Help with Paint Research

During my investigations of egg tempera paint I discovered many websites detailing a step by step process on creating it and a small history of egg tempera paint. That is great and all, but I am struggling to find any other information besides that. So if anyone knows any good websites for finding more different information then I would appreciate it. Anyways, the information I found is decent so far. For example, if you visit this website, http://www.geocities.com/CollegePark/Library/2036/tempera.html, you will find a decent amount of information detailing the different part of paint. First you have a liquid. In tempera paint the liquid is water. Then you have the binder which acts as an adhesive for the pigment, liquid, and the canvas you are painting on. For egg tempera the binder is egg yolk. Egg yolk mixed with water makes a powerful binder. Finally, the pigment gives the paint its color. In prehistoric times, humans used natural pigments such as charcoal and cadmium for color. Then it describes the different ways one could create a binder which could then be sued to create paint. Of course, there are still many unanswered questions such as, why egg yolk is such a powerful adhesive. I understand why a binder and pigment are needed for paint but why a liquid? Also, the paper mentions emulsions, but all they state is that egg yolk is an emulsifier. What is and emulsifier? To add to that is another small section on the history of prehistoric pigments.