That it was observed at an early period that metallic oxides, and particularly that of iron, which most frequently occurs, communicate various colours to glass, has been already proved[565]. It needs therefore excite no wonder that men should be induced to make experiments on colouring glass with various minerals, and especially such as contained iron. Now, since manganese, as already said, has a great resemblance to iron-stone, it was also occasionally employed; and it was soon found that this supposed species of iron-stone, according as it is used in greater or less quantity, gives to glass many beautiful shades of a violet, red, and dark brown colour. As it was necessary that the artist should weigh the manganese, in order to proportion it to the vitreous mass, according to the required colour, it is possible that the glass, when a very small quantity had been added, was found to be colourless. This observation must have been made with the greater satisfaction, and more readily turned to advantage, the higher colourless glass, which approached nearest to rock crystal, was at that time esteemed[566].
The period however when this great improvement in one of the most useful arts was fortunately introduced, cannot with certainty be determined; but it is very probable that it was practised in the time of Pliny. Were not this the case, what should have induced him, more than once, to remark that the magnet was employed in glass? Under this name the ancients certainly comprehended manganese; which, in general, had a resemblance to the magnet, and was considered as such by Agricola, Kircher, and others, at a more modern period. Pliny[567], in one passage, speaks of a kind of magnet which was found in Cantabria, not in veins, but interspersed or in nuclei; and he adds that he did not know whether it was useful in glass-making, because no one had ever tried it. This use of manganese then must at that time have been very common, since it occurred so readily to a writer in speaking of a supposed magnet.
Another passage of Pliny has been supposed to allude to manganese, but in my opinion with much less probability. It is that where he says Alabandicus flows in the fire, and is fused at the glass-houses[568]. But by that term he seems to understand a kind of marble, according to the opinion of Isidorus, by whom the word is repeated. As a calcareous earth it was perhaps added to promote the fusion of the sand. Camillus Leonardus, however, considered the Alabandicus as manganese[569].
It is not improbable that the ancients employed manganese, if not for glazing, at any rate for painting their pottery or earthenware, as soon as they became acquainted at the glass-houses with its susceptibility of being converted into a coloured vitreous mass.
But this is far from being proved, though count Caylus, Genssane and others positively assert that the so-called Etruscan vases and lamps were painted with the same manganese that we use for our earthen-ware.
Those who attempt to trace out the history of the arts must be very cautious not to admit, without sufficient proof, that what the ancients accomplished was effected by the same means as those employed by us for the same purpose. This, in some cases, may be true; but in many others false. Thus, they made a beautiful kind of blue and red glass, without being acquainted with our cobalt and mineral purple; and they performed very long sea voyages without our compass. It is the duty of the historian either to point out the means which the ancients employed, whether they were the same or not as those used at present, or to acknowledge that their processes are unknown to us. Those who invariably follow this rule will sometimes discover that, in ancient times, men were able to accomplish the same objects and to produce the same effects, by means totally different from those used at present; and then the question will sometimes arise, Which of the means, the old or the new, are the cheapest, the most convenient, and the surest? This leads to technological problems, the solution of which, notwithstanding the great superiority we possess in those auxiliaries of the arts, natural history, chemistry, &c., is impossible. I have indulged in these observations, in mentioning the celebrated Caylus, because I well know that he has often erred in not attending to them. I acknowledge and respect the service of this eminent man; but I am convinced that by the boldness of his assertions he acquired greater confidence and more celebrity than he deserved.
The colours on the Etruscan vases have a resemblance indeed to those on our stone-ware, but it is also true that they might be produced by oxide of iron.
The substances used by the ancient potters can be determined only by the testimony of the ancients or by experiments; but the former is not to be found; and the latter have never been made, though they would not be difficult to any chemist who might choose to sacrifice a few vessels of that kind.
The question how the use of manganese was first found out, occurred even to Pliny; and his opinion on that subject deserves to be quoted, especially as it was long considered as true by Albertus Magnus, Caneparius, and many later writers. To understand it one must know that it was at first believed that the magnet, as it attracts iron, could attract other bodies also; and it was conjectured that other minerals might possess a similar property. Some imagined that they had found magnets for gold and silver. In the oldest times men had so erroneous an opinion of the art of glass-making, that they conceived that glass was obtained from sand, as metal from its ore; and Pliny thinks that they then conjectured that a magnet could attract glass as well as it does iron. Now as manganese, on account of its similarity, was considered to be a magnet, it was consequently subjected to experiments, which gave rise to the beneficial discovery that it renders glass colourless.
This use of it then has been retained through every age to the present time, and it is mentioned by all those authors who have written on glass-making. Avicenna[570] makes so complete a distinction between it and the magnet, that he treats of each in a particular section, though he says nothing of its employment in the glass-houses; but indeed as a physician he had no opportunity of doing so. Albertus Magnus[571], however, who lived a century later, Roger Bacon, Basilius Valentine, Camillus Leonardus, Biringoccio, Mercati, Neri and many others have spoken in the plainest terms of this application.