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, crystal[304]. 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 vitreæ_[305]. Trebellius Pollio relates
in how whimsical a manner Gallienus punished a cheat who had sold to
his wife a piece of glass for a jewel[306]: 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 festivals[307]. 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 made[308].
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 emeralds[309]; 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
mention[310]. For colouring crystal and glass, so as to resemble
stones, Porta[311], Neri[312], 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
shown[313]. 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 blackish[314].
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, and to precipitate it
by a solution of tin, when it assumed the form of a purple-coloured
powder. This substance, which must be mixed with the best frit, is
called the precipitate of Cassius, gold-purple, or mineral-purple[315].
This Cassius, from whom it takes its name, was called Andrew, and
because both the father and the son had the same christian name, they
have been often confounded with each other. The father was secretary to
the duke of Schleswig, and is not known as a man of letters; but the
son is celebrated as the inventor or preparer of the gold-purple, and
of a bezoar-essence. He took the degree of doctor at Leyden, in 1632,
practised physic at Hamburg, and was appointed physician in ordinary
to the bishop of Lubec. As far as I know, he never published anything
respecting his art; but this service was rendered to the public by his
son, who was born at Hamburg, and resided as a physician at Lubec.
He was the author of a well-known treatise, now exceedingly scarce,
entitled Thoughts concerning that last and most perfect work of nature,
and chief of metals, gold, its wonderful properties, generation,
affections, effects, and fitness for the operations of art; illustrated
by experiments[316].
From this work, it will be easily understood why the author does not
give himself out as the inventor of the gold-purple[317], which he
is commonly supposed to be, at which Lewis is much astonished. It is
seen also by it that Leibnitz calls him improperly a physician at
Hamburg, having probably confounded the father and son together[318].
Upon the whole, it is not proved that any of the Cassius’s was the
inventor of the above precipitate, else it would certainly not have
been omitted[319] in this treatise; and mention of gold-purple is to be
found in the works of several old chemists[320].
Something of this kind has, doubtless, been meant by the old chemists,
when they talk of red lions, the purple soul of gold, and the golden
mantle; but what they wished to conceal under these metaphors, I am
not able to conjecture. In the year 1606, when Libavius published his
Alchemy, the art of making ruby-glass must have been unknown. He indeed
quotes an old receipt for making rubies; and conjectures, that because
the real stones of the same name are found in the neighbourhood of gold
mines, they may have acquired their colour from that metal; and that by
means of art, glass might be coloured by a solution of gold[321]. The
later chemists, however, and particularly Achard, found no traces of
gold, but of iron, in that precious stone[322].
Neri, who lived almost at the same time as Libavius[323], was better
acquainted with the gold-purple, though his receipt is very defective.
According to his directions, the gold solution must be evaporated,
and the residue suffered to remain over the fire until it becomes of
a purple colour. One may readily believe that this colour will be
produced; but glass will scarcely be coloured equally through by this
powder, and perhaps some of the gold particles will show themselves
in it. Kunkel affirms, and not without reason, that something more
is necessary to make rubies by means of gold; but he has not thought
proper to tell us what it is[324].
Glauber, who wrote his Philosophical Furnace[325] about the middle of
the seventeenth century, appears to have made several experiments with
the gold-purple. He dissolved the metal in aqua regia; precipitated
it by liquor of flint, and melted into glass the precipitate, which
contained in it abundance of vitreous earth[326].
None, however, in the seventeenth century, understood better the
use and preparation of gold-purple than John Kunkel, who, after
being ennobled by Charles XI., king of Sweden, assumed the name of
Löwenstiern. He himself tells us, that he made artificial rubies in
great abundance, and sold them by weight, at a high price. He says,
he made for the elector of Cologne a cup of ruby-glass, weighing not
less than twenty-four pounds, which was a full inch in thickness, and
of an equally beautiful colour throughout. He employed himself most on
this art after he engaged in the service of Frederic William, elector
of Brandenburg, in the year 1679. At that time he was inspector of
the glass-houses at Potsdam; and, in order that the art of making
ruby-glass might be brought to perfection, the elector expended 1600
ducats. A cup with a cover, of this manufacture, is still preserved at
Berlin. Kunkel, however, has nowhere given a full account of this art.
He has only left in his works a few scattered remarks, which Lewis has
collected[327].
