The wonderfully bright colours used in medieval miniatures in manuscript books needed some sort of adhesive to make sure that they stuck to the surface of the animal skin. This was usually provided by eggs. Both the white of the egg and the egg yolk can be used, but rarely were they used together. Naturally, the white of an egg can sometimes be stringy or quite thick and gelatinous, and this is difficult to work with. So the egg is beaten to a light froth and then left for a while, and then the liquid underneath the froth is used, mixed with the pigment, and that makes it stick to the paper. Or the egg yolk can be used. The white needs to be removed from the egg yolk, and I'm doing this by passing it from palm to palm and wiping my hands to get rid of all the excess egg white. And then we need to remove the yolk sack so that the yolk itself remains on its own and can be used with the paint. This is then mixed with the paint, and this makes the paint stick to the surface.