In a general way, it may be said of the Oriental glass that penetrated into Europe in the early Middle Ages, that the type is given by carvings in rock crystal. We can point to no example of sculptured glass that can be compared to the magnificent vases carved out of that mineral that we may see in the Louvre or in the treasury of St. Mark’s. I should be inclined to place the district where this branch of glyptic art flourished, whether we consider works of rock crystal or of glass, somewhere in what may be called Upper Western Asia—in Armenia, Georgia, or Western Persia—and to refer many of the extant examples to a date rather before than after the Arab conquest. But all this, of course, is pure conjecture.

Of quite another type was the glass made, it would seem without interruption during all this period, in various parts of Syria. The industry appears by this time to have passed in great measure into the hands of the Jews. Benjamin of Tudela in the twelfth century found Jewish glass-makers at Antioch and at Tyre. It was they, apparently, who carried on the old traditions in the manufacture of artificial pastes, coloured to imitate precious stones. The fusible glass containing lead of which such pastes were made had indeed been from an early date associated with the Jews—‘Vitrum plumbeum, JudÆum scilicet,’ says Heraclius. The demand for such work must have increased immensely with the prevailing fashion of incrusting reliquaries, the covers of books, and various personal ornaments with large coloured jewels, real or false (generally the latter), cut en cabochon.

It is chiefly in connection with such work that there arose a curious literature, if that term may be used for the barbarous treatises in question. Already in Roman times we hear of writings that describe the manufacture of artificial gems: Pliny says that he purposely abstains from mentioning the names of these works—he would not help to spread so objectionable an industry. But at that time and even later it was in Egypt that treatises of the kind chiefly originated. The mysteries of glass-making were there early associated with more dangerous arts. It is mainly to writers on magic—white or even black—and to those on alchemy that we must turn to find the earliest examples of those strange recipes for the manufacture, and especially the colouring, of glass, of which I shall have more to say later on. This connection between the arts of the glass-maker and of the alchemist arose from many causes, some of them obscure. For example, the vessels used in the experiments of the alchemists were from an early date made of glass. Again, the strange changes of colour observed when glass was stained by copper or by gold were regarded as steps to the great discovery itself. So that from the days of the Ptolemies in Egypt, if not from an earlier date, down to the time of the German alchemists and Rosicrucians of the seventeenth and eighteenth centuries, we find along with the grotesque and cryptic formulas for the preparation of gold an almost continuous chain of recipes, equally absurd for the most part, for the colouring of glass. In addition to this, many of these treatises, although professing to deal with the general problem of the transmutation of matter, are in reality concerned with the more practical questions of making plausible imitations of gold, silver, and precious stones—they are, in fact, handbooks for the fraudulent goldsmith.

This is especially the case with the earliest example of the class that has come down to us, the famous papyrus of Leyden, which alone has survived the destruction that the Roman law again and again attempted to enforce in the case of all books of magic. M. Berthelot, whom I follow for these early writers,^[83] calls this papyrus the working notes of an ‘artisan faussaire et d’un magicien charlatan.’ This little work, found long ago at Thebes, is a Greek manuscript of the third century; it contains, however, little or nothing about glass, and is of interest merely as an early specimen of this class of composition.

Other Greek treatises of a similar character, which are either lost or survive only in extracts or translations, are attributed to Zosimus, a writer of the third century, who had a section on glass; to Synesius, a Cyrenaic bishop (400 A.D.), married, and half a pagan; and to Olympiodorus, a priest of Isis, who in the sixth century kept up some of the Hellenic traditions. The late Byzantine scholiasts drew up summaries of these treatises and of many others; an important manuscript of this class at Venice gives a list of fifty-two such works. But these Byzantine summaries are of little value to us; all grip of fact is completely lost in the mystical jargon of the school.

Of much greater interest are the many series of practical formulas written in Latin, beginning with the Compositiones ad Tingenda, known to us from a manuscript of the eighth century. Here we find a section upon the colouring of artificial stones, their gilding and polishing, and upon the colouring of glass generally—how it is rendered milky by means of tin, red by cinnabar (?), by litharge (?), and by a substance called calcocecaumenon, the latter word doubtless a corruption of the Greek equivalent of the Æs Æstum, or burnt bronze, the well-known mediÆval source of an opaque red. Further on recipes are given for other colours to be applied as varnishes. There is also a chapter on the making of glass and some summary account of glass-furnaces, interesting solely as the earliest example of the many such descriptions that have come down to us. In fact, all these writers copied one from the other, summarising or amplifying. The same recipes, more or less intelligently expressed, turn up again and again: we can trace them in Theophilus, and even in such comparatively modern writers as Neri and Kunckel.

A later treatise, the MappÆ clavicula (ninth or tenth century), follows closely upon the Compositiones. As regards glass, we find headings—for that is unfortunately all that remains of this section—on unbreakable glass, on the soldering of glass, on the art of tracing trees and fruits of all kinds upon a flask, on an indelible manner of painting on glass, and finally, three sections on the fabrication of pearls.

