: wooden panels (oak), prepared with animal glue and then animal glue with chalk. Chalk in those days wasn't as white as today (because of purity), so it would be a greyish surface. To prevent the paint from being absorbed too much into the chalk/glue layer, an isolation layer was used. This layer consisted of oil [it doesn't mention what kind of oil, but probably linseed oil or walnut oil] with lead as a siccative. Sometimes this layer contained some red pigment (probably red ochre), so this layer might have had a warmer tone.
: Drawings have been found under and on the isolation layer. It is speculated that a sketch was made directly on the glue/chalk layer, with black chalk, which was then fixed with the isolation layer. On this layer the final drawing was made with black paint/glue, applied with pen or brush.
: Van Eyck used drying oils with lead ("lead-oil") and protein (probably from eggs) for most colors. Lead-oil has a tedency to turn yellow and to crack, but the emulsification with egg probably prevented this. For the finest details he uses tempera (possibly with egg white). The same medium was found in parts where he used lapis lazuli, probably to prevent this expensive pigment to turn green because of yellowing.
In the glazes he used pure lead-oil (without the protein). These glazes were very thin, to prevent cracking. Because they contained no lead white (which acts like a drier) it did not crack.
The paint layers (examples from "The lamb of God")
- Blue in Mary's mantle: First layer consisted of azurite with lead white. Second layer consisted of a glace of pure ultramarine (lapis lazuli).
- Red in God's Red mantle: First layer consisted of vermilion with a little lead white. Second layer consisted of red lac with lead white. The final layer consisted of a glaze of pure red lac. In some places some black or blue (lapis lazuli) pigments were found, but the top layer always consisted of pure red lac.
- Green in St.John's mantle: First layer consisted of lead white with carbon black. The next layer consisted of lead white with "cupper green". The final layer consisted of a glaze of cupper resinate. [This probably wrong and should be verdigris.]
In general one can say that the first layers were rather grey, but usually with some of the final color. The next layer contained more color, less lead white and was less opaque. The glaze was applied with pure lead-oil and was very thin.
Stay tuned for "Rubens"