mirrors glass silver surface process solution silvering water thin silvered
MIRROR. It is only since the early part of the 16th century that mirrors have become articles of household furniture and decoration. Previous to that time - from the 12th to the end of the 15th century - pocket mirrors or small hand mirrors carried at the girdle were indispensable adjuncts to ladies' toilets. The pocket mirrors consisted of small circular plaques of polished metal fixed in a shallow circular box, covered with a lid. Mirror cases were chiefly made of ivory, carved with relief representations of love or domestic scenes, hunting, and games, and sometimes illustrations of popular poetry or romance. Gold and silver, enamels, ebony, and other costly materials were likewise used for mirror cases, on which were lavished the highest decorative efforts of art workmanship and costly jewelling. The mirrors worn at the girdle had no cover, but were furnished with a short handle. In 625 Pope Boniface IV. sent Queen Ethelberga of Northumbria a present of a silver mirror; and there is ample evidence that in early Anglo-Saxon times mirrors were well known in England. It is a remarkable fact that on many of the sculptured stones of Scotland, belonging probably to the 7th, 8th, or 9th century, representations of mirrors, mirror cases, and combs occur.
The method of backing glass with thin sheets of metal for mirrors was well known in the Middle Ages at a time when steel and silver mirrors were almost exclusively employed. Vincent de Beauvais, writing about 1250, says that the mirror of glass and lead is the best of all " quia vitrum propter transparentiam melius recipit radios." It is known that small convex mirrors were commonly made in southern Germany before the beginning of the 16th century, and these continued to be in demand under the name of bull's-eyes (Ochsen-Augen) till comparatively modern times. They were made by blowing small globes of glass into which while still hot was passed through the pipe a mixture of tin, antimony, and resin or tar. When the globe was entirely coated with the metallic compound and cooled it was cut into convex lenses, which of course formed small but well-defined images. It appears that attention was drawn to this method of making mirrors in Venice as early as 1317, in which year a " Magister de Alemania," who knew how to work glass for mirrors, broke an agreement he had made to instruct three Venetians, leaving in their hands a large quantity of mixed alum and soot for which they could find no use.
It was, however, in Venice that the making of glass mirrors on a commercial scale was first developed; and that enterprising republic enjoyed a rich and much-prized monopoly of the manufacture for about a century and a half. In 1507 two inhabitants of Murano, representing that they possessed the secret of making perfect mirrors of glass, a knowledge hitherto confined to one German glasshouse, obtained an exclusive privilege of manufacturing mirrors for a period of twenty years. In 1564 the mirror-makers of Venice, who enjoyed peculiar privileges, formed themselves into a corporation. The products of the Murano glass-houses quickly supplanted the mirrors of polished metal, and a large and lucrative trade in Venetian glass mirrors sprang up. They were made from blown cylinders of glass which were slit, flattened on a stone, carefully polished, the edges frequently bevelled, and the backs "silvered" by an amalgam. The glass was remarkably pure and uniform, the "silvering" bright, and the sheets sometimes of considerable dimensions. In the inventory of his effects made on the death of the great French minister Colbert is enumerated a Venetian mirror 46 by 26 inches, in a silver frame, valued at 8016 livres, while a picture by Raphael is put down at 3000 livres.
The manufacture of glass mirrors, with the aid of Italian workmen, was practised in England by Sir Robert Mansel early in the 17th century, and about 1670 the duke of Buckingham was concerned in a glass-work at Lambeth where flint glass was made for looking-glasses. These old English mirrors, with bevelled edges in the Venetian fashion, are still well known.
The Venetians guarded with the utmost jealousy the secrets of their varied manufactures, and gave most exceptional privileges to those engaged in such industries. By their statutes any glassmaker carrying his art into a foreign state was ordered to return on the pain of imprisonment of his nearest relatives, and should he disobey the command emissaries were delegated to slay the contumacious subject. In face of such a statute Colbert attempted in 1664, through the French ambassador in Venice, to get Venetian artists transported to France to develop the two great industries of mirror-making and point-lace working. The ambassador, the bishop of Beziers, pointed out that to attempt to send the required artists was to court the risk of being thrown into the Adriatic, and he further showed that Venice was selling to France mirrors to the value of 100,000 crowns and lace to three or four times that value. Notwithstanding these circumstances, however, twenty Venetian glass-mirror makers were sent to France in 1665, and the manufacture was begun under the fostering care of Colbert in the Faubourg St Antoine, Paris. But previous to this the art of blowing glass for mirrors had been actually practised at Tour-la-Ville, near Cherbourg, by Richard Lucas, Sieur de Nehou, in 1653; and by the subsequent combination of skill of both establishments French mirrors soon excelled in quality those of Venice. The art received a new impulse in France on the introduction of the making of plate glass, which was discovered in 1691. The St Gobain Glass Company attribute the discovery to Louis Lucas of Nehou, and over the door of the chapel of St Gobain they have placed an inscription in memory of " Louis Lucas qui invents in 1691 le methode de couler les places et installa la manufacture en 1695 dans he château de Saint Gobain."
