Sep
15
Antique Coloured Glass
September 15, 2009 |
Coloured
Glass
Amphoriskos in blue G1.ass with trailed
decoration
Egypt, (.1450-1350 B.C. Ht. 115 mm (45 in.)
Colouring glass by deliberately adding metal-
lic oxides is a technique almost as old as the
making of glass itself, and it appears at least
a thousand years before the first clear
colourless glass is found. A recipe for a glaze
exists on a tablet found near Tell ‘Umar on
the Tigris, which dates back as early as the
17th century B.C. A series of clay cuneiform
tablets found in the library established by
Ashurbanipal at Nineveh in the 7th century
B.C. gives detailed directions for preparing
coloured glass. From analyses it has been
proved that in general, ancient glasses were
roughly akin to modern-day soda-lime glass,
though they contained a much higher amount
of magnesia, particularly in Mesopotamia.
Whether from Egypt, Mesopotamia or else-
where, ancient glass used exactly the same
colouring agents. Iron and manganese were
often unintentionally added with the raw
materials, but oxides like copper, cobalt and
antimony were intentional additions. There
has been little change in the basic metallic
oxides used until the modern period.
Blue Glass: Shades of blue were the first
colours to be used by glass-makers, the
metal oxides of cobalt and copper being
the usual colouring agents. Although cobalt
oxide is used less frequently than copper
oxide to produce blue, it has remained
present as a colouring oxide throughout
the whole glass-making period. The earliest
known piece of cobalt blue glass comes
from Eridu in Mesopotamia, and has been
dated to at least 2000 B.C. The cobalt used
was probably brought from Persia or the
European countries bordering the Med-
iterranean. Only a minute amount of it, of
the order of 005 per cent, is needed to
impart a deep blue colour to glass. Blue
Egyptian glass was made with the aid of
both cobalt oxide and copper oxides, but
the blue colour in Egyptian glasses which
contain cobalt is not always entirely due to
that, for they often contain a small pro-
portion of copper or manganese as well.
Generally speaking, around 1500 B.C.
copper was used by the Egyptian glass-
makers to make bluish-green glass, and
cobalt was used to make pure blue or
violet-blue glass. The workers added the
oxides to their basic raw materials of sand,
soda and lime. Unlimited supplies of
ordinary quartz sand and calcareous sand
were available to them; soda, in the form
of natron, a natural soda, could easily be
obtained in vast desert deposits at Wadi
Natrun, a depression in the Libyan desert
some 40 miles to the west of Cairo, and at
El Kab in Upper Egypt, both of which are
mentioned in ancient Egyptian records.
Limestone was also available in unlimited
quantities, since the hills bordering the
Nile for some 500 miles from Cairo to
beyond Esna are composed of it. Dolomitic
limestone and dolomite also occur to some
extent, and lime could moreover be
obtained from burned sea shells. The
Mesopotamian tablets in some places
specify ground red shells from the sea as
an ingredient.
from tests it has been proved that
Egyptian glasses were probably produced
at temperatures of io6o°C. The trail it >n
of making blue glass was to continue
throughout the whole glass-making period.
Dark blue seems to have been a favourite
colour in Europe in the 7th century A.D.
This dark-age blown glass is, like ihe
Roman glass before it, of the soda-litfnc
variety. Glass-makers in western Europe
were still apparently able to obtain from
the Mediterranean countries the soda-
charged marine plant-ash used by the
Romans. It was not until around 1000
A.D. that glass-makers switched to potash
glass, using as alkali burnt bracken apd
other woodland plants obtained locally,
though the colouring oxides remained
liasn ally the same. The thick trellis trailing
on the shoulder of the jar illustrated v as
typical of dark-age decoration, when t tie
skills of painting, gilding, cutting, indei it-
ing and tooling had ceased.
standing cup in blue glass
Venice, mill-1 =;111 century. Ht. 165 mm (65 in.)
The next notable achievement in the
production of blue glass came with the rise
of the glass-making industry in Venice.
Glass-making had been practised there
continuously since the end of the 10th
century, but it was the Renaissance that
gave the stimulus needed to bring about a
period of intense development in the
Venetian glass industry. In 1291 the glass-
houses of Venice were transferred across
the Lagoon to the island of Murano
because of the general fear of fire spreading
from the furnaces. By 1400 the Venetians
had perfected a glass material that achieved
such clarity it was known as Venetian
cristallo after rock crystal (see Clear Glass),
and they had rediscovered the secret of
colouring glasses, including a very beauti-
ful deep blue colour. The Venetians made
a soda-lime glass, using for silica the
quartz-like pebbles obtained from the bed
of the river Ticino, and for alkali, the ash
of sea and salt-marsh plants.
covered goblet in blue and colourless glass
Nuremberg, Germany, late 17th century
Ht. 406 mm (t6 in.)
The Venetian glass-makers’ ability to
produce gloriously rich colours, particu-
larly blue, green and turquoise, was taken
with them to the glass-houses they helped
to establish in such places as Hall-in-the-
Tyrol and the Netherlands. In the 17th
century the French and Nuremburg crafts-
men introduced a new blue colour. The
colouring oxides remained the same, but
the composition of the glass was different.
The new German glass material included
potash instead of soda, and a large propor-
tion of chalk. For a time the new glass
continued to be used in the tradition of
soda-lime glass, for carrying out baroque
forms inspired by Venetian products.
However, it did not take the German
glass-makers long to realise that the new
glass was a perfect medium for cut and
engraved work, and soon all other decora-
tive techniques, including the art of
colouring glass, took a back seat in
preference to the art of cutting and
engraving glass.