We must now turn to the materials from which, according to Theophilus, the glass was prepared. Beechwood logs are dried and burned, and the ashes are carefully collected so as to be free from earth. Two parts of these ashes are mixed with one part of clean sand.[90] The mixture is roasted on an upper hearth and stirred with an iron trowel, so that it may not liquefy, for the space of a night and day. Note here that the ashes of the beechwood are used directly without any previous lixiviation; such ashes would contain, besides some alumina, more or less lime and silica, and these substances would pass into the glass. The glass pots are conical in form, curved inwards round the mouth, and they have a small lip. They are filled with the frit in the evening, and for the whole night a fire of dried logs is kept burning. There follows what is probably our earliest account of the process by which the gathering on the blowing-iron is converted either into a sheet of glass or into a hollow glass vessel. In the first case the _fistula_ or blowing-iron is dipped into the molten metal and turned round so that a mass of glass gathers on it. You blow gently through the tube, beating the glass at times against a flat stone that stands by the furnace.[91] You heat again the end of the long vesicle of glass, and with a piece of wood open out the aperture which now appears at the extremity to the full width of the glass tube. We have here a somewhat primitive method of forming a cylindrical _manchon_. The cylinder is now reheated in what is apparently a separate oven—the dilating oven; it is slit lengthways and opened out with an iron forceps and a piece of wood. When the glass has been smoothed out into sheets it is taken to the annealing oven, where the sheets are ranged on end against the wall and gradually cooled. It is somewhat of a surprise to find this ‘cylinder process’ for making a sheet of glass described by Theophilus, while not a word is said of the older process of ‘flashing’ or ‘spinning.’ There is some reason to believe that the knowledge of the former process was never lost in Germany. It was, however, only in the seventeenth or eighteenth century that the preparation of crown glass by means of cylinders came into general use in other parts of Europe.