Manufacture. - The term "silvering," as applied to the formation of a metallic coating on glass for giving it the properties of a mirror, was till quite recently a misnomer, seeing that till about 1840 no silver was used in the process. Now, however, a large proportion of mirrors are made by depositing on the glass a coating of pure silver, and the old amalgamation process is comparatively little used.
The process of amalgamation consists in applying a thin amalgam of tin and mercury to the surface of glass, which is done on a perfectly flat and horizontal slab of stone bedded in a heavy, ironbound wooden frame, with a gutter running round the outer edge. On the surface of this table, which mast be perfectly smooth and level, is spread a sheet of thin tin-foil, somewhat larger than the glass to be operated on, and after all folds and creases have been completely removed, by means of stroking and beating with a covered wooden rubber, the process of " quickening" the foil is commenced. A small quantity of mercury is rubbed lightly and quickly over the whole surface, and the scum of dust, impure tin, and mercury is taken off. Mercury is then poured upon the quickened foil, until there is a body of it sufficient to float the glass to be silvered (about inch deep), and, the edge at one of the sides having been cleared of the scum peculiar to mercury, the glass (scrupulously cleaned simultaneously with the above operations) is slid from that side over the surface of the mercury. Weights are placed over the surface until the greater part of the amalgamated mercury is pressed out, the table is then tilted diagonally, by means of dumb-screws, and all superfluous mercury finds its way to the gutter. The glass is left twenty-four hours under weights ; it is then turned over silvered side up, and removed to a drainer with inclining shelves, where by slow degrees, as it dries and hardens, it is brought to a vertical position, which in the case of large sheets may not be arrived at in less than a month. This process yields excellent results, producing a brilliant silver-white metallic lustre which is only subject to alteration by exposure to high temperatures, or by contact with damp surfaces ; but the mercurial vapours to which the workmen are exposed give rise to the most distressing and fatal affections.
In 1835 Baron Liebig observed that, on heating aldehyde with an ammoniacal solution of nitrate of silver, in a glass vessel, a brilliant deposit of metallic silver was formed on the surface of the glass. To this observation is due the modern process of silvering glass. In practice the process was introduced about 1840; and it is now carried on, with several modifications, in two distinct ways, called the hot and the cold process respectively. In the former method there is employed a horizontal double-bottomed metallic table, which is heated with steam to from 35° to 40° C. The glass to be silvered is cleaned thoroughly with wet whiting, then washed with distilled water, and prepared for the silver with a sensitizing solution of tin, which is well rinsed off immediately before its removal to the silvering table. The table being raised to the proper temperature, the glass is laid, and the silvering solution at once poured over it, before the beat of the table has time to dry any part of the surface of the glass. The solution used is prepared as follows : - in half a litre of distilled water 100 grammes of nitrate of silver are dissolved ; to this there is added of liquid ammonia (sp. gr. 0.880) 62 grammes; the mixture is filtered, and made up to 8 litres with distilled water, and 7.5 grammes of tartaric acid dissolved in 30 grammes of water are mixed with the solution. About 2.5 litres are poured over the glass for each superficial metre to be silvered. The metal immediately begins to deposit on the glass, which is maintained at about 40° C. (104° F.), and in little more than half au hour a continuous coating of silver is formed. The silvered surface is then cleaned by very cautiously wiping with a very soft chamois rubber, and treated a second time with a solution like the first, but containing a double quantity of tartaric acid. This solution is applied in two portions, and thereafter the glass is once more carefully cleared of all unattached silver and refuse and removed to a side room for backing up.
