MEDIÆVAL TREATISES ON GLASS
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 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!’
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.
[Illustration:
PLATE XIX
MEDIÆVAL GLASS FURNACE
FROM AN ILLUMINATED MANUSCRIPT OF RABANUS MAURUS]
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.