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 Sculpturâ 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!’