Why did egg tempera become a durable painting medium?

Egg tempera is considered the most durable form of the tempera painting medium because it dries quickly to form a tough, resilient film. This film acts as a protective skin over the support, making the finished work generally unaffected by changes in humidity and temperature.1

The durability of the medium is further enhanced by the traditional preparation of the support. Artists typically use rigid surfaces, such as wood panels, coated with multiple thin layers of gesso—a mixture of plaster or gypsum and size—which creates a stable, absorbent ground. Once applied, the paint becomes sufficiently water-resistant within minutes, allowing for stable overpainting. Because of these properties, egg tempera has been a reliable medium for centuries, used extensively for icons and altarpieces that have survived for generations.12