In silvering by the cold process advantage is taken of the power of inverted sugar to reduce the nitrate of silver. This process has been adopted for the silvering of mirrors for astronomical telescopes, notably for Leverrier's great telescope in the Paris Observatory. For ordinary mirror silvering the following is the process recommended by If. E. 13enrath. Two solutions are prepared, the first of which contains the silver salt, and the second the sugar preparation. For the silver solution 800 grammes of nitrate of silver and 1200 grammes of nitrate of ammonium are dissolved in 10 litres of water, and 1.3 kilos of pure caustic soda in 10 litres of water, and of each of these solutions 1 litre is added to 8 litres of water, which is allowed to rest till the sediment forms and then decanted. The second solution - inverted sugar - is prepared by dissolving 150 grammes of loaf sugar with 15 grammes of vinegar in 0.5 litre of water, and boiling tire solution for half an hour. After cooling it is made up with water to 4200 cubic centimetres. The silvering is done on horizontal tables in a well-lighted and moderately heated apartment, and the glass is cleaned with scrupulous care. For each square centimetre of glass operated on 15 cubic centimetres of the silver solution above described are measured out, and from 7 to 10 per cent. of the solution of inverted sugar is added, both being quickly stirred together and poured rapidly and evenly over the glass. The reduction immediately begins, and the solution exhibits tints passing through rose, violet, and black, till in about seven minutes it again becomes transparent and the deposit of metal is complete. This first deposit is extremely thin, and allows the transmission of bluish rays. The exhausted solution with floating and unattached dust-like granules of silver is carefully wiped off, the silvered surface washed with distilled water and again treated with the mixed solutions to the extent of half the quantity used in the first application. The finished surface is wiped and washed in the most thorough manner, - for the least trace of caustic soda left would destroy the mirror. The further processes are the same in both methods of silvering.
The deposit of silver on glass is not so adherent and unalterable under the influence of sunlight and sulphurous fumes as the tin-mercury amalgam, and moreover real silvered glass has a slightly yellowish tinge. These defects have been overcome by a process introduced by M. Lenoir, which consists of brushing over the silvered surface with a dilute solution of cyanide of mercury, which instantaneously forming a kind of amalgam renders the deposit at once much whiter and more firmly adherent than before. To protect the thin metallic film from mechanical injury and the chemical action of gases and vapours, it is coated with shellac or copal varnish, over which when dry are applied two coatings of red-lead paint.
Platinum Mirrors. - A cheap process of preparing mirror glass is to some extent prosecuted in France, whereby a thin but very adherent deposit of platinum is formed on the glass. A solution of chloride of platinum with a proportion of litharge and borate of lead dissolved in essential oil of spike is applied with a brush to well-cleaned glass, which is then placed on edge in a muffle furnace, and the platinum is thus burned in, forming an exceedingly thin but brilliant metallic backing having a somewhat grey lustre. It is used only for the lids of cheap boxes, toys, ornamental letters, &c.
Magic Mirrors. - Hand mirrors of metal are still in common use in Oriental countries, and especially in Japan and China they continue to be the prevalent form of looking-glass. In the former country indeed bronze mirrors are articles of the greatest importance in the generally meagre furnishing of houses, and besides possess a religious significance. They have been known and used from the most remote period, mention of them being found in Chinese literature of the 9th century. The (reputed) first made Japanese mirror, preserved at Is6, is an object of the highest veneration in Japan, and an ancient mirror, connected with which is a tradition to the effect that is was given by the sun-goddess at the foundation of the empire, is a principal article of the Japanese regalia. The mirrors of Japan vary in form and size, but in general they consist of thin disks, from 3 to 12 inches in diameter, of speculum metal with handles cast in one piece. The polished face of the mirror is slightly convex in form, so that a reflected image is seen proportionately reduced in size ; the back of the disk is occupied with characteristic Japanese ornamentation and inscriptions in bold relief, and its rim is also raised to the back. Much attention has been attracted to these mirrors by a singular physical peculiarity which in a few cases they are found to possess. These are known as magic mirrors from the fact that when a strong beam of light is reflected from their smooth and polished surface, and thrown on a white screen, an image of the raised ornaments and characters on the back of? the mirror is formed with more or less distinctness in the disk of light on the screen. This peculiarity has at no time been specially observed by the Japanese, but in China it attracted attention as early as the 11th century, and mirrors possessed of this property sell among the Chinese at ten or even twenty times the price sought for the ordinary non-sensitive examples. The true explanation of the magic mirror was first suggested by the French physicist Person, who observed that the reflecting surface of the mirrors was not uniformly convex, the portions opposite relief surfaces being plane. Therefore, as he says,'' the rays reflected from the convex portion diverge and give but a feebly illuminated image, while, on the contrary, the rays reflected from the plane portions of the mirror preserve their parallelism, and appear on the screen as an image by reason of their contrast with the feebler illumination of the rest of the disk." That such differences of plane in the mirror surface arise is an accidental circumstance due to the manner in which it is prepared, a process explained by Professors Ayrton and Perry, by whom ample details of the history, process of manufacture, and composition of Oriental mirrors have been published. A preliminary operation in polishing the surface consists of scoring the cast disk in every direction with a sharp tool. The thicker portions with relief ornament offer more resistance to the pressure of the tool than the thin flat portions, which tend to yield and form at first a concave surface, but this by the reaction of its elasticity rises afterwards and forms a slightly convex surface while the more rigid thick portions are comparatively little affected. This irregularity of surface is inconspicuous in ordinary light, and does not visibly distort images • but when the mirror reflects a bright light on a screen the unequal radiation renders the minute differences of surface obvious. The ingenious theory of Person has been established by experiments communicated by M. Govi to the academy of Turin in 1864-65, and more recently by investigations of MM. Berth' and Dubosq. See Annales de Chinnie et de Physique (5th ser., vol. xx.). (J. PA.) Ancient Mirrors.
