Glass

by Edward Dillon

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.