I have already discussed one of the recipes of Heraclius (or Eraclius) when describing the cemetery glasses. All that we know of this writer is that he was a monk, and that he probably wrote in Rome, not later than the tenth century. The twenty-one little sections that make up his two books are written in hexameters, and treat of The Colours and Arts of the Romans. A third and much larger book in prose, that is found in some manuscripts, is of a considerably later date and of quite a different nature.^[84] I will now briefly summarise what the true Heraclius has to say about glass in his two metrical books.

In the third section we are told that earthenware may be glazed with a preparation of pounded Roman glass, mixed with water and gum and then carefully refired. The fourth section—De Sculptur Vitri—describes a method by which glass may be first softened by smearing it with a mixture of fat worms and vinegar, sprinkled over with the blood of a fasting goat that had been fed with ivy; the glass may then be cut with a hard stone called pirites. This association of goat’s blood and ivy occurs more than once in the old recipes; for these strange ingredients there may have been originally a cryptic interpretation, but we should perhaps rather take the pretended necessity of their employment as a sign that the art of cutting glass had been lost. Then in section v. follows the account of the writer’s attempt at imitating the gilt glass of the catacombs, which I have already analysed (p. 92). The description of the manner of cutting (secari) the cristallum in section xii. is more practical; we are told of a plate of lead mounted on iron, over which a certain hard powder is sprinkled. But here, too, the virtue of goat’s blood is not forgotten; by its means the diamond may be made to yield to iron. In section xiv. a process is described by which Roman glass may be melted and cast into moulds of chalk to form ‘fair shining gems.’ Heraclius has been called an ignorant quack, but he well represents the views of his time. Compared with him, Theophilus, who wrote in the north of Germany some hundred or two hundred years later, seems almost a modern.

More important to us than any of these Western sources of information before the time of Theophilus, are certain Syriac manuscripts preserved in the British Museum and at Cambridge. For our knowledge of the contents of these I am again indebted to M. Berthelot (La Chimie au Moyen Age, vol. ii.). In the sixth and seventh centuries Syria had taken a commanding position, both commercially and artistically. The trade between the west and the east, when not interrupted by the wars between the Greeks and the Sassanians, passed through Antioch, and after the Arab conquest the seat of the Caliphs was for a time at Damascus, a Syrian town. In the history of glass, from the very earliest times down to the Middle Ages, Syria, as represented by the coast towns at least, has vied with Egypt for the premier position; the two countries have always been closely connected, and at more than one time they were under the same ruler. When we come to study the glorious Saracenic glass of the thirteenth and fourteenth centuries, we shall find that Syria has perhaps a better claim than Egypt to be regarded as the original seat of the manufacture.

These Syriac and Arabo-Syriac manuscripts (the later sections are chiefly in Arabic) form part of the material from which the Arabs learned the arts of the Greeks. They claim for the most part to be translations from Greek, above all Alexandrian Greek, writers, from Zosimus and especially from the pseudo-Democritus. They deal with alchemy, that is to say with ‘applied chemistry’ and the subsidiary arts. There is, perhaps, more of local knowledge and practical experience in them than appears at first sight, or than M. Berthelot seems to allow: it was the fashion then to sail under the colours of the great men of old.

Beside some scattered references to the subject in other places, we find in the thirteenth section of the second part of the British Museum manuscript a chapter devoted entirely to glass—it can hardly be earlier than the ninth century. To make glass, we are told, add ten parts of alkali to ten of sand, grill the mixture in an oven till it is ‘clean as pure wool.’ Here we have the preliminary fritting described. Heat in a crucible till the substance can be drawn out like gum, ‘then make of it what you will—cups, bottles, boxes—as the Lord may permit.’ If the vessels thus made tend to split during the manufacture, ‘lay upon them a thread of melted glass. Shape the head and other parts, then put back the vessels in the furnace to reheat, and withdraw them gradually [that is to say, anneal the glass carefully as a final process].... If you wish the glass to be white, throw in some female magnesia [i.e. oxide of manganese, see p. 77], if blue, add four mithgals of burnt antimony.’ The method of ‘cleansing’ glass by means of manganese had doubtless been handed down from Roman times, and the ‘burnt antimony’ is probably to be interpreted as a roasted ore of cobalt. For producing other colours, mention is made of various substances, but I am unable to give any reasonable interpretation of this part; we hear of tin, lead, and borax—the preparation of a fusible enamel would seem to be implied. Finally, we are told—‘Do what is to be done, according to the will of the Lord Sabaoth!’

