's rounded form.
Some rare portraits were painted directly on the wood; but the surface to be painted was mostly covered with a white coating, the texture of which was smooth and dense, sometimes tinted by adding blue or black pigments. In most of the cases studied, this coating was made up of calcium sulfate, or plaster, to which was added a varying quantity of an albuminoid binder - this is gesso. Gesso had been know in Egypt since the earliest days of antiquity; it was first identified in King Djoser's funerary suite in Saqqara, where it was used to attach elements of blue decorative faience to the walls. Statues and statuettes from the Old, Middle and New Kingdoms were also covered with gesso before being polychromed. The painters of the New Kingdom did not alter the formula in preparing the walls of the tombs they were decorating. During the Ptolemaic and Roman eras, the funerary masks were made of molded gesso or, more rarely, molded on a core of papyrus or a mass of sized cloth. The carefully polished wood, covered with a coat of gesso, which was allowed to dry, then, after being smoothed, was ready to be painted. With a few brush strokes, in black or in red ochre, the artist quickly sketched his model. Some of these sketches, the first expression of creation, have been found on the very back of the panel.
Canvas was another medium. It was used prior to the advent of the Romans, as attested by small votive paintings on linen, exhumed in Deir el-Medina and Deir el-Bahara, representing the deceased before his table of offerings or his arrival at the portal of the Amenti; these date from the 18th to the 21st Dynasties. But the oldest example of painting on canvas, currently preserved in the Turin Museum, dates back to the first half of the fourth millennium BC; found in fragments at Gebelein, it represents the preparations for a funeral ceremony. In the Fayyum ateliers, sized or unsized linen was used nearly as often as wood. Its suppleness leads one to believe that it was reserved for funerary portraits only, with no other function than simply to be placed upon the mummy.