Page:EB1911 - Volume 18.djvu/603

 Medieval and Modern Mirrors.—Small metallic mirrors with a highly polished surface were largely used during the middle ages: pocket mirrors or small hand mirrors carried at the girdle being indispensable adjuncts to ladies’ toilets. The pocket mirrors consisted of small circular plaques of polished metal, usually steel or silver, 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 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 of Beauvais, writing about 1250, says that the mirror of glass and lead is the best of all, “quia vitrum propter transparentiam melius recipit radios”; and a verre à mirer is mentioned in the inventories of the dukes of Burgundy, dating from the 15th century. A gild of glass-mirror makers existed at Nuremberg in 1373, and 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 formed small but well-defined images. As early as 1317, 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 the republic enjoyed a 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 glass-house, 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 French minister Colbert, a Venetian mirror, 46 by 26 in., in a silver frame, is 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 glass-works 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 manufactures, and gave exceptional privileges to those engaged in such industries. By their statutes any glass-maker 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 him. In face of such a statute Colbert attempted in 1664 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 Béziers, pointed out that this was to court the risk of being thrown into the Adriatic, and, further, that Venice was selling to France mirrors to the value of 100,000 crowns and lace to three or four times that value. Nevertheless, twenty Venetian glass-mirror makers were sent to France in 1665, and the manufacture was begun in the Faubourg St Antoine, Paris. But previous to this the art of blowing glass for mirrors had been 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 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 inventa en 1691 le méthode de couler les glaces et installa la manufacture en 1695 dans le 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, but a tin amalgam, 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. A sheet of thin tin-foil, somewhat larger than the glass to be operated on, is spread out on a flat table, and after all folds and creases have been completely removed 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 in. deep), and the glass (scrupulously cleaned simultaneously with the above operations) is slid over the surface of the mercury. Weights are placed over the surface until the greater part of the amalgamated mercury is pressed out, and the table is then tilted so that 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, where it dries and hardens. This process, when elaborated, 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.

The “silver on glass” mirror may be regarded as a discovery of J. von Liebig, who in 1835 observed that by heating aldehyde with an ammoniacal solution of silver nitrate in a glass vessel a brilliant deposit of metallic silver was formed on the surface of the 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., and the reduction of the ammoniacal silver solution is effected with tartaric acid.

In silvering by the cold process advantage is taken of the power of sugar to reduce the silver nitrate. This method has been generally adopted for the silvering of mirrors for astronomical telescopes. G. W. Ritchey (“The Modern Reflecting Telescope,” Smithsonian Contributions to Knowledge, xxxiv. 40) used the process devised by Brashear in 1884. The glass disk is mounted on a rocking-table, and most carefully cleaned with nitric acid, potash, and finally with distilled water. The reducing solution (which improves on keeping) is made up from 200 parts of water, 20 of loaf sugar, 20 of alcohol and 1 of nitric acid (commercial pure). The silver solution is prepared as follows: 2 parts of silver nitrate are dissolved in 20 parts of water, and strong ammonia added until the brown solution becomes clear. A solution of parts of potash (pure by alcohol) in 20 of water is now added, and then ammonia until the solution is again clear. A solution of part of silver nitrate in 16 of water is added until the liquid is straw-coloured; it is then filtered. Quantities of the solutions, such that the sugar equals one half the nitrate, are taken, then diluted, mixed, and poured on to the plate, which is gently rocked. The liquid goes muddy-brown, and in 3 to 4 minutes it begins to clear, a thick deposit being formed in about 5 minutes. The solution is poured off, and water run on, the streaks of precipitate being removed by lightly held cotton wool. The washing is repeated, and then water is allowed to remain on the film for one hour. The water is then run off, and the plate is washed several times with alcohol, and then dried by an air fan. The film is now burnished with a chamois leather pad, and finally with the finest jewellers’ rouge, the silver surface being the reflecting surface of the mirror.