There follows on this what is perhaps the earliest extant description of a glass furnace. ‘The furnace of the glass-makers should have six compartments, of which three are disposed in stories one above the other.... The lower compartment should be deep, in it is the fire; that of the middle story has an opening in front of the central chambers—these last should be equal, disposed on the sides and not in the centre (?), so that the fire from below may rise towards the central region where the glass is and heat and melt the materials. The upper compartment, which is vaulted, is arranged so as uniformly to roof over the middle story; it is used to cool the vessels after their manufacture.’^[85] We have also the description of a smaller furnace, which is perhaps that in which the more fusible glass for enamels and minor objects of verroterie was melted. Finally, an oven with a floor of brick-earth is mentioned, for fritting the sand and alkali. In spite of much that is obscure in this description, we can trace in it the general type of furnace which, doubtless handed down from Roman times, has survived in places with few important changes to the present day.

And here I may call attention to a contemporary drawing of a mediÆval glass furnace—a source of information as unique as it is unexpected. This is to be found in a manuscript of an encyclopÆdic work, De Originibus Rerum, compiled by Rabanus Maurus, one of the earliest of the schoolmen. Rabanus lived in the Benedictine monastery of Fulda, in the first half of the ninth century. The manuscript in question, which is attributed to the year 1023, has been carefully reproduced by the monks of Monte Cassino where it is preserved. The full-paged miniature is to be found in a chapter headed De Vitro; I can, however, discover nothing in the text that throws any light on our subject. In the illustration we see to the left a nearly naked workman who holds a mass of some green material, perhaps the frit; another man is blowing through a tube what is probably meant for the unfinished cup; to the left a chalice-like vessel, perhaps the model, is depicted. Notice, too, in the pediment of the roof (probably to be regarded as the annealing oven) a cup with a knob for stem, and hemispherical foot. Cups of a similar form, apparently in these cases of metal, are found in other illustrations of the manuscript (Plate XIX.).

M. Berthelot has reproduced in his earlier work (La Chimie des Anciens) several rough pen-sketches of the apparatus used by the mediÆval alchemists, taken from the St. Mark’s manuscript mentioned above. These drawings help us in a measure to understand the important place taken by glass vessels of various forms in the researches of these early experimental workers. Still more interesting are the illustrations in the Syriac manuscript from which I have just quoted; in these, the modern chemist may recognise many familiar forms. The glass vessels have chiefly reference to processes of distillation. The most important is the alembic, a form easily made; the neck of a long pendulous paraison has only to be heated on one side near the base, when it falls over of itself to assume the well-known shape. We see also flasks, standing in water or sand baths, within which various substances are digesting; in other cases the contents are volatilising into the turban-shaped aludels placed above them.^[86]

But in all this strange literature, which, starting from the banks of the Nile in the first centuries of our era, spread over the Byzantine empire and was so eagerly absorbed by the first Arab conquerors, the interest in glass is only of a secondary nature,—the great question was the transmutation of matter and the consequent preparation of gold. Glass, as I have said, was of importance chiefly as a means to that end.

It was far otherwise with the writer whose work we must now examine. Theophilus, the author of the Schedula Diversarum Artium, was, it would seem, a monk in the monastery of Helmershausen, not far from Paderborn, in the old Saxon land. The earliest manuscript of his work probably dates from the twelfth century; it is preserved in the famous library at WolfenbÜttel. The treatise itself may perhaps be referred to the end of the eleventh or to the beginning of the next century; but in spite of this early date the style of the book is modern compared with the mediÆval compilations we have lately been considering. That the German monk Rugerus, or Rogherus, should have assumed the Greek name Theophilus is itself a significant fact. He was, it would seem, a hard-working goldsmith and a ‘skilled artificer’ in many branches of the arts. He drew his inspiration from the Byzantine East on the one hand, and on the other from the younger civilisation that was beginning to centre in the new kingdom that was growing up in and around the Isle de France. To these sources we must perhaps add the older Cluniac tradition: from Tuscan artists also he had something to learn.^[87]

‘Theophilus, an humble priest, servant of the servants of God, addresses his words to all who desire by the practical work of their hands and by the pleasing meditation of what is new, to put aside and trample under foot all sloth of mind and wandering of spirit....’ In this book they will find ‘all that Greece possesses in the way of divers colours and mixtures, all that Tuscany knows of the working of enamels [electrorum operositate] or of niello [nigellum], all that Arabia has to show of works ductile, fusible, or chased, all the many vases and sculptured gems and ivory that Italy adorns with gold, all that France prizes in costly variety of windows, all that in gold, silver, copper and iron or in subtle working of wood and stone is extolled by inventive [sollers] Germany.’ We are here in a healthy northern atmosphere, far removed from the shuffling statements and ambiguous formulas of the oriental alchemists.^[88]

The second book of the Schedula is concerned exclusively with glass, but most of the thirty-one sections deal with the preparation of stained glass for windows. In a curious passage to be found in the prologue of this book, Theophilus tells us that he has ‘approached the atrium of the Holy Wisdom [AgiÆ SophiÆ] and beheld the cellula adorned with every variety of divers colours, showing the nature and use of each.’^[89]

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.