Glass is a substance in so many ways connected with the conveniences and amenities of our daily life, and the word calls up so many varied associations, that I must here at the very beginning make clear with what a comparatively small proportion of the manifold applications of the substance I have to deal.

In the first place, this is an art history, so that with methods of manufacture and practical uses we are only concerned so far as they may influence or help to explain points of artistic interest. Again, even on the artistic side, it is not with every branch of the varied applications of glass that we shall be occupied in this work. By an anomaly of the English language, whose vocabulary for matters connected with the arts is so strangely deficient, we have come to understand by the term ‘glass,’ when used without further explanation, what is called in the trade ‘hollow ware,’ the verrerie of the French; in other words—vessels of glass. The term may also be extended to include various minor applications of the material—beads, small ornaments, etc., what the French call verroterie. But the application of glass to windows, especially when coloured and stained glass is in question, to say nothing of work in mosaic, is usually, although not always, held to lie outside this narrower connotation of the word.

Now it happens that for us this restriction is in every way convenient. For though the material basis is the same, it is evident that both the artist who works in mosaic and the designer of stained windows are concerned, each in his department, with artistic problems only incidentally connected with the material in which they work. In other words, the art element in both these crafts only becomes prominent at a stage when the actual preparation of the glass is completed. It is, however, certainly a pity that there is no English word which would not only clearly connote the class of objects with which I have here to deal, but which would at the same time distinctly comprise nothing beyond.

I have now explained the somewhat restricted and artificial sense of the word glass that I propose to accept in this work. But for a moment let us pass to the other extreme, and going beyond the ordinary connotation of the term include in it the glazes of pottery—the word ‘glaze’ is in its origin the same as glass—as well as the many forms of enamel. In all these cases we are dealing with substances of similar composition. They may all probably be traced back to a common origin, so that from an evolutionary point of view we have here an instance of the development of the complex and varied from the simple and single. Looking at the question in another way, the art of the enameller, using the term in a restricted sense, may be held to be subsidiary both to that of the potter and of the glass-worker; while many of the problems that arise in treating of the glazes of fictile wares—questions as to fusibility, or as to the colours employed and the changes of these colours during the firing—turn up again in the manufacture of glass. We shall see that experience gained in following the processes of one art may serve to throw light upon the difficulties and problems of the other.

Historically the connection between glass and pottery is not so close. In some degree the prevalence of one art has tended to oust the other, or to relegate it to an inferior position. The Greeks, who carried the potter’s art to such perfection, knew little about glass—it was long an exotic substance for them. The Romans, on the other hand, who in the first centuries of our era first fully appreciated and developed the capacities of glass, produced little pottery of artistic interest. In the sixteenth century, in Umbria and Tuscany, where the finest majolica was made, we hear nothing of the manufacture of glass, while on the other hand the fayence of Venice, at this time pre-occupied with her glass, was of subsidiary importance. If we turn to the home of porcelain, in China glass has always held a subordinate position, while in Japan it was until recent days practically unknown.

Were a comparison to be made between the development of the various minor arts, it would be difficult to find a wider contrast than that between the history of porcelain and that of glass. The knowledge of porcelain was confined for nearly a thousand years to China, the country where it was first made, and where it was slowly brought to perfection. Let loose, as it were, in the West early in the eighteenth century, it had then a short period of glory, but before the end of the century the art had already fallen upon evil days. The manufacture of glass, on the other hand, had long been carried on in Egypt, and perhaps in other Eastern lands, by a primitive process, although it only became an article of general use after the discovery of the blowing-iron. When and where this discovery was made we do not know—perhaps somewhere in Syria or Mesopotamia, in the third or second century before Christ. The art of blowing glass was known, no doubt, if not fully developed, at the time when the kingdoms of the Ptolemies and of the SeleucidÆ fell under the rule of the Romans. By them it was before long brought to perfection and carried into every corner of the West, so that by the second or third century of our era the production of glass in Europe was probably greater than at any subsequent time, at least until quite recent days. Nor was the art of glass-making completely extinguished by ‘the advance of the barbarians.’ Indeed, some of the Germanic tribes not impossibly brought with them a knowledge of the process not only of preparing but also of blowing glass, picked up on their journeyings through East Europe, or perhaps even learned in Western Asia. This was an instance of the passage to the North and West of the arts of civilisation, by what we may call the back-road of Europe, in opposition to the high-roads that led directly from Italy by way of the Rhone and the Rhine.

