For in another context we find the _hsien hung_ contrasted with _fan hung_, which is the usual term for overglaze iron red, and the description already given of the application of _pao shih hung_ leaves no doubt whatever that it was an underglaze colour. The two terms are probably fanciful names for two variations of the same colour, or perhaps for two different applications of it, for we know that it was used as a pigment for brushwork as well as in the form of a ground colour incorporated in the glaze. The secret of the colour seems to have been well kept, and the general impression prevailing outside the factories was that its tint and brilliancy were due to powdered rubies, the red precious stone from the West which gave the name to the _pao shih hung_.[29] It is known that in some cases such stones as cornelian (_ma nao_) have been incorporated in the porcelain glazes in China to increase the limpidity of the glaze. This is reputed to have happened in the case of the Ju yao, but neither cornelian nor ruby could serve in any way as a colouring agent, as their colour would be dissipated in the heat of the furnace. The real colouring agent of the _chi hung_ is protoxide of copper. If there were nothing else to prove this, it would be clear from the fact hinted in the _Po wu yao lan_ that the failures came out a brownish or blackish tint. This colour has always proved a difficult one to manage, and in the early part of the last dynasty, when it was freely used after the manner of the Hsüan Tê potters, the results were most unequal, varying from a fine blood red to maroon and brown, and even to a blackish tint. The peculiar merits of the Hsüan Tê red were probably due in some measure to the clay of which the ware was composed, and which contained some natural ingredient favourable to the development of the red. At any rate, we are told[30] that in the Chia Ching period (1522–1566) “the earth used for the _hsien hung_ ran short.”