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III.—On the Process of Decay in Glass, and, incidentally, on the Composition and Texture of Glass at different periods, and the History of its Manu- facture. By JAMES EOWXER, Esq., F.S.A., Local Secretary for the County of Durham. Bead November 30, 1876. GLASS is, in many respects, one of the most remarkable substances in the world. No known substance combines such varied uses with such matchless beauties. Por innumerable domestic purposes it has for centuries been considered a necessity of daily life. Without glass innumerable paths in science and the arts would never have been explored; and in these paths progress has been made in proportion as the methods of making glass have been improved. On the other hand the peculiar beauties inherent in or incident to this material are so great that at no period in history has man been able to grasp completely more than one of them at once. The Venetians realized above all others the marvellous capacity of glass for being wrought into all kinds of beautiful forms ; our Gothic forefathers developed beyond all others its capabilities in respect of colour; the Phoenicians and Romans did wonders both in form and colour, but were nevertheless inferior to the Venetians in the former, and to our Gothic fore- fathers in the latter; we, in our day, excel in developing to the utmost (wonderful talent that it is!) the crystalline transparency and brilliancy of glass, but it is in this direction only that we have any true art or artists—in form and colour we do comparatively nothing. Thus, in each instance, the full realization of a period has been but as it were a passing glimpse—it has never been found possible to retain it, so as to carry it into the full realization of another period; even as the highest natural beauty is but for a moment—it increases until maturity, and then immediately begins to fade. So that we are now, after more than two thousand five hundred years of labour, but on the threshold of what is VOL. XLVI. K 66 The Process of Decay in Glass. conceivable in this kind. What would have been the result had all the excellences of past periods ever been united ? What if each were pushed yet further ? And why not ? No such delicate or exquisite forms can be pro- duced in any other material as are produced in glass; and this material furnishes the most delicate and subtle, as well as the most rich and brilliant, effects of colour that can be obtained by art. The faculties of man, not the capabilities of the material, alone place a limit to that which is attainable. As if still further to excite our wonder and admiration, age and the process of decay, which render other things loathsome, repulsive, and deformed, invest this substance with additional splendour. It is the process just mentioned which will form the principal matter, or central point, of the present inquiry; but it will be found necessary to consider also, at some length, the larger and perhaps still more interesting question of the quality and texture of glass at different periods, and the history of its manu- facture. It is curious that, hitherto, the subject should never have been thoroughly investigated. Though the appearances are strikingly curious and varied, seeming of themselves almost without preliminary inquiry to invite, and promising to reward, an attentive study, the subject has remained an unsolved pi'oblem on the border-land of several sciences, belonging to all but claimed by none. To Antiquaries it has seemed too chemical or mechanical a question, and to chemists and physicists too antiquarian a one. It is true that some years ago Sir David Brewstera had his attention directed to the subject, but he confined himself to one form of decay only, and to the appearances merely, rather than to the explanation of them; being content to suggest that glass decays because the atoms of which it consists, having been " forced into union without natural a Notes on the Decay of Glass, especially upon that of the Ancient Glass found at Nineveh, Appendix to Layard, Second Expedition, 1853; and On the Decomposed Glass found at Nineveh and other places, Transactions of the British Association, 1860, p. 9. Both are important communications, and should be read carefully in connection with this subject. It so happens that my illustrative example No. 9 came from a specimen formerly in the possession of Sir David Brewster, and greatly prized by him at a time when specimens of Roman glass were much more difficult to procure than at present. Having noticed the windows of stables often coated with a precisely similar iridescence (Ex. 13), and conjecturing it was caused by ammoniacal fumes, he formed the idea that the glass might have been buried near a Roman stable, and thus acted upon by the decomposing excretions of the horses. Sir David himself scraped off some of the films, and gave them as a curiosity, with the preceding explanation, to the Reverend H. G. W. Aubrey, of Hale Rectory, Salisbury, at that time a neighbour in Scotland, who had the kindness to present them to me. The Process of Decay in Glass. 67 affinity," " tend to resume their primitive state and separate themselves." Others, thinking only of Gothic glass, in situ in churches, and observing the greater amount of decay on the south than the north side of buildings, have attributed its decomposition, some—to the moon,a others—to the sun,b the peculiar immunity from decay possessed by yellow stained glass being explained by its power of arresting the chemical rays of the sun: as though glass did not decay faster when buried far beneath the surface in damp soils than when in situ, or there were no such thing as decay of Phoenician, Egyptian, Assyrian, Babylonian, Roman, Celtic, or Saxon glass in graves, tombs, or other localities where no ray of light ever penetrated. Some, finding lichens growing in profusion on old glass, have attributed its decay to their presence,0 whereas more careful observation would have shown myriads of specimens of decay without the presence of a single lichen, and that the glass must first be decayed before the lichen can find upon its surface a resting-place for its tapestries of gold and silver. Mere dirt, again, and dust, both on the outside and inside of windows, and confervoids, often seen on the inside of windows in damp situations, are not unfrequently mistaken for decay, and vice versd. Lastly, some, and indeed by far the great majority, of observers, recognising a process which they could not understand, have avoided the difficulty of making an explanation by attri- buting the appearances to corrosion, rust, or the like, terms which explain nothing. Let us inquire, then, what are really the phenomena of decay in glass ? What, in this case, more exactly, do we mean by such terms as corrosion and rust ? How are these processes originated ? And in what do they result ? Unlike the rusting of iron or bronze, which is substantially the same under whatever circumstances produced, the rusting or decay of glass takes place in several modes, which differ widely in appearance according to the circumstances under which they are produced. The primary modes of decay in glass are two, —filmy and granular: but of the latter there are four secondary forms, which I propose to call respectively—superficial creeping, deep creeping, spotty or pitting, and splitting or crackling granular. a Bernard Palissy, writing in 1563, says the glaziers in Poitou and Bretagne attributed the decay of their church-windows to the moon. b This view was advocated by at least one of the speakers in the discussion which followed the reading of this Paper. c " A diminutive moss or lichen, which absolutely decompounds the substance of the glass in vermicular lines." Whitaker, Loidis and Eltnete, ii. 322, note. See also Winston, Inquiry, &c. 1867, i. 23. K2 68 The Process of Decay in Glass. I.—DESCRIPTION OF THE DIFFERENT MODES OF DECAY IN GLASS. A. FILMY, BLISTERING, OR IRIDESCENT DECAY. This form of decay is characteristic of the transparent or crystalline makes of glass; of the early Assyrian and Babylonian, for instance, if we may take as examples the vessel bearing the name of Sargon, King of Assyria, and other specimens in the British Museum;" of the later Phoenician and Egyptian, of which there are many specimens in the British Museum; of the Greek; of Eoman, whether bottle-glass, window-glass, or the glass of artificial gems,b down to the latest period of which we have specimens (catacomb glass); of some of the earliest Teutonic glass (Saxon); probably also of Saracenic, Persian, and Byzantine glass, though specimens of these makes are so extremely rare, and usually so well kept, that it is impossible to speak with certainty; certainly how- ever of the finest and most perfectly made Venetian glass; of modern crown glass, the inside of panes in stables, for instance, and (in a less degree, but very commonly) the outside of panes in dwelling-houses ; of modern waste glass of all kinds thrown on ash-pits or manure heaps, the source, probably, of those frag- ments of iridescent glass which may be picked up in almost any garden or field;c of modern bottle-glass buried in certain soils, or in the mud at the bottom of ponds, lakes, and rivers, or in ships sunk in the sea; of old microscope and telescope lenses, termed by opticians "tarnish," "sweating," "rust," or "mildew," incapable of being washed off or rubbed off,.

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