But in the West the manufacture, though continuously carried on in many spots, was after the fall of the Western Empire relegated to the woods,—for nearly a thousand years little glass was produced of any artistic interest. Indeed, but few examples of this forest or green glass of the Middle Ages have survived to our time. During all this long interval, in one direction only, in the West, was any advance made. Within this period falls the great development of stained glass: we must turn to the glorious windows of the cathedrals of France and other Western lands, to see what the glass-workers of the time were capable of producing. In the East, on the other hand, in the lands ruled from Constantinople or influenced by Byzantine civilisation, what we know of the glass of the early Middle Ages is almost confined to the mosaic coverings of the walls of the contemporary churches. But just as distinctly as the glass in the windows of the Gothic churches, this mosaic work, for the reason we have already given, falls outside our limits.

It was not till the end of the twelfth century that any important advance was made in our narrower department of ‘hollow ware.’ Among the many beautiful things made during that glorious season of artistic production that had its start about this time in Egypt (or perhaps, rather, in the lands between the Persian Gulf and the Mediterranean)—except it be the inlaid metal work—there is nothing that now interests us so much as the enamelled glass, the beautiful ware that culminated in the magnificent Cairene mosque lamps of the thirteenth and fourteenth centuries. The art of enamelling on glass passed over to Venice in the fifteenth century, perhaps earlier, and there in the next century the manufacture of the famous cristallo was finally achieved, and complete mastery was obtained in the working of this pure white glass. A fresh start was now given to the industry in the north by means of the Venetian glass-workers, who were sought for in every country to teach their new methods.

In Germany alone did some of the traditions of the old forest-workers of ‘green-glass’ survive. By the end of the seventeenth century the German glass, in some respects to be regarded as a compromise between the old and new, had become the most important in Europe. For a hundred years the products of ‘the mountain fringe of Bohemia’ held the premier position, but towards the end of the eighteenth century this place was taken by the facetted flint-glass of England. It is certainly remarkable that it is only of quite recent years that any such prominent position could be claimed for France, which heretofore had been content to follow in the wake first of Venice and then of Germany and of England. At the present day, however, this at least may be said—that France is almost the only country where any really artistic work in glass, apart from the reproduction of old patterns and old methods, is being produced.

This hasty sketch of the history of glass-making will help us to understand why it is that in following the development of the art in so many lands, and for a period of more than three thousand years, there is no need to linger for any time except at a few of the more important Étapes. Indeed such a procedure is forced upon us, for much of the road is quite barren, other parts are unexplored, while for whole stages we pass through prosaic districts where we find little of artistic merit to detain us.

The periods, then, of real importance in the history of glass, either from the cultur-historisch or from a purely artistic point of view, are separated by long intervals, during which little of interest was produced. The primitive glass of Egypt, the varied productions of the first centuries of the Roman Empire, the enamelled glass of the Saracens, and the Venetian glass of the Renaissance—this exhausts all that we find either of commanding historic interest or of superlative artistic merit. What follows—the German and the Netherlandish glass of the seventeenth and eighteenth centuries—is still of some importance under both these heads. I can hardly say so much of the English glass of the eighteenth century; but this glass must not be neglected—it is English, and it is highly prized by many enthusiastic collectors.

It will be seen that there is a long gap between the first and second of our critical periods—between the beginning of the primitive Egyptian and the earliest Roman glass. This gap will be filled, in some measure, by some account of the rare surviving specimens of glass that can claim an Assyrian origin, of the glass pastes of the MycenÆan age, and of the few examples of glass that can be strictly classed as Greek of the classical age. So again of the second long hiatus—the interval of nearly a thousand years between the period of the Roman glass and that of the Saracens,—this may be partly filled by the few scanty pieces that have come down to us from Sassanian and Byzantine times. To this period belongs also the glass of the Germanic tribes of northern Europe, above all that of our Anglo-Saxon ancestors.

