A History of Inventions, Discoveries, and Origins. Johann Beckmann

      COLOURED GLASS.—ARTIFICIAL GEMS.

       Table of Contents

It is probable that there was no great interval between the discovery of the art of making glass, and that of giving it different colours. When the substance of which it is formed contains, by accident, any metallic particles, the glass assumes some tint; and this happens oftener than is wished; nay, a considerable degree of foresight is necessary to produce glass perfectly colourless; and I am of opinion that this skill has not been attained till a late period in the progress of the art. Even in Pliny’s time the highest value was set upon glass entirely free from colour, and transparent, or, as it was called, crystal304. From the different colours which glass acquired of itself, it was easy to conceive the idea of giving it the tinge of some precious stone: and this art, in ancient times, was carried to a very great extent. Proofs of this may be found in Pliny, who, besides others, mentions artificial hyacinths, sapphires, and that black glass which approached very near to the obsidian stone, and which in more than one place he calls gemmæ vitreæ305. Trebellius Pollio relates in how whimsical a manner Gallienus punished a cheat who had sold to his wife a piece of glass for a jewel306: and Tertullian ridicules the folly of paying as dear for coloured glass as for real pearls. The glass-houses at Alexandria were celebrated among the ancients for the skill and ingenuity of the workmen employed in them. From these, the Romans, who did not acquire a knowledge of that art till a late period, procured for a long time all their glass ware. The learned author of Recherches sur les Égyptiens et les Chinois, in the end of his first volume, relates more of these glass-houses than I know where to find in the works of the ancients; but it is certain that coloured glass was made even in those early ages. The emperor Adrian received as a present from an Egyptian priest, several glass cups which sparkled with colours of every kind, and which, as costly wares, he ordered to be used only on grand festivals307. Strabo tells us, that a glass-maker in Alexandria informed him that an earth was found in Egypt, without which the valuable coloured glass could not be made308.

Seneca, in his ninetieth epistle, in which he judges too philosophically, that is, with too little knowledge of the world, in regard to the value of labour, mentions one Democritus who had discovered the art of making artificial emeralds309; but in my opinion this discovery consisted in giving a green colour by cementation to the natural rock crystal: and this art I imagine was treated of in that book, the name of which Pliny, through an over-anxious care lest the deception should become common, does not mention310. For colouring crystal and glass, so as to resemble stones, Porta311, Neri312, and others have, in modern times, given directions which are, however, not much used, because the crystal is thereby liable to acquire so many flaws that it cannot be easily cut afterwards, though, as Neri assures us, these by attention may sometimes be avoided.

      It is worthy of remark, that in some collections of antiquities at Rome, there are pieces of coloured glass which were once used as jewels. In the Museum Victorium, for example, there are shown a chrysolite and an emerald, both of which are so well executed, that they are not only perfectly transparent and coloured throughout, but neither externally nor internally have the smallest blemish, which certainly could not be guarded against without great care and skill.

What materials the ancients used for colouring glass, has not been told to us by any of their writers. It is, however, certain that metallic oxides only can be employed for that purpose, because these pigments withstand the heat of the glass furnaces; and it is highly probable that ferruginous earth, if not the sole, was at least the principal substance, by which not only all shades of red, violet, and yellow, but even a blue colour, could be communicated, as Professor Gmelin has shown313. Respecting the red, of which only I mean here to speak, there is the less doubt, as, at present, sometimes an artificial, and sometimes a natural, iron ochre is often employed for that purpose. For common works this is sufficient; but when pure clear glass, coloured strongly throughout with a beautiful lively red, free from flaws, and in somewhat large pieces, is required, iron is not fit, because its colour, by the continued heat necessary for making glass, either disappears or becomes dirty and almost blackish314.

In the last century, some artists in Germany first fell upon the method of employing gold instead of iron, and of thereby making artificial rubies, which when they were well set could deceive the eye of a connoisseur, unless he tried them with a diamond or a file. The usual method was to dissolve the gold in aqua regia,