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.

In the older times, when rare and valuable pigments were used, as well
as large quantities of pure gold, the materials to be employed and their
value were stipulated for in the contract made between the painter and
his employer before the picture was begun, and an artist's remuneration
at that time was much of the nature of a salary, calculated on an
approximate guess at the time he might need for the work. That was, of
course, a survival from the time of the Byzantine artists, to whom gold
and silver and paints were weighed out by the ecclesiastics for whom
they painted, and had to be accounted for in the finished picture. There
is a story told of an artist's apprentice, who made a considerable sum
of money by selling the washings of his master's brushes when the latter
was using a great quantity of ultramarine; and that shows the costliness
of mere paints at that time. As for the more valuable materials, the
great altar picture in Saint Mark's, in Venice, is entirely composed of
plates of pure gold enamelled in different colours, and fastened in a
sort of mosaic upon the wood panel as required, the lights and shades
being produced by hatching regular lines through the hard enamel with a
sharp instrument. The whole technical history of painting lies between
that sort of work and the modern painter's studio.

Before oil painting became general, artists were largely dependent on
commissions in order to do any work except drawing. Fresco needed a
wall, and work done in that ma