Some notice must also be taken of a few districts situated on bypaths, of the glass from countries that lie away from the main centres of production—these latter centres, I may note, until comparatively recent times are mostly to be found in close connection with the basin of the Mediterranean. To these outlying districts we must finally turn to examine the glass of Persia, of India, and above all the glass of China.

An interesting chapter, nay, a separate work, might be devoted to the classification and history of a class of objects of which the manufacture has been carried on continuously and with few changes from the time of the Middle Empire in Egypt—of beads, I mean, and other allied applications of glass, included in the French term verroterie. But, however great the claims to attention of such objects, their interest is rather archÆological than artistic, and it will be sufficient to treat of them incidentally along with the, for us, more important class of ‘hollow ware’ produced with the aid of the glass-blower’s tube.

Properties and Composition of Glass

Christopher Merret, our earliest English writer on glass, sets down the properties of the material under twenty-six heads, ‘by which we may easily differentiate it from all other bodies.’ From these I will select some four or five which will be sufficient for our purpose. Thus, of glass, he says: ‘’Tis a concrete of salt and sand or stones. ’Tis artificial. It melts in a strong fire. When melted ’tis tenacious and sticks together.... When melted it cleaves to iron, etc. ’Tis ductile whilst red-hot, and fashionable into any form, but not malleable, and it may be blown into a hollowness’ (Art of Glass, 1662). Here we have briefly expressed the real differentiÆ of glass. It is rather by these properties than by any virtue of transparency or of definite chemical composition that glass is to be distinguished from all other bodies; and it is only by duly taking advantage of these properties that the preparation of a vessel of glass is rendered possible.

In passing from a liquid to a solid state there intervenes a viscous stage when the glass may be gathered at the end of an iron rod; the ductile, tenacious mass may now be drawn out into long threads, whose length and fineness are only limited by the difficulty of maintaining the requisite temperature. Again, if the rod upon which the mass is gathered is hollow, the glass may be blown out into a vesicle or bulb, the starting-point from which an endless variety of objects, bottles, cups, tubes, or even flat sheets of glass, may be subsequently formed. Until advantage was taken of this remarkable property of glass—its capability, I mean, of being blown out into a hollow vesicle when in a viscid condition—the art of the glass-maker was in a primitive stage. We may compare the glass prepared without the aid of the blowing-tube—that of the ancient Egyptians, for instance—to the pottery made by hand before the invention of the potter’s wheel.

In dealing with the practical side of our subject—the materials from which glass is made, how these materials are first fritted and then fused together, and how the fused mass is subsequently dealt with—the best plan will be to approach the questions in each case from the point of view of the time and country. But as, on the one hand, for classical times, our sources of information for these practical details are but scanty, and as, on the other, I am not concerned with the industrial developments of the nineteenth century, it will be well to postpone any fuller treatment of such matters until I come to speak of the glass of late MediÆval and Renaissance times. I shall then be able to make use of contemporary accounts which will throw light on the processes of manufacture.

A few preliminary notes on the chemical and physical properties of glass may, however, not be out of place.

Glass, Merret tells us, is ‘a concrete of salt and sand or stones.’ This, in modern scientific language, we should express by saying that it is a combination of silica with an alkali. But these substances alone are not enough. You cannot make a glass fit for practical use from a pure quartz sand with the addition of nothing else than a salt of potash or soda. Such a glass—a simple alkaline silicate—would indeed be transparent, but it would be difficult to work and very fragile. In all cases there is need of a second base, and this, to speak generally, should be either lime or oxide of lead. The latter base we may for the present neglect; speaking generally, it is the presence of lime that gives the working qualities and the requisite toughness. These, then, are the essential materials for the preparation of glass. Other substances may be present; alumina, for example, or one or other of the oxides of iron, but as a rule the presence of these latter bases is not desired—the glass would be better without them.

