For the composition of one of these figures of men or animals a large number of units was required, and in order that it might preserve its fidelity it was necessary not only that the separate pieces should exactly coincide but that they should be fixed and fitted with extreme nicety. At Babylon they were attached to the wall with bitumen. On the posterior surface of several enamelled bricks in the Louvre a thick coat of this substance may be seen; it has preserved an impression of all the roughnesses on the surface of the crude mass to which it was applied. It is impossible to decide whether this natural mortar was allowed to fill the joints between one enamelled square and another or not. None of these bricks have been found in place, and none, so far as we know, unbroken. The coat at the back may have rendered the adherence so complete that no further precaution was necessary. In Assyria, so far at least as Khorsabad is concerned, they were content with less trouble. The bricks forming the enamelled archivolt of which we have spoken are attached to the wall with a mortar in which there is but little adhesive power.[367] It offered no resistance when M. Place stripped the archway in order that he might enrich his own country with the spoils of Sargon. But for an accident that sent his boats to the bottom of the Tigris not far from Bassorah this beautiful gateway would have been rebuilt in Paris.[368] To fit all these squares into their proper places was a delicate operation, but it was rendered easy by long practice. Signs, or rather numbers, for the guidance of the workmen, have been noticed upon the uncovered faces of the crude brick walls.[369] Still more skill was required for the proper distribution of a figure over the bricks by whose apposition it was to be created. No retouches were possible, because the bricks were painted before firing. The least negligence would be punished by the interruption of the contours, or by their malformation through a failure of junction between a line upon one brick and its continuation on the next. There was but one way to prevent such mistakes, and that was by preparing in advance what we should call a cartoon. On this the proposed design would be traced over a network of squares representing the junctions of the bricks. The bricks were then shaped, modelled, and numbered; each was painted according to the cartoon with its due proportion of ground or figure as the case might be, and marked with the same number as that on the corresponding square in the drawing.[370] The colour was laid separately on each brick; this is proved by the existence on their edges of pigment that has overflowed from the face and been fired at the same time as the rest.