The writing material was clay. Papyrus, it is true, was occasionally used, but it was expensive, while clay literally lay under the feet of everyone. While the clay was still soft, the cuneiform or "wedge-shaped" characters were engraved upon it by means of a stylus. They had originally been pictorial, but when the use of clay was adopted the pictures necessarily degenerated into groups of wedge-like lines, every curve becoming an angle formed by the junction of two lines. As time went on, the characters were more and more simplified, the number of wedges of which they consisted being reduced and only so many left as served to distinguish one sign from another. The simplification reached its extreme point in the official script of Assyria.
At first the clay tablet after being inscribed was allowed to dry in the sun. But sun-dried clay easily crumbles, and the fashion accordingly grew up of baking the tablet in a kiln. In Assyria, where the heat of the sun was not so great as in the southern kingdom of Babylonia, the tablet was invariably baked, holes being first drilled in it to allow the escape of the moisture and to prevent it from cracking. Some of the early Babylonian tablets were of great size, and it is wonderful that they have lasted to our own days. But the larger the tablet, the more difficult it was to bake it safely, and consequently the most of the tablets are of small size. As it was often necessary to compress a long text into this limited space, the writing became more and more minute, and in many cases a magnifying glass is needed to read it properly. That such glasses were really used by the Assyrians is proved by Layard's discovery of a magnifying lens at Nineveh. The lens, which is of crystal, has been turned on a lathe, and is now in the British Museum. But even with the help of lenses, the study of the cuneiform tablets encouraged short sight, which must have been common in the Babylonian schools. In the case of Assur-bani-pal this was counteracted by the out-of-door exercises in which he was trained, and it is probable that similar exercises were also customary in Babylonia.
A book generally consisted of several tablets, which may consequently be compared with our chapters. At the end of each tablet was a colophon stating what was its number in the series to which it belonged, and giving the first line of the next tablet. The series received its name from the words with which it began; thus the fourth tablet or chapter of the "Epic of the Creation" states that it contains "one hundred and forty-six lines of the fourth tablet (of the work beginning) 'When on high unproclaimed,' " and adds the first line of the tablet which follows. Catalogues were made of the standard books to be found in a library, giving the name of the author and the first line of each; so that it was easy for the reader or librarian to find both the work he wanted and the particular chapter in it he wished to consult. The books were arranged on shelves; M. de Sarzec discovered about 32,000 of them at Tello in Southern Chaldea still in the order in which they had been put in the age of Gudea (2700 B.C.).