The ancients painted on wood, on tanned skins, on lava, on marble, on slate, on metals, on walls covered with lime mortar, and on stone itself, after having saturated it with a sizing of resin made to penetrate by heating of a chafing-dish. Their portable panels were very thick and very solid. It is related that at the siege of Rhodes the soldiers made use of pictures by Apelles for a table. In the middle ages the same substances were used to paint on, but especially wood, chiefly oak and poplar. The joins of boards which formed the panel, and its coating, were made of animal glue (Taurocoll), or of flourpaste mixed with plaster or chalk, or even with a paste made of cheese and lime, the recipe for which Theophilus gives in a celebrated manuscript. This last paste is by far the most solid. It has been proved that on old panels altogether rotten the coating remained intact, and that even at the places where the joins were solidified by bands of canvas prepared with this paste, the wood which it covered had been preserved. Sometimes, for valuable pictures, they gummed all over the surface of the panel a leather coated with resin or covered over with plates of gold. But as they could not make those made, they were valuable enough to tempt the cupidity of the iconoclasts, and nearly all the pictures painted on those foundations considered indestructible were just the ones that were first destroyed. The exclusive use of wood panels was preserved in Italy till the time of Raphael, and in the Low Countries much later, till the time of Rubens. At that time so strong was the conviction that panels for painting should be scrupulously chosen, that government had monopolised their manufacture. Only perfectly dry and faultless wood was used, the workmanship was of the best, and it was forbidden, under penalty of fine, to paint on other panels than those of the government, for the following reasons:,"that the genius of an artist is the patrimony of his country; that it is the duty of the country to guarantee the longest possible duration to its masterpieces, and that, to make this sure, equal precautions should be taken with regard to all pictures; as a painter, however celebrated he may become, always begins by being unknown, he may by chance be modest, and consequently ignorant of the ultimate value of the work which he undertakes, and therefore the possibility of his compromising the work by negligence or economy should not be permitted.",,This law, which to-day would make many people smile, was rigorously enforced during the zenith of the Dutch school, and it is perhaps to it that we owe the preservation of many of the finest gems in European museums. ,,