Re: Casein Opacity?
Tricky indeed, while they grow more opaque as they dry, they also become more matte looking. They can be buffed after cured to a glossy look though, and can be used in a very transparent manner by adding very large amounts of water to paint ratios. It is surprising how strong the film is even when diluted by up to 95% water to paint ratios (depending on the surface preparation).
At that level of dilution they are quite transparent, but building up can be tricky then because a paper surface will want to buckle. On a traditional gesso primed panel though, I have found I can build up watercolor-like glazes of color pretty efficiently. They remain fairly transparent at that dilution level, and seem to adhere a bit better than Watercolor would (still having a very slight film of Casein affixing them to the surface).
One can also increase the transparency by adding more of the Casein binder itself, which works to keep the paint body more, it is similar to adding a few drops of oil to oil paints (or adding “clear” acrylic mediums to acrylic paint). This makes the paint a bit more mobile and transparent, but not at the cost of making glazing more difficult (as can happen with extreme water dilution).