The first chapter treats of the construction of the glass furnace, and enters at once into practical details. A German writer (A. Friedrich, Alt-Deutsche Gläser) has illustrated the furnaces of Theophilus by means of a diagram, and attempts to show how they differ from those described by the pseudo-Heraclius. All we can say is, that while the furnace of the later writer consisted distinctly of three parts—the main furnace with the glass pots in the centre, the fritting oven on one side, and the annealing oven on the other—in the earlier type of Theophilus there is no separate building for the fritting, which, it would seem, was done on the roof of the main furnace. In both cases the ovens form a compact group, heated by one fire. In the earlier furnace there were as many as eight pots, with corresponding openings, but these pots were probably much smaller than those of the thirteenth-century oven.
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.
Theophilus proceeds in the tenth section to describe how a vase of glass is prepared, and we have here again our earliest description of the process by which the gathering on the blowing-iron is manipulated so as this time to become a hollow vessel. In this case, he tells us, after blowing out your gathering of glass, instead of making an opening at the further end as in the case of the preparation of cylinders, you separate the bulb from the rod with a stick of moistened wood, and make the rod adhere to the lower end of the glass.[[92]] After reheating the glass, you now, with a piece of wood, widen and shape as you desire the opening where the tube was first attached. The foot is then shaped and hollowed. (If this foot is to be regarded as a separate piece, it is not quite clear how it is attached to the vessel.) The handles are fastened on by means of a string of glass taken from the pot with a slender rod of iron, and by similar means the surface may finally be decorated with threadings of glass. Theophilus then describes how a simple flask with a long neck may be prepared by swinging the bulb over your head, and then, as it cools, letting it hang down from the end of the tube; the vessel is then separated by a piece of moist wood; in this case no second rod is needed. No mention is made of the use of shears for cutting the semi-molten glass; they are replaced in a measure by shaping tools of wood.
In the twelfth section we are told of the remains of glass mosaics of various colours found in old pagan buildings, and how from these little cubes enamels are made to be set in gold, silver, and copper. In like manner it is by means of fragments of divers little vessels (vascula)—sapphire, purple, or green—that the French colour the costly glass so admired in their windows. This is a statement of no little interest.
Section xiii. treats of the manner in which the Greeks decorate the glass cups made from ‘sapphire stones’ with gold and silver leaf, covering the foil with a layer of very fusible colourless enamel. The passage is obscure, and I can only say in passing that I do not think that the process described can be identified with that adopted by the makers of the Roman cemetery glass. In the next section is described the Greek method of decorating glass vessels with the same colours—green, red, and white—that are used in the cloisonné enamels (electra). With these colours laid on pretty thickly, as well as with a preparation of gold, ground in a mill, they paint birds and beasts or little rosettes and knots in circles.[[93]] The Greeks make also bowls of purple and light blue, and flasks with longish necks, twisting around them threads of white glass, of which too the handles are made.
It may be inferred from these two sections that Theophilus probably regarded all the artistically coloured and enamelled vessels of his time as of Byzantine origin. He knows nothing about the constituents of the fusible enamels. The pseudo-Heraclius, on the other hand, has a chapter (viii.) telling how glass is made from lead (calcined previously to a powder) and how such glass is coloured. In another section the same writer refers to the ‘plumbeum vitrum Judæum scilicet,’ which is ground on a slab and used as an enamel to paint on glass.
Most of the remaining sections of Theophilus’s second book are concerned with the preparation of coloured glass for windows, but the last of all, ‘On Rings,’[[94]] describes carefully a method of making articles of verroterie with a small furnace and little crucibles. Lead is here mentioned casually as a constituent of the glass, and this, I think, is the only reference to this substance to be found in Theophilus’s chapter on glass. Here as elsewhere we may note that the word sapphirus is used as the equivalent of a blue glass paste (coloured probably by cobalt), and that it is referred to as a material that is at hand already prepared. Such cakes or slabs appear to have been an article of commerce from a period of remote antiquity. Something not unlike them has been found in Babylonian excavations (p. [40]). Similar cakes of coloured glass are still exported to China from the Bohemian glassworks.