The manufacture of these bricks was carried on as it is with us at the present day. The fairly pure clay was well kneaded and pressed into a rectangular wooden frame laid on a rough reed matting. Nebuchadnezzar’s bricks almost invariably show the impress of the matting on one side, while the bricks of the other monarchs appear to have been made without this underlay. The frames were frequently grooved on one or more of their inner sides, which caused corresponding ridges on the narrow edges of the bricks. We can thus distinguish bricks with 1, 2 (see Fig. 71), or even 7 of these ridges. In Nebuchadnezzar’s first building period the bricks had no ridges, then only one, while in his latest buildings, such as the Principal Citadel, there are seven. It thus happens that no 7–ridged brick has a 6–line stamp, as by that time they were disused. Besides their number, the ridges vary in breadth, depth, and position. The sign of early manufacture is that they are placed in the centre of the side, and are of greater breadth, while later they are placed near the corners. Thus we have ample material for dividing them, not only according to the places where they were made but also as to their age. In the course of the 43 years’ reign of Nebuchadnezzar, it is obvious that with the gradual multiplication of brick factories the necessity of being able to distinguish between their several productions increased in like measure. The bricks are not always accurately separated from each other in the buildings, according to their marks, but on the whole the stamps, in addition to the ridges on the sides, enable us to distinguish the relative ages of the various walls. [Illustration: FIG. 53.—Aramaic addition on Nebuchadnezzar brick. ] It is evident from the bricks themselves that the burning was done in ovens, which can scarcely have differed materially from the brick-kilns used to-day both here and in Bagdad. They are built outside the town, where the clay is good and fuel—the low bushes of the desert—is abundant. They form great fantastic groups of buildings, to which the people attach tales of horror. With the Persians it was a favourite method of execution to throw persons into these heated ovens, and when one sees the flickering glare from their mouths rising up against the evening sky of Babylon, one is unconsciously reminded of the striking account in the third chapter of Daniel of the three men, Shadrach, Meshach, and Abednego, in the fiery furnace. Herodotus states that the manufacture of bricks for the town walls was always carried on close to the site where they were to be used. This may have been done in exceptional cases, but ordinarily the ovens were certainly farther outside.