Putting aside, then, for the present the glass in which lead is a constituent, as well as that in which the soda is replaced by potash, it is remarkable how little difference of composition we find in examples of glass of the most divergent origin. Let us compare the composition of a Roman ‘lachrymatory’ with that of a piece of modern English plate-glass. In a hundred parts we find—

These examples are indeed two extreme terms of a long but continuous series. A sample of Saracenic glass of the fourteenth or of Venetian glass of the sixteenth century, would yield on analysis much the same result.^[1]

This, then, may be regarded as the normal composition of such glass as I shall have to deal with in this history. The main question has generally been—How can the sand or silica, the premier element in glass, be best converted into a substance which shall in external aspect resemble as closely as possible the native rock crystal (itself pure silica), but which at the same time shall be not only fusible, but after fusing pass on cooling through a plastic condition when it may be expanded into a vesicle and otherwise worked up into various shapes? Long practical experience has shown that this can be best effected by adding to the sand materials containing both soda and lime, and as far as possible nothing beyond these bases. A glass thus compounded we may take as our normal type, but, as I have said, the soda may in certain cases be replaced by potash and the lime by lead oxide.

Silica in any case is the essential element in glass, and in any normal glass there may be present from 60 to 75 per cent. If, however, the bases with which it is combined have a high combining number—and this is especially the case with lead—the percentage of silica may fall below the former figure. Thus, in a bottle glass with 12 per cent. of iron oxide and alumina^[2] the proportion is reduced to 54 per cent., and in a flint glass with 43 per cent. of lead oxide there is only 45 per cent. of silica.

It was once the fashion among English writers on glass to classify the substance under the heads of crown-glass, bottle-glass, broad-glass, plate-glass, flint-glass, etc.; but such a classification, not very logical in itself, would be of no use to us.^[3]

Glass, of course, varies in optical properties, in hardness, and in fusibility, but I do not think that any useful classification could be based directly on these properties. But there is one distinction of the greatest importance technically and geographically, and this is between the glass of maritime countries in which the alkali is soda, and that of inland and forest districts where the soda is replaced by potash. In the first group, by far the most important—I have indeed regarded such glass as the normal type—may, it would seem, be placed not only the ‘primitive’ glass of the Eastern Mediterranean, but probably all the glass of the Romans. To it belongs also the glass of the Saracens and the greater part of the artistic glass of the Renaissance, including the Venetian glass, although in this last the soda is often in part replaced by an appreciable quantity of potash. The potash group, on the other hand, includes the old voirre À fougÈre of the French and the wald-glas of the Germans. In addition, almost the whole of the glass of higher quality made in later days in Germany and in the Bohemian borderlands belongs essentially to this last class. Finally, it may be mentioned that in the case of the abnormal family where the lime is replaced by oxide of lead, the alkali is invariably potash. Of this family our English flint-glass is the most important member.

With regard to the hardness of glass, Merret mentions as the thirteenth property possessed by that substance, ‘that it only receives sculpture or cutting from a Diamond or Emery stone.’ But such a statement would be likely to give an exaggerated idea of the hardness of glass. If we take the scale of hardness used by the mineralogist, it will be found that there are few kinds of glass that do not fall between the fifth and sixth divisions of that scale. In other words, it would be difficult to find a specimen of glass on which a crystal of apatite (phosphate of lime) would make any impression, whereas all glass in ordinary use is readily scratched by felspar. It is possible, however, that some kinds of Bohemian glass may equal the latter mineral in hardness; it is indeed a common statement that certain Bohemian or German ‘combustion-tubes’ will strike fire with steel. On the other hand, the presence of lead tends to make a soft glass; our cut flint is perceptibly softer than common window-glass, and perhaps the most important defect of the paste used to imitate precious stones—such paste may contain as much as 50 per cent. of lead oxide—is to be found in its comparative softness.

At the same time, the greater the amount of lead in a glass, the greater its dispersive power on the light that passes through it. Hence the brilliancy and fire of flint-glass, and still more of artificial gems.

Apart from the varieties containing lead, samples of glass differ little in weight; the specific gravity may range between 2·4 and 2·8. That of flint-glass, on the other hand, varies from 3 to 3·8; indeed in some optical glasses containing a large percentage of lead, and again in the paste used for false jewellery, the specific gravity may be as high as 4·5 or even 5.

The high melting-point, or more definitely the high softening-point, of certain kinds of Bohemian and German glass, makes them invaluable in the laboratory of the chemist. On the other hand, the ready fusibility of glass containing lead was, as we shall see, one of the causes that promoted the adoption of such a glass in our furnaces.

