I have never experienced a greater disaster in the studio than when I attempted to use wax applied over hide glue. Hide glue melts at 130 degrees Fahrenheit, wax melts at 140 degrees Fahrenheit. Perhaps you can begin to see the problem. Once a board is heated to receive the wax, usually 170 degrees Fahrenheit, the hide glue will have melted into a liquid gel on the surface.
That is exactly what happened when I attempted to use hide glue as a gesso. When I attempted to apply wax to the surface, the wax gummed into itself and refused to bond with the hide glue.
When heated, hide glue melts and can be combined with pigments to create a primer. When it cools, it solidifies into a marvelous painting surface for watercolor and tempera. The cracked board from Phase II was coated on the back side with hide glue, a tonal under painting was created and then the board was placed onto the griddle to heat. Sadly, when the board came up to temperature, the hide glue primer melted and created a slick, impermeable surface to which wax refused to adhere. The wax balled up into globules and floated over the surface of the melted hide glue. When wiped down, the under painting smeared but remained mostly intact. A positive result of the adventure was that the hide glue sealed a crack in the wood.
Direct painting of colored wax on a hide glue primer did not work. Would it work on a tempera painting that had been created on hide glue? The next test applied a clear wax overlay onto a tempera under painting. Although the all wax application failed on the hide glue primer, could hide glue work with a watercolor under painting and wax overlay? One final test of hide glue was attempted. For this test a digital reconstruction was made of a fragment of a painting. A sketch was created on a hide glue primed board and the portrait painted in watercolor. Would the watercolor provide enough of a barrier between the wax and hide glue for a wax overlay to be possible? The result was only semi-successful. When the water color portrait was heated to temperature for application of the wax (170 degrees Fahrenheit), the hide glue primer began to break down and started to create a slurry beneath the watercolor. Only a very delicate and light hand in application of wax saved the picture from a complete meltdown and some detail was lost.
The final question is whether or not a traditional egg tempera painting on hide glue primer can hold up to the heat and allow a clear wax overlay. I think the odds are pretty good that it will. The water color almost survived. Egg tempera, once it is cured, will be a completely solid surface. I have painted a traditional Byzantine Icon and will wait 6 months for it to cure. Once cured, I will put it into the oven, heat it to 170 degrees Fahrenheit and apply a clear wax overlay.
Stylization over time has increased but there are still some similarities in the structure of the face, most notably the chin which is almost a perfect circle. The highlighting of the face is similar, the eyes have gotten smaller and the exaggerated stylization of the forehead is a modern invention.