No one knows exactly when eggs were first used in fresco painting, nor does it matter much. Some people used the yolk and the white together, some only one or the other, but the egg was, and is, always mixed with water. Some artists now put gum tragacanth into the mixture. It is then used like water in water-colour work, but is called 'tempera' or 'distemper.' The effect of the egg is to produce an easy flow of the colour with so little liquid that the paint does not run on the surface, as it easily does in ordinary water-colours. The effect of the yellow yolk of the egg upon the tints is insignificant, unless too much be used. By using egg, one may paint upon ordinary prepared canvas as easily as with oils, which is impossible with water-colour. As for the early paintings upon panels of wood, before oils were used, they were meant to be portable imitations of fresco. The wood was accordingly prepared by covering it with a thin coating of fine white cement, or stucco, which was allowed to dry and become perfectly hard, because it was of course impossible to lay it on fresh every day in such small quantities. The vehicle used could therefore not be water, which would have made the colours run. The most common practice of the Byzantine and Romanesque schools seems to have been to use warm melted wax in combination with some kind of oil, the mixture being kept ready at hand over a lighted lamp, or on a pan of burning charcoal. There are artists in Europe, still, who occasionally use wax in this way, though generally mixed with alcohol or turpentine, and the result is said to be very durable. Sir Joshua Reynolds painted many pictures in this way. With regard to using oils on a dry surface in wall painting, instead of fresco, Lionardo da Vinci tried it repeatedly with the result that many of his wall paintings were completely lost within thirty or forty years after they had been painted. The greatest of those which have survived at all, the Last Supper in Milan, has had to be restored so often that little of the original picture remains untouched. The enormous value of linseed oil and nut oil as a vehicle was apparent as soon as it was discovered in Holland. Its great advantages are that, unlike water or egg, it will carry a large quantity of colour upon the canvas at the first stroke, that it dries slowly, so that the same ground may be worked over without haste while it is still fresh, and that it has a very small effect in changing the tints of the original paints used. One may see what value was attached to its use from the fact that those who first brought it to Italy worked in secret. Andrea Castagno, surnamed the Assassin, learned the method from his best friend, Domenico Veneziano, and then murdered him while he was singing a serenade under a lady's window, in order to possess the secret alone. But it soon became universally known and made a revolution in Italian painting.