Thus we find that the potash-lime glass of Bohemia, containing a high percentage of silica, excels in hardness and resistance to heat; on the other hand, the various kinds of glass containing lead are soft and easily fusible, and at the same time they combine a high specific gravity with a wide dispersive power. What we may call the maritime or soda-lime glass takes an intermediate place in all these respects. This is indeed an additional reason for regarding this great family of ‘Mediterranean’ glass as the normal type.

The two essential elements, then, required by the glass-maker are, in the first place, silica, and secondly an alkali, in each case as pure as possible, and in a convenient form for mixing and fusing together. I do not propose here to do more than indicate the source of these materials.

The silica has at all times been derived either from solid quartz, whether in the form of rock crystal or of the white pebbles from the beds of Alpine rivers, or more often from sand obtained either by excavation or from the seashore.

In the case of the alkali, the maritime people of the South extracted their soda, for the most part, from the ashes of certain plants growing in salt marshes near the sea. Most of these maritime plants belong to the natural order of the ChenopodiaceÆ, the goose-foot or spinach tribe, and we find among them various species of Salsola, Chenopodium, Salicornia, etc. These plants were all included in old days under the vague name of kali. The roughly lixiviated ashes exported from Spain were known in the trade as barilla; those from the Levant as roquetta.^[4] In other instances the impure alkaline carbonates were found ready at hand—as in the case of the natron deposits not far from Cairo. In the North the principal source of soda was till recent days the varech or kelp, cast up on the west coast of France and of Scotland.

The inland folk, on the other hand, had to find the alkali for their glass in the ashes of plants. This ‘potash’ was obtained by lixiviating the ashes of various trees and bushes—in Germany the ashes of beechwood, in France those of the bracken or fougÈre, were most in favour.

The quality of the glass depended in great measure upon the care taken in the preparation of the soda or potash. But the more impure ashes had this advantage: the amount of lime, to say nothing of the iron oxide and alumina, that they contained, rendered unnecessary in many cases the addition of any further basic material; even the comparatively pure Spanish barilla contained as much as seven per cent. of lime. In other cases that base had to be added, generally in the form of a more or less impure limestone.

Of the furnaces and of the various operations that come into play in the preparation of the glass I shall treat as the occasion arises in the following chapters. As, however, in this book we are—at least after the ‘primitive glass’ has been dealt with in the next chapter—almost exclusively concerned with vessels of ‘hollow ware’ made by a blowing process, it may be well to indicate, in this introductory chapter, the nature of this process, and to give the names of the principal tools used. These implements—apart from quite modern improvements with which I am not concerned here—are of the simplest nature, and have undergone little change during the last five hundred years—perhaps I might say since the days of the Romans.

The molten glass is collected on the extremity of the blowing-iron to form a ‘gathering.’ This gathering, while still in a soft condition, is rolled upon the ‘marver’ into a cylindrical mass. By blowing down the tube this mass is now distended to form a hollow pear-shaped vesicle, for which it will be convenient to adopt the French term paraison. It is from this paraison that a start is made to form by a ‘spinning’ or ‘flashing’ process a sheet of broad or crown glass; again, the vesicle may be made to assume a cylindrical shape, and then opened out to form larger sheets of glass; or finally—and this is for us the most important—by holding the blowing-iron to which the bulb of glass is attached in a vertical position (or sometimes by swinging it over the workman’s head), and then by shaping it by means of certain simple tools, the paraison is started on the course by which it will finally be converted into a bottle or into a bowl-shaped vessel. I will here only dwell on one point. It is evident that so long as the glass is attached to the blowing-iron, although a simple bulb-shaped vessel may be formed, there is so far no means of shaping or finishing the upper portion. Before this can be done the further extremity of the paraison must be attached by means of a small gathering of molten glass to a light tapering rod of iron, the ‘punto’ or ‘pontil.’ The vessel—for so the paraison may now be called—is at this stage removed from the blowing-iron. This is done by ‘wetting it off’ by means of a rod of moistened iron. The glass vessel, now attached by its base to the pontil, is reheated, and the further treatment taken in hand by a workman seated on a stool with long projecting arms, on which (or on the knee of the workman) the pontil is rotated. The shaping is chiefly done by an iron instrument called the ‘procello,’ or spring-tool, formed like a pair of sugar-tongs by two blades connected by an elastic bow. Finally, the edges are finished off by shears and scissors of various forms, which cut the hot glass as if it were a piece of soft leather. The now finished vessel is removed from the pontil by wetting the point of attachment, and is taken to the annealing oven.

