We can now form some idea of how far the art of making porcelain had advanced at the time when the tide of the Mongol invasion swept over the country. Our knowledge of the wares made at this time must be derived chiefly from the imitations of the older porcelain made at a later period, but in such a conservative country as China this reservation is of no great importance. We must remember that in all these wares there was no other decoration than that given by the glaze as applied to the variously moulded or incised surface of the paste. The nature of the glaze was therefore of pre-eminent importance. The range of colour, except in the rare _flambé_ vases, was in the main confined to shades of blue and green, and even of these colours pronounced tints are rare. All the colours at the command of the potters of these days were derived from the oxides of iron and copper. And yet with such simple elements, what an infinite variety! It has been truly said by a French writer that the beauty of the glaze is the _qualité maîtresse de la céramique_, and it is partly a recognition of this claim that has led so many French and American collectors, of late, to follow the example of the Chinese and Japanese connoisseurs, and to give so marked a preference to monochrome porcelains, which owe their charm to the [Illustration: _PLATE IV._ CHINESE] merits of the glaze alone. But the specimens we find in these collections are with but few exceptions of much later date. The price that a fine piece of Sung ware, above all if it has a good pedigree and comes from a known collection, has always commanded in China has sufficed, at least until quite lately, to keep such specimens in their native country.