The History of Chemistry (Vol.1&2) - Thomas Thomson

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glasses were most highly prized, and that glass was rendered colourless then as it is at present, by the addition of a certain quantity of oxide of manganese. Colourless glass was very high priced in Pliny’s time. He relates, that for two moderate-sized colourless drinking-glasses the Emperor Nero paid 6000 sistertii, which is equivalent to 25l. of our money.

      Pliny relates the story of the man who brought a vessel of malleable glass to the Emperor Tiberius, and who, after dimpling it by dashing it against the floor, restored it to its original shape and beauty by means of a hammer; Tiberius, as a reward for this important discovery, ordered the artist to be executed, in order, as he alleged, to prevent gold and silver from becoming useless. But though Pliny relates this story, it is evident that he does not give credit to it; nor does it deserve credit. We can assign no reason why malleable substances may not be transparent; but all of them hitherto known are opaque. Chloride of silver, chloride of lead and iron constitute no exception, for they are not malleable, though by peculiar contrivances they may be extended; and their transparency is very imperfect.

      Many specimens of the coloured glasses made by the ancients still remain, particularly the beads employed as ornaments to the Egyptian mummies. Of these ancient glasses several have been examined chemically by Klaproth, Hatchett, and some other individuals, in order to ascertain the substances employed to give colour to the glass. The following are the facts that have been ascertained:

      1. Red glass. This glass was opaque, and of a lively copper-red colour. It was probably the kind of red glass to which Pliny gave the name of hæmatinon. Klaproth analyzed it, and obtained from 100 grains of it the following constituents: Silica 71·0 Oxide of lead 10·0 Oxide of copper 7·5 Oxide of iron 1·0 Alumina 2·5 Lime 1·5 93·5 75 No doubt the deficiency was owing to the presence of an alkali. From this analysis we see that the colouring matter of this glass was red oxide of copper.

      2. Green glass. The colour was light verdigris-green, and the glass, like the preceding, was opaque. The constituents from 100 grains were, Silica 65·0 Black oxide of copper 10·0 Oxide of lead 7·5 Oxide of iron 3·5 Lime 6·5 Alumina 5·5 98·0 76 Thus it appears that both the red and green glass are composed of the same ingredients, though in different proportions. Both owe their colour to copper. The red glass is coloured by the red oxide of that metal; the green by the black oxide, which forms green-coloured compounds, with various acids, particularly with carbonic acid and with silica.

      3. Blue glass. The variety analyzed by Klaproth had a sapphire-blue colour, and was only translucent on the edges. The constituents from 100 grains of it were, Silica 81·50 Oxide of iron 9·50 Alumina 1·50 Oxide of copper 0·50 Lime 0·25 93·25 77 From this analysis it appears that the colouring matter of this glass was oxide of iron: it was therefore analogous to the lapis lazuli, or ultramarine, in its nature.

      Davy, as has been formerly noticed, found another blue glass, or frit, coloured by means of copper; and he showed that the blue paint of the ancients was often made from this glass, simply by grinding it to powder.

      Klaproth could find no cobalt in the blue glass which he examined; but Davy found the transparent blue glass vessels, which are along with the vases, in the tombs of Magna Græcia, tinged with cobalt; and he found cobalt in all the transparent ancient blue glasses with which Mr. Millingen supplied him. The mere fusion of these glasses with alkali, and subsequent digestion of the product with muriatic acid, was sufficient to produce a sympathetic ink from them.78 The transparent blue beads which occasionally adorn the Egyptian mummies have also been examined, and found coloured by cobalt. The opaque glass beads are all tinged by means of oxide of copper. It is probable from this that all the transparent blue glasses of the ancients were coloured by cobalt; yet we find no allusion to cobalt in any of the ancient authors. Theophrastus says that copper (χαλκος, chalcos) was used to give glass a fine colour. Is it not likely that the impure oxide of cobalt, in the state in which they used it, was confounded by them with χαλκος (chalcos)?

      IV.—VASA MURRHINA.

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      The Romans obtained from the east, and particularly from Egypt, a set of vessels which they distinguished by the name of vasa murrhina, and which were held by them in very high estimation. They were never larger than to be capable of containing from about thirty-six to forty cubic inches. One of the largest size cost, in the time of Pliny, about 7000l. Nero actually gave for one 3000l. They began to be known in Rome about the latter days of the republic. The first six ever seen in Rome were sent by Pompey from the treasures of Mithridates. They were deposited in the temple of Jupiter in the capitol. Augustus, after the battle of Actium, brought one of these vessels from Egypt, and dedicated it also to the gods. In Nero’s time they began to be used by private persons; and were so much coveted that Petronius, the favourite of that tyrant, being ordered for execution, and conceiving that his death was owing to a wish of Nero to get possession of a vessel of this kind which he had, broke the vessel in pieces in order to prevent Nero from gaining his object.

      There appear to have been two kinds of these vasa murrhina; those that came from Asia, and those that were made in Egypt. The latter were much more common, and much lower priced than the former, as appears from various passages in Martial and Propertius.

      Many attempts have been made, and much learning displayed by the moderns to determine the nature of these celebrated vessels; but in general these attempts were made by individuals too little acquainted with chemistry and with natural history in general to qualify them for researches of so difficult a nature. Some will have it that they consisted of a kind of gum; others that they were made of glass; others, of a particular kind of shell. Cardan and Scaliger assure us that they were porcelain vessels; and this opinion was adopted likewise by Whitaker, who supported it with his usual violence and arrogance. Many conceive them to have been made of some precious stone, some that they were of obsidian; Count de Veltheim thinks that they were made of the Chinese agalmatolite, or figure stone; and Dr. Hager conceives that they were made from the Chinese stone yu. Bruckmann was of opinion that these vessels were made of sardonyx, and the Abbé Winckelmann joins him in the same conclusion.

      Pliny informs us that these vasa murrhina were formed from a species of stone dug out of the earth in Parthia, and especially in Carimania, and also in other places but little known.79 They must have been very abundant at Rome in the time of Nero; for Pliny informs us that a man of consular rank, famous for his collection of vasa murrhina, having died, Nero forcibly deprived his children of these vessels, and they were so numerous that they filled the whole inside of a theatre, which Nero hoped to have seen filled with Romans when he came to it to sing in public.

      It is clear that the value of these vessels depended on their size. Small vessels bore but a small price, while that of large vessels was very high; this shows us that it must have been difficult to procure a block of the stone out of which they were cut, of a size sufficiently great to make a large vessel.

      These vessels were so soft that an impression might be made upon them with the teeth; for Pliny relates the story of a man of consular rank, who drank out of one, and was so enamoured with it that he bit pieces out of the lip of the cup: “Potavit ex eo ante hos annos consularis, ob amorem abraso ejus margine.” And what is singular, the value of the cup, so far from being injured by this abrasure, was augmented: “ut tamen injuria ilia pretium augeret; neque est hodie murrhini alterius præstantior indicatura.”80 It is clear from this that the matter of these vessels was neither rock crystal, agate, nor any precious stone whatever, all of which are too hard to admit of an impression from the teeth of a man.

      The lustre was vitreous to such a degree that the name vitrum murrhinum was given to the artificial fabric, in Egypt.

      The splendour was not very great, for Pliny observes, “Splendor his sine viribus nitorque verius quam splendor.”

      The colours, from their depth and richness, were what gave these vessels their value and excited admiration. The principal colours were purple and white, disposed in undulating

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