This black varnish is particularly lustrous and deep, but varies under different circumstances. Great difference of opinion has always existed as to its nature, and the method by which it was brought to such perfection by the Greeks. The variations in its appearance are due partly to differences of locality and fabric, partly to accidents of production. It is seen in its greatest perfection in the so-called Nolan amphorae of the severe red-figure period; and at its worst in the Etruscan and Italiote imitations of Greek fabrics. On the vases found at Vulci it shows a tendency to assume a greenish hue, as opposed to the blue-black of the Nolan vases, while variations in the direction of red, brown, and (on late South Italy fabrics) grey are of frequent occurrence. It is probable that these gradations of quality are mainly due to the action of fire, according as a higher or lower temperature was employed. On the other hand, the ashen-grey hue which vases of all periods sometimes assume[791] seems to be due to the direct action of fire in contact with them, and this may perhaps be explained by supposing that they had been burnt on a funeral pyre. This varnish also varies in the thickness with which it was laid on, as can be easily detected with the finger. Although the chemical action of the earth sometimes causes the black varnish to disappear entirely, leaving only the figures faintly indicated on the red-clay ground, there has never yet been found any acid which has any effect upon it.[792] Various opinions have been promulgated, from Caylus downwards, as to the elements of which it is composed.[793] Brongniart[794] has analysed it with the following results:— Silicic acid 46·30 50·00 Clay earth 11·90 Iron oxide 16·70 17·00 Chalk 5·70 Magnesia 2·30 Soda 17·10 Copper traces. It is unnecessary here to enter in detail into the numerous other theories of its composition, but so far it cannot be said that any certainty has been attained. Turning now to the methods by which the black varnish was applied, we find it necessary to distinguish between the two classes of black-figured and red-figured vases; some vases, of course, are completely covered with it, having no painted design, but these do not enter into the question.