In the year 1684, earlier than Cassius, John Christian Orschal
wrote his well-known work, Sol sine veste[328], in which he treats,
more intelligibly than any one before him, of the manner of making
ruby-glass. He, however, confesses that Cassius first taught him to
precipitate gold by means of tin; that Cassius traded in glass coloured
with this precipitate, and that a good deal of coloured glass was then
made at Freysingen, but that the art was kept very secret. As Orschal
deserves that his fate should be better known, I shall here mention the
following few particulars respecting him. About the year 1682 he was at
Dresden, in the service of John Henry Rudolf, from whom he learnt many
chemical processes, and particularly amalgamation, by which he gained
money afterwards in Bohemia. After this he was employed at the mines in
Hesse; but he brought great trouble upon himself by polygamy and other
irregularities, and died in a monastery in Poland.
Christopher Grummet, who was Kunkel’s assistant, wrote, in opposition
to Orschal, his known treatise, Sol non sine veste, which was printed
at Rothenburg, in 1685[329]. In like manner, an anonymous author
printed against Orschal, at Cologne, in 1684, another work, in
duodecimo, entitled Apelles post tabulam observans maculas in Sole sine
veste. The dispute, however, was not so much concerning the use of
gold-purple, as the cause of the red colour, and the vitrification of
gold.
It is worthy of remark, that Kunkel affirms he could give to glass a
perfect ruby-red colour without gold; which Orschal and most chemists
have however doubted. It is nevertheless said, that Krüger, who was
inspector of the glass-houses at Potsdam, under Frederic William king
of Prussia, discovered earlier the art of making ruby-glass without
gold, and that a cup and cover of cut glass made in this manner is
still preserved at Berlin.
Painting on glass and in enamel, and the preparation of coloured
materials for mosaic work, may, in certain respects, be considered as
branches of the art of colouring glass; and in all these a beautiful
red is the most difficult, the dearest, and the scarcest. When the old
master-pieces of painting on glass are examined, it is found either
that the panes have on one side a transparent red varnish burnt into
them, or that the pieces which are stained through and through, are
thinner than those coloured in the other manner[330]. It is therefore
extremely probable that the old artists, as they did not know how to
give to thick pieces a beautiful transparent red colour, employed
only iron, or manganese, which pigment, as already observed, easily
becomes in a strong heat blackish and muddy[331]. Enamel-painters,
however, were for a long time obliged to be contented with it. A red
colour in mosaic work is attended with less difficulty, because no
transparency, nay rather opacity, is required. At Rome those pieces are
valued most which have the beautiful shining red colour of the finest
sealing-wax. We are told by Ferber that such pieces were at one time
made only by a man named Mathioli, and out of a kind of copper dross;
at present (1792), there are several artists in that city who prepare
these materials, but they are not able to give them a perfect high
colour[332].
[Of late years the interesting art of painting on glass has attracted
considerable attention; lovers of the fine arts, antiquaries, and
chemists, have contributed to its perfection, and have sought to
ascertain by what methods their predecessors were able to give those
beautiful and brilliant tints to their productions, many of which have
been so wantonly destroyed by the barbarity of the last century[333].
One of the most ingenious essays that has been written on the subject,
is that published by an anonymous correspondent in the Philosophical
Magazine for December 1836, which we subjoin in elucidation of our
present knowledge on the subject.
_On the Art of Glass-Painting._ _By_ a Correspondent.
