The Romans of the early Christian period had covered the floors and the walls of their temples and houses with mosaics; pictures made of coloured bits of glass. But this art had been exceedingly difficult. It gave the painter no chance to express all he wanted to say, as all children know who have ever tried to make figures out of coloured blocks of wood. The art of mosaic painting therefore died out during the late Middle Ages except in Russia, where the Byzantine mosaic painters had found a refuge after the fall of Constantinople and continued to ornament the walls of the orthodox churches until the day of the Bolsheviki, when there was an end to the building of churches. Of course, the mediaeval painter could mix his colours with the water of the wet plaster which was put upon the walls of the churches. This method of painting upon "fresh plaster" (which was generally called "fresco" or "fresh" painting) was very popular for many centuries. To-day, it is as rare as the art of painting miniatures in manuscripts and among the hundreds of artists of our modern cities there is perhaps one who can handle this medium successfully. But during the Middle Ages there was no other way and the artists were "fresco" workers for lack of something better. The method however had certain great disadvantages. Very often the plaster came off the walls after only a few years, or dampness spoiled the pictures, just as dampness will spoil the pattern of our wall paper. People tried every imaginable expedient to get away from this plaster background. They tried to mix their colours with wine and vinegar and with honey and with the sticky white of egg, but none of these methods were satisfactory. For more than a thousand years these experiments continued. In painting pictures upon the parchment leaves of manuscripts the mediaeval artists were very successful. But when it came to covering large spaces of wood or stone with paint which would stick, they did not succeed very well. At last, during the first half of the fifteenth century, the problem was solved in the southern Netherlands by Jan and Hubert van Eyck. The famous Flemish brothers mixed their paint with specially prepared oils and this allowed them to use wood and canvas or stone or anything else as a background for their pictures. But by this time the religious ardour of the early Middle Ages was a thing of the past. The rich burghers of the cities were succeeding the bishops as patrons of the arts. And as art invariably follows the full dinner-pail, the artists now began to work for these worldly employers and painted pictures for kings, for grand-dukes and for rich bankers. Within a very short time, the new method of painting with oil spread through Europe and in every country there developed a school of special painting which showed the characteristic tastes of the people for whom these portraits and landscapes were made.