That the art of glass-making may have arisen from an accident, such as
that mentioned by Pliny[562], I am ready to admit; but by what accident
were artists made acquainted with the use of manganese, a mineral
the outward appearance of which seems to announce nothing that could
be useful to the glass-maker? It is not found in such abundance as
to allow us to suppose that it naturally presented itself; nor do we
know that any older application of it may have induced the ancients to
employ and examine it in such a manner that the present use of it might
be accidentally discovered. In general, it resembles some kinds of
iron-stone, which it was considered to be till a very late period. That
iron, however, colours glass must have been very early remarked; and
therefore it could occur to no one to employ manganese for depriving
frit[563] of its colour. It produces this decoloration only when it is
added sparingly, and according to a determinate proportion; otherwise
it gives to the glass a violet colour, something similar to that of the
amethyst.
The application of manganese was certainly taught by accident, and
not by theory. But in regard to the question, why it frees glass from
its dirty colour, it must be admitted, if we readily acknowledge the
truth, that we can offer only hypotheses; as the old chemists called
in the aid of phlogiston, and the new that of oxygen[564]. Did a false
hypothesis, then, conduct to this discovery? That this was the case,
has been asserted by old as well as more modern writers, and is no
doubt possible. Thus Kepler, from an erroneous hypothesis in regard to
the revolution of the planets, discovered the ratio of their motion,
according to their distance from the sun; and such instances may be
adduced in favour of hypotheses which have done more harm than good.
But, in my opinion, in examining the origin of the ancient arts, we
ought not to give credit to any cause assigned for an invention until
no other can be found. In regard to the art in question, I think I can
mention one which, at any rate, has probability in its favour, and
which I shall here submit to the reader’s decision.
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.
It is seen by the words quoted from different authors, that the name,
which as far as I know occurs first in Albertus Magnus, was written
in a great many different ways: _magnesia_, _magnosia_, _magnasia_,
_manganensis_, _mangadesum_, and in French _magalaise_, _méganaise_,
_magnese_. One might imagine that it is derived from magnet, partly
on account of the similarity of the two substances, and partly on
account of its supposed power to attract glass. Besides, its other name
_sidera_ seems to have a reference to the Greek word for iron. Mercati,
however, deduces the term from _mangonizare_, because potters besmear
their wares with this mineral; but I suspect that the name was common
before that use of the substance was known. It is to be observed that
to this word various other significations have been given. Sometimes
it seems to denote common iron-stone, and sometimes pyrites. What the
gold-makers understood by it will be best discovered by consulting
the works of their followers. _Braunstein_ also, the German name, the
earliest mention of which occurs perhaps in the writings of Basilius
Valentine, denoted at first every kind of ferruginous earth employed
by the potters for painting. Thus Schwenkfeld gave the name of
_Braunstein_ and _Braunfarbe_ to a kind of bloodstone[572].
For a long time the manganese imported from Piedmont was in Germany
accounted the best, and therefore was much sought after by the artists
of Nuremberg. Afterwards, a kind brought from Perigord, a place in
Guyenne, and named _pierre de Périgueux_, or _lapis petracorius_, was
highly esteemed. Wallerius gives this as a peculiar species; and in
my opinion he is right. Its distinguishing characters are, that it
resembles a burnt coal or cinder; has a somewhat shining surface, and
on the fracture appears to be finely striped and a little coloured.
A piece which I have in my possession exhibits all these marks. This
species has been mentioned by very few of the new mineralogists.
Germany, however, for some centuries past has employed its own
manganese, which even in the time of Biringoccio was sent as an article
of commerce to Italy.
[The distinctness of the metal contained in the manganese of commerce
from iron was first proved by the experiments of Pott in 1740, by Kaim
and Winterl in 1770, and by Scheele and Bergman in 1774. Soon after
this the metal itself was obtained in an isolated state by Gahn, who
gave to it the name of _magnesium_, which term however was subsequently
applied to the metal contained in magnesia, and the word _manganese_
has been adopted to designate both the metal and the black ore. In
addition to its application in the manufacture of glass, it is now very
extensively used in the decomposition of common salt for the production
of chlorine for bleaching. Some salts of the lower oxides of manganese
have lately been used in calico-printing as a source of brown colours.]