With the help of these models, moulds could be made for each separate brick. They were probably of burnt pottery similar to the moulds made for the abundant terra-cottas of Babylonia. The mould would form one side of the frame in which the brick was struck, and, according to the regular method of bonding, a course of whole bricks (33 × 33 centimetres) would be followed by a course of half bricks (33 × 16½). Thus the ground of the reliefs and the wall surface were actually identical, and there is not even a projecting base on which the paws of the great beasts might appear to rest, as would be the case with stone reliefs. This is art in clay, a specialised art, distinguished from all other kinds of relief. The edges of the figures do not project more or less squarely as they do in Assyrian alabaster reliefs (Fig. 17 A), but in an obtuse angle (Fig. 17 B). Also there are no even upper surfaces as there are on Assyrian stone carvings. Both peculiarities would considerably facilitate the withdrawal of the tile from the mould. The same conception of art influenced the marvellous, highly developed, glyptic art of Babylonia. The style of the gem reliefs during the time of Hammurabi was also transferred to stone, while the older Babylonian stone reliefs distinctly show their direct derivation from the previous flat bas-reliefs, to which Assyrian art of the later period still adhered. Previous to our excavations no example of the plastic art of the time of Nebuchadnezzar was known. The brick when moulded and before it was enamelled was burnt like any ordinary brick; the contours were then drawn on it with black lines of a readily fusible vitreous composition, leaving clearly marked fields. These were filled with liquid coloured enamels, the whole dried and then fused, this time apparently in a gentler fire. As the black lines had the same fusing-point as the coloured portions they often mixed with the colours themselves, thus giving the work that marvellous and harmonious brilliancy and life which we admire to-day. With the Persian enamels which we shall meet with in connection with the Persian buildings these black lines have a higher melting-point and therefore remain distinct and project above the coloured enamels after the firing. The bricks had then to be arranged according to the design. In order to facilitate this and to ensure an accurate distribution of them on the building site, the bricks were marked on the upper side in rough glaze with a series of simple signs and numerals. The sign on the side of a brick and on that which was to be placed next it are identical. We shall learn more of the system in the Southern Citadel, where it was employed in the enamelled decorations of the great court.