The Greek potter never made any legitimate advance beyond the Red-figured Style of the fifth century. In the early part of the fourth century there is no appreciable change of style; the technique is a little more perfect, the aim is a little less vigorous. The series of Panathenaic amphoræ[101] (those large jars painted with figures of Athena and athletic subjects intended for prizes at the Panathenaic games) continues unbroken, and their design changes little because they have to correspond with a conventional type. The custom was that they should have their figures in black, and accordingly the painter obeyed the custom by leaving parts of his vase in the natural red of the burnt clay, and treating those parts as [Illustration: PLATE LXXV. A NIOBID _Anderson_] panels on which he painted his figures in black.[102] One change we notice: vases are no longer signed by the artist. We conclude from this that pottery is no longer assigned to known masters like Hieron and Douris for decoration, but more mechanically produced in large numbers by humble craftsmen in factories. This would correspond with the increased professionalism which characterises this period in all departments of life. Towards the end of the century--that is, in the days of Alexander--it appears that vases were more frequently made in metal; not that we have any metal vases surviving, but the earthenware takes forms which can only be explained as imitation of metal. Thus the surface is often raised in relief, and vases are apparently cast in moulds.