Gold in Ruby Glass:
A 197Au Mössbauer Study
S. Haslbeck1, K.-P. Martinek2, L. Stievano3
and F. E. Wagner1
1Physik-Department
E15, Technische Universität München,
85748 Garching, Germany.
2F. X. Nachtmann Bleikristallwerke GmbH, 94566 Riedlhütte,
Germany
3Laboratoire de Réactivité de Surface UMR 7609, Université
Pierre et Marie Curie, Paris, France
e-mail: fwagner@ph.tum.de
Gold was used, though rarely, for colouring glass already in antiquity. A fine example
is the Lycurgus cup, which is attributed to the Late Roman Period (about AD
400) and which is now in the British Museum (I. Freestone et al., Gold Bulletin 40 (2007)
270). On a larger scale gold ruby glass came into use in the late17th century, when
sizeable ruby glass vessels were first made by the German Johann Kunckel in his glass
works at Potsdam (S. Frank, Glass Technology 25 (1984) 47). The pictures below show
two modern commercial flashed cups from the production of the Nachtmann glass works
at Riedelhütte, Germany (left) and a 19th century wine glass together with a beaker (right)
made recently at Nachtmann according to an old recipe given by Kunkel.
The red colour of Gold ruby glass is caused by nanoparticles of metallic gold in the
silicate glass matrix. Particles with sizes between a few and about 100 nm yield a fine
colour. Due to surface plasmon excitation in which the electrons vibrate collectively with
respect to the atomic cores, such particles exhibit a strong light absorption around 550
nm, in the green region of the visible spectrum. Red as well as blue light is absorbed
much less, and as a consequence the glass attains its typical magenta or ruby red
colour. The picture on the left (C. Burda et al., Chem. Rev. 105 (2005) 1025) illustrates
this and also shows that the optical absorption, and hence the hue of the colour,
depends on particle size. Additives influence the hue of the glass, as is illustrated by the
examples shown below on the right of fragments of a soda lime glass (70% SiO2, 22%
Na2O, 8% CaO, 200 ppm Au) with different additives.
No additivess
0.2 wt.% Sn
20 ppm Sn
5 wt.% Pb
1 wt.% Sb
9 ppm Se
Only about 200 ppm of gold are needed to impart an intense red colour to the glass. In
fact, higher concentrations are counterproductive because of the low solubility of gold in
the silica glass melt, which leads to the formation of gold inclusions that are too large to
contribute to the red colour. The gold is added to the raw materials – usually silica, soda
or potash and lime – as a gold compound like KAu(CN)2, and the mixture is then melted
at around 1400ºC. From the melt vessels can be formed by blowing or pressing. After
rapid cooling, the glass is then still colourless. Only on annealing at temperatures around
600 ºC for minutes or hours does it strike red, because only then do the metallic
nanoparticles form. This is illustrated by the two dishes shown below: The one on the left
is still in the quenched state, while the one on the right was annealed. Such dishes are
used in the glass industry to make flashed glasses like those shown on the previous page
by blowing colourless glass into them and then shaping and engraving the desired items.
Points of interest that are difficult to
elucidate concern the mechanism of
formation of the gold nanoparticles and the
chemical state of the gold before the glass
strikes red. Mössbauer spectroscopy with
the 77 keV γ-rays of 197Au has been
successfully used to answer the latter
question and to make some contribution to
the former one, though the measurements
are time-consuming because of the low
gold concentration in the glass.
QS[m/s]
6
Glas
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The measurements were performed at 4.2 K in a liquid helium bath cryostat with 197Pt
(T1/2 = 19 h) sources, which were made by neutron irradiation of metallic 196Pt. A typical
Mössbauer spectrum of a quenched, i.e., colourless gold ruby glass is shown below. It
can be decomposed into a single line at −1.23 mm/s attributable to metallic gold, and a
quadrupole doublet with a splitting of about 6.2 mm/s and an isomer shift of about 1.0
mm/s. This doublet represents the gold dissolved in the glass matrix. Comparing its
Mössbauer parameters with the established correlations between isomer shifts and
quadrupole splittings of Au(I) and Au(III) compounds, one concludes that the dissolved
gold is fairly ionic Au(I).
1
2
Since the glass is still colourless, the metallic gold
must form particles that are too big to yield colour.
1
0
These particles may already be present in the melt
A
u
(
I
)
because of the low gold solubility.
8
A
u
(
I
I
I
)
4
2
0
2
4
2
0
2
4
I
S
[
m
m
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]
6
8
The gold Mössbauer spectra shown
here (F. E. Wagner et al., Nature
407 (2000) 691) represent three
different ruby glasses before (left)
and after quenching (right). The
glass on top is a lead-rich crystal
glass from the production of the
Nachtmann glass works (46% SiO2,
39% PbO, 6% K2O, 4% Na2O, 2%
As2O3, 1% Sb2O3, 1% SnO2