About contemporary tempera

The binder of modern tempera is an artificial emulsion.
There are source references (Vasari) that artificial emulsion was known as early as the Middle Ages (egg yolk was combined with oil). However, it was not more widely used.
Modern tempera is much more durable than the old one, since it consists of oil and resins.
The old tempera was water-soluble for a long time, and therefore to protect it and make it resistant to moisture, it had to be covered with varnish.
Nowadays tempera immediately after painting is not soluble and does not require varnish, unless this is the intention of the painter.