In this very summary account of the processes involved in making, say, a flask of simple shape, I have only dwelt upon such instruments and methods as have for several centuries been in general use.

The Decay of Glass

Before ending this preliminary chapter, a few words may be said of the changes that take place in glass in the course of time from the action of the surrounding medium.^[5] These changes are in the main due to the moisture and carbonic acid contained either in the soil or in the atmosphere. Perhaps what is most striking in this action is on the one hand the apparently capricious and irregular way in which the glass is attacked, and on the other the great beauty of the iridescent effects that so often accompany the process of decay.

As to the apparent irregularity in the progress of the superficial decay, it would seem that, apart from differences in the chemical composition of the glass, much depends upon the preservation of the original smooth ‘epidermis.’ Once this is impaired, whether by accidental scratches or by the growth of fungus or lichen, the carbonic acid or the ammonia salts contained in the air or soil find, in the presence of moisture, a secure lodgment, and the work of decay proceeds rapidly. Thus in the case of the little flasks of primitive glass of which I shall have to speak in the next chapter, in one example it may be found that the smooth skin of the glass has for more than three thousand years remained absolutely intact, while in another specimen from a neighbouring tomb the glass not only on the surface, but far into the interior, has taken on a talc-like or porcelainous consistency, and the brilliant colours have for the most part disappeared.

There is no need to enter into the details of the chemical processes involved in this process of decay. Suffice to say that the action is one of the same nature as that which has played so important a part in the geological changes of the earth’s surface, especially in the disintegration of the granitic rocks. It depends upon the power possessed by carbonic acid, in the presence of moisture, of decomposing the silicates of the alkalis. The soluble carbonate of soda or of potash thus formed is then quickly washed out from the surface of the glass. There remains, in the form either of iridescent scales or of an opaque pearly crust, a layer consisting not perhaps of pure silica, but of an acid silicate of lime, alumina, or lead as the case may be.

Now a piece of clear glass may appear to the eye to be devoid of internal structure. But the ‘metal’ has, we know, in every case been subjected during the manufacture to a complicated series of involutions and doublings, to say nothing of the subsequent inflation if the glass has been subjected to a blowing process. When decay sets in—something similar may at times be seen in the case of a piece of wrought iron—this complicated formation is in part revealed, for it is evident that upon it the lines taken by the decay are in a measure dependent. On blown glass especially, the disintegration of the surface tends to result in a scaly formation resembling that of the shell of an oyster. As a result of the decomposition of light in its passage through these fine superficial films, and of the partial reflection from the back of the scales at various depths, we get those unsurpassed iridescent effects that we associate above all with the glass of the Romans. That these brilliant hues are dependent entirely upon the physical structure is well shown by the total disappearance of the colours when the surface of a piece of iridescent glass is moistened, as well as by their reappearance when the glass is again dried.

Lead of glass is much less liable to such changes, but where in such glass decay has once set in, the whole mass may be converted into a white horny substance.

In other cases the surface of a piece of clear white glass will become gradually filled with a series of minute intersecting fissures, which in time may penetrate the whole mass. When this change has been fully developed we get a true crackle-glass, not to be confounded with the frosted glass of Venice mentioned in Chapter XIII. This fissuring of the glass-mass in its various stages may be traced in many of the specimens of Venetian, Netherlandish, and English glass at South Kensington. When fully developed the effect is at times very beautiful.

The tints of coloured glass may, it would seem, change in the course of time. Colourless glass also, from which the greenish shades derived from protoxide of iron have been removed by the addition of binoxide of manganese, is above all liable to assume in the course of time a purple tint under the action of sunlight. Again, if sulphur be present in glass, as is the case where sulphate of soda has been employed as a source of the alkali, the soda salt may be reduced by any protoxide of iron that is present. The sulphide of sodium and the sesqui-oxide of iron thus gradually formed will both of them tend to give a yellowish tint to the glass.^[6]

Changes of this nature may occasionally have come about in the stained glass of the windows of our Gothic churches—the flesh-tints, which we know were produced in early days by manganese, may in the course of time have become of a more pronounced purple hue.