It is a singular fact, that the art of glass-painting, practised with
such success during the former ages from one end of Europe to the
other, should gradually have fallen into such disuse, that in the
beginning of the last century it came to be generally considered as
a lost art. In the course of the eighteenth century, however, the
art again began to attract attention, and many attempts were made to
revive it. It was soon found by modern artists, that by employing the
processes always in use among enamel-painters, the works of the old
painters on glass might in most respects be successfully imitated;
but they were totally unable to produce any imitation whatever of
that glowing red which sheds such incomparable brilliancy over the
ancient windows that still adorn so many of our churches[334]. For
this splendid colour they possessed no substitute, until a property,
peculiar to silver alone among all the metals, was discovered, which
will presently be described. The art of enamelling on glass differs
little from the well-known art of enamelling on other substances. The
colouring materials (which are exclusively metallic) are prepared by
being ground up with a _flux_, that is, a very fusible glass, composed
of silex, flint-glass, lead, and borax: the colour with its flux is
then mixed with volatile oil, and laid on with the brush. The pane
of glass thus enamelled is then exposed to a dull red heat, just
sufficient to soften and unite together the particles of the flux, by
which means the colour is perfectly fixed on the glass. Treated in
this way, gold yields a purple, gold and silver mixed a rose-colour,
iron a brick-red, cobalt a blue[335]; mixtures of iron, copper and
manganese, brown and black. Copper, which yields the green in common
enamel-painting[336], is not found to produce a fine colour when
applied in the same way to glass, and viewed by transmitted light; for
a green therefore recourse is often had to glass coloured blue on one
side and yellow on the other. To obtain a yellow, silver is employed,
which, either in the metallic or in any other form, possesses the
singular property of imparting a transparent stain, when exposed to
a low red heat in contact with glass. This stain is either yellow,
orange, or red, according to circumstances. For this purpose no flux
is used: the prepared silver is merely ground up with ochre or clay,
and applied in a thick layer upon the glass. When removed from the
furnace the silver is found not at all adhering to the glass; it is
easily scraped off, leaving a transparent stain, which penetrates to a
certain depth. If a large proportion of ochre has been employed, the
stain is yellow; if a small proportion, it is orange-coloured; and by
repeated exposure to the fire, without any additional colouring matter,
the orange may be converted into red. This conversion of orange into
red is, I believe, a matter of much nicety, in which experience only
can ensure success. Till within a few years this was the only bright
red in use among modern glass-painters; and though the best specimens
certainly produce a fine effect, yet it will seldom bear comparison
with the red employed in such profusion by the old artists.
Besides the enamels and stains above-described, artists, whenever
the subject will allow of it, make use of panes coloured throughout
their substance in the glass-house melting-pot, because the perfect
transparency of such glass gives a brilliancy of effect, which
enamel-colouring, always more or less opake, cannot equal. It was to
a glass of this kind that the old glass-painters owed their splendid
red. This in fact is the only point in which the modern and ancient
processes differ, and this is the only part of the art which was
ever really lost. Instead of blowing plates of solid red, the old
glass-makers used to _flash_ a thin layer of red over a substratum of
plain glass. Their process must have been to melt side by side in the
glass-house a pot of plain and a pot of red glass: then the workman,
by dipping his rod first into the plain and then into the red glass
pot, obtained a lump of plain glass covered with a coating of red,
which, by dexterous management in blowing and whirling, he extended
into a plate, exhibiting on its surface a very thin stratum of the
desired colour[337]. In this state the glass came into the hands of
the glass-painter, and answered most of his purposes, except when the
subject required the representation of white or other colours on a red
ground: in this case it became necessary to employ a machine like the
lapidary’s wheel, partially to grind away the coloured surface till the
white substratum appeared.
The material employed by the old glass-makers to tinge their glass red
was the protoxide of copper, but on the discontinuance of the art of
glass-painting the dependent manufacture of red glass of course ceased,
and all knowledge of the art became so entirely extinct, that the
notion generally prevailed that the colour in question was derived from
gold[338]. It is not a little remarkable that the knowledge of the
copper-red should have been so entirely lost, though printed receipts
have always existed detailing the whole process. Baptista Porta (born
about 1540) gives a receipt in his Magia Naturalis, noticing at the
same time the difficulty of success. Several receipts are found in the
compilations of Neri, Merret and Kunckel, from whence they have been
copied into our Encyclopædias[339]. None of these receipts however
state to what purposes the red glass was applied, nor do they make any
mention of the _flashing_. The difficulty of the art consists in the
proneness of the copper to pass from the state of protoxide into that
of peroxide, in which latter state it tinges glass green. In order to
preserve it in the state of protoxide, these receipts prescribe various
deoxygenating substances to be stirred into the melted glass, such as
smiths’ clinkers, tartar, soot, rotten wood, and cinnabar.
One curious circumstance deserves to be noticed, which is, that glass
containing copper when removed from the melting-pot sometimes only
exhibits a faint greenish tinge, yet in this state nothing more than
simple exposure to a gentle heat is requisite to throw out a brilliant
red. This change of colour is very remarkable, as it is obvious that no
change of oxygenation can possibly take place during the _recuisson_.