The mirror of classical antiquity (ra-roa-rpov, speculum) was a thin disk of bronze slightly convex on one side and polished, usually provided with a handle, sometimes mounted on a stand in the form of a female figure (see COSTUME, vol. vi. p. 453, fig. 1), sometimes fixed inside a circular bronze case. The common size is that of an ordinary hand mirror. Examples large enough to take in the whole figure appear to have been rare. Mirrors of glass are mentioned, and though none of them have been found their existence need not be questioned altogether, since the process of silvering occasionally employed on bronze mirrors suggests that an analogous process may have been applied to glass. But the very large number of mirrors still existing from antiquity shows that bronze was the regular material employed. The alloy known as speculum, producing a very hard metal w ith great reflecting power, is comparatively seldom met with. Silver mirrors are mentioned, but none have as yet been found.
The principal feature of ancient mirrors, especially those of Etruria, is the design incised on the back (sec ETRURIA, vol. viii. p. 643). While twelve incised specimens are all that are as yet known from Greece, the number found in Etruria must be nearly a thousand. As a rule the subjects incised are taken from Greek mythology and legend, the names of the persons represented being frequently added in Etruscan letters and orthography. In most cases the style of drawing, the types of tire figures, and the manner of composing the groups are true to the characteristics of Greek art. Some may have been imported from Greece •, but the greater number appear to have been more or less faithfully imitated from such designs as occurred on the almost innumerable printed Greek vases which the Etruscans obtained from Greece. Even where distinctly Etruscan figures are introduced, such as the heroes Mins and Ceclius Vibenna on a mirror in the British Museum, Greek models are followed. The characteristics of Greek art here referred to date from a little before 400 B. C., and last for some time after. In this period would fall the majority of the Etruscan mirrors, and to this period also belong the Greek incised mirrors, among which may be mentioned for their beauty one representing Leucas and Corifithus, inscribed with their names (engraved, Monuments Grecs, 1873, pl. 3, published by the Association pour Pencouragement des Etudes Grecques), and another in the British Museum (Gazette Arch., ii. pl. 27), on the back of which is a figure of Eros which has been silvered over. With this last-mentioned mirror was found the bronze case used to contain it, on the back of which is a group of Aphrodite and Eros in repoussee. It was found in Crete. But most of the Greek mirrors and mirror-cases having artistic designs are from Corinth. One bears the name of the artist, 'Aroxxas bruin (engraved, Arch. Zeitung, 1862, pl. 166, fig. 1).
Archaic art (about 500 n.c.) is represented by a mirror in the British Museum from Sunium in Attica. The mirror itself is quite plain, but the stand is composed of a draped female figure, above whose head float two cupids. From Etruria there is a comparatively small number with archaic incised designs. It may be concluded that the luxury of mirrors enriched with incised designs was not freely indulged before 400 B.C. in Etruria and never to any extent in Greece. A special centre of incised mirrors was the Latian town of Prteneste (Palestrina), and it is of interest in regard to some of the mirrors found there that they have inscriptions in early Latin. Artistically they have a purely Greek character. Plain mirrors are found wherever Greek and Roman civilization spread, and it may be seen from a specimen found in Cornwall, now in the British Museum, that the Celtic population of England had adopted the form and substance of the mirror from their conquerors. This specimen is enriched with a Celtic pattern incised. The shape of the handle testifies to native originality. Mirrors were used in Greece, perhaps rarely, for divination, as appears, for example, from Pausanias (vii. 21, 5), the method being to let the mirror down into a well by means of a string till it reached close to the surface of the water. When it was pulled up after a little it was expected to show the face of the sick person on whose behalf the ceremony was performed. This was at Patras.
The principal publications on ancient mirrors are Gerhard, Etruskisthe Spiegel, Be, lin, 1843-67, 4 vols., containing 430 plates; for the Greek mirrors, Mylonas, 'EXammai, Kivrorrpa, Athens, 1876, and Dumont, Bullet. de Correcp. Heflin., 1877, p. 108; see also Friederichs, Kleinere Kunst and Industrie fm Alterthum, DUsseldorf, 1871, p. 18 sq.; and Marquardt and Mommsen, Handbuch der rornischen Allerthiitner, vii. pt. 2, p. 670. (A. S. m.)