More from The Practice of Tempera Painting:
"When the coat of paint lies on a ground of equal value with itself, or on two or three coats of similar color and value superimposed, the surface underneath it is no more reflecting than the paint itself. Light passes into the paint layer and it reflected according to its color and value, and the effect produced is of opacity. Any color in tempera can be made to look opaque by putting on repeated coats of it."
Well, this seems reasonable. I realize that this series is a rudimentary attempt to grasp tempera principles. I'm quite unsure of what's to be learned here but I know I have to give it a try.
A side effect is that I'm approaching mixes with more attention. Most dry pigments seem to need a bit of grinding to subdue excessive granulation. I can't go to the muller for such a tiny bit of pigment so instead the palette knife becomes the grinder. I cannot remember where I've seen an image of a baby muller, just a little thing to be held with fingertips, perfect for that tiny pigment pile. I've found 2" models but something a bit smaller might be nice.
Thompson, Daniel V. Jr. The Practice of Tempera Painting. New York. Dover Publications. 1962. (Yale University Press. 1936.) p. 101.