Another important factor in forming the Flemish school was the influence of the guilds. In the fourteenth century, the painter was a craftsman and as rigidly bound by the laws of his guild as any carpenter or mason. He was apprenticed to a master for perhaps five years, during which he was taught the secrets of the craft. He learned to choose the wood for his panel and make it ready for use. He mixed the fine plaster with which to cover the wood, and the durability of his picture depended on the care he used in this and the evenness of the coating. For every implement with which he worked, every colour that entered into his picture, he must depend upon himself. He must prepare his own oils and varnishes. If he wished to make a drawing, he often was obliged to work with the silver-point, and to prepare his paper himself; if he drew in chalk or charcoal, he had to make his own selection of materials. After the apprenticeship came the years of wandering, when the young painter could work for any master he pleased, could travel as far afield as he chose, and in this way gain experience and a store of valuable impressions. When he returned to his home, he was admitted to the painters' guild, provided he could satisfy its officers that he was competent; if so, he could take his position as a master of the craft. Even then he was not free from the supervision of the fraternity. His master's oath bound him to honesty and to do his work "as in the sight of God." Its officers inspected his materials and his output, and if either was found to be below the standard he was punished. Every contract must be fulfilled to the letter, and the guild officers were the arbiters in case of any dispute. Finally, all his implements were marked with the sign of the guild. Pictures of the cities of Flanders in the fifteenth century bear witness to their artistic splendour. Says an English writer of Bruges at that time, "The squares were adorned with fountains; its bridges with statues in bronze; the public buildings and many of the private houses with statuary and carved work, the beauty of which was heightened and brought out by gilding and polychrome; the windows were rich with storied glass, and the walls of the interiors adorned with paintings in distemper, or hung with gorgeous tapestry." It was in surroundings such as these and under the stimulus of competition with his brother craftsmen that Hubert van Eyck made his great discovery of a manner of using oil in painting large pieces that would make it possible to equal the brilliant colours of the illuminators. The Flemings kept the secret of the new process so well that it was not disclosed to Italian artists until toward the end of the fifteenth century.