To carry out his intention of holding up a mirror to reality in all its details, van Eyck had to improve the technique of painting. He was the inventor of oil-painting…If he used oil instead of egg (tempera), he could work much more slowly and accurately. - The History of Art, E.H. Gombrich pg 240
The Story of Art, E.H. Gombrich pg 239
if we want to understand the way in which northern art developed, we must appreciate this infinite care and patience of Jan van Eyck. The southern artists of his generation; the Florentine masters of Brunelleschi’s circle, had developed a method by which, nature could be represented in a picture with almost scientific accuracy. They began with the framework of perspective lines, and they built up the human body through their knowledge of anatomy and of the laws of foreshortenning. Van Eyck took the opposite way. He achieved the illusion of nature by patiently adding detail till his whole picture became like a mirror of the visible world. The difference between northern and Italian art remained important for many years. It is a fair guess to say that any work which excels in the representation of the beautiful surface of things, of flowers, jewels or fabric, will be by a northern artist, most probably by an artist from the Netherlands; while a painting with bold outlines, clear perspective and a sure mastery of the beautiful human body, will be Italian. - The Story of Art, E.H. Gombrich pg 239