Egg tempera is wonderful -- just translucent enough to show the colours underneath new brushstrokes, but opaque enough for each layer to hold its own.
Here's the piece I'm experimenting with:
It's about a third done, I think. Large areas are going to receive another two or three layers of feathery furry texture. The whole piece is about 230 X 395 mm. Rather large for a miniature? ... except that there is no good reason for a miniature to be small. (The post on that is below.)
This vellum is an interesting piece. It had cockled during production and when it went into the sanding machine (so I understand from the manufacturers) the cockles sanded preferentially, giving it a curiously spotted appearance, which, with the veining, I've been working with to develop the background and the leopard spots on the 'dad' figure.
Haven't decided how to handle the white circles yet. They are gorgeous in real life, showing the vellum surface. But for reasons that will become clear below, I'm thinking they need to be coated with at least a glaze of egg and water.
I'm using a combination of broad thin coatings of paint like glazes, applied with a size 6, and short hairline brushstrokes to build up a furry texture, applied with a 000. Here you can see part of the bottom edge of the cloud with close-up of the little furry lines. There are a lot of these already, and an awful lot more to come.
Yellow ochre over lapis lazuli produces a pleasing very dark stone grey. A small quantity of titanium white mixed into rust and brushed in hairlines over blue layers generates a startlingly luminous violet. So startling in the context of what I wanted to paint that I scraped that section off. The dark section pictured below left shows a much more restrained spectrum including less violent purple.
The paint dries tender to start with, and can pick up scratches and scuffmarks in its first few days. After two months, it seems tougher.
More on flexible supports.
The layers of paint all over the vellum -- tempera in front, watercolour on the back -- have stabilised the skin so it is not flexing/cockling like past pieces painted with watercolour/gouache alone on one side. The vellum is not entirely flat, but it does lie evenly on the easel now with a heavy, almost floppy quality that can't be achieved using watercolours. I think this is because the built-up layers of egg pigment (1) provide independent structure, and, perhaps, (2) prevent environmental humidity from reaching the vellum. So if vellum were coated back and front with egg tempera, the paint itself might reduce the skin's tendency to expand and contract, and so help prevent flaking.
But this is all speculation so far!
In any case, it is accepted that egg tempera should not be left on a flexible support because in time it becomes brittle, and it will then flake off if the support flexes. However, it is not always clear whether it is assumed that gesso is also involved. So I shall frame carefully and also hope that my theories -- about egg binding well onto animal skin, and the paint 'hydro-proofing' the vellum -- will be borne out.
Searching "egg tempera on vellum" gets any number of hits for artists and courses ... but not much about technical how-to or conservation issues. I'll be chasing up more info in the next weeks.
Meanwhile I have a lot of very tiny lines to paint.