The art of tinging glass by protoxide of copper and flashing it on
crown-glass, has of late years been revived by the Tyne Company in
England, at Choisy in France[340], and in Suabia in Germany, and in
1827 the Academy of Arts at Berlin gave a premium for an imperfect
receipt. To what extent modern glass-painters make use of these new
glasses I am ignorant; the specimens that I have seen were so strongly
coloured as to be in parts almost opake, but this is a defect which
might no doubt be easily remedied[341].
I shall now conclude these observations by a few notices respecting
glass tinged by fusion with gold, which, though never brought into
general use among glass-painters, has I know been employed in one or
two instances, flashed both on crown- and on flint-glass. Not long
after the time when the art of making the copper-red glass was lost,
Kunkel appears to have discovered that gold melted with flint-glass
was capable of imparting to it a beautiful ruby colour. As he derived
much profit from the invention, he kept his method secret, and his
successors have done the same to the present day. The art, however, has
been practised ever since for the purpose of imitating precious stones,
&c., and the glass used to be sold at Birmingham for a high price under
the name of _Jew’s glass_. The rose-coloured scent-bottles, &c., now
commonly made, are composed of plain glass flashed or coated with a
very thin layer of the glass in question. I have myself made numerous
experiments on this subject, and have been completely, and at last
uniformly, successful, in producing glass of a fine crimson colour.
One cause why so many persons have failed in the same attempt[342],
I suspect is that they have used too large a proportion of gold; for
it is a fact, that an additional quantity of gold, beyond a certain
point, far from deepening the colour, actually destroys it altogether.
Another cause probably is, that they have not employed a sufficient
degree of heat in the fusion. I have found that a degree of heat, which
I judged sufficient to melt cast-iron, is not strong enough to injure
the colour. It would appear, that in order to receive the colour, it is
necessary that the glass should contain a proportion either of lead, or
of some other metallic glass. I have found bismuth, zinc, and antimony
to answer the purpose, but have in vain attempted to impart any tinge
of this colour to crown-glass alone.
Glass containing gold exhibits the same singular change of colour
on being exposed to a gentle heat, as has been already noticed with
respect to glass containing copper[343]. The former when taken from the
crucible is generally of a pale rose-colour, but sometimes colourless
as water, and does not assume its ruby colour till it has been exposed
to a low red-heat, either under a muffle or in the lamp. Great care
must be taken in this operation, for a slight excess of fire destroys
the colour, leaving the glass of a dingy brown, but with a blue
transparency like that of gold-leaf. These changes of colour have been
vaguely attributed to change of oxygenation in the gold; but it is
obviously impossible that mere exposure to a gentle heat can effect any
chemical change in the interior of a solid mass of glass, which has
already undergone a heat far more intense. In fact I have found that
metallic gold gives the red colour as well as the oxide, and it appears
scarcely to admit of a doubt, that in a metal so easily reduced, the
whole of the oxygen must be expelled long before the glass has reached
its melting-point. It has long been known that silver yields its colour
to glass while in the metallic state, and everything leads one to
suppose that the case is the same as to gold.
There is still one other substance by means of which I find it is
possible to give a red colour to glass, and that is a compound of tin,
chromic acid, and lime; but my trials do not lead me to suppose that
glass thus coloured will ever be brought into use.
* * * * *
With respect to the production of artificial gems, they are now made
abundantly of almost every shade of colour, closely approximating to
those which occur in nature, excepting in hardness and refractive
power. They are formed by fusing what is called a base with various
metallic oxides. The base varies in composition: thus, M. Fontanieu
makes his by fusing silica with carbonate of potash, carbonate of
lead and borax. M. Donault Wieland’s consists of silica, potash,
borax, oxide of lead, and sometimes arsenious acid. Hence the base
differs but little in composition from glass. By fusing the base with
metallic oxides, the former acquires various tints. Thus with oxide of
antimony the oriental topaz is prepared; with oxide of manganese and a
little purple of cassius, the amethyst; with antimony and a very small
quantity of cobalt, the beryl; with horn silver (chloride of silver),
the diamond and opal: the oriental ruby is prepared from the base, the
purple of cassius, peroxide of iron, golden sulphuret of antimony,
manganese calcined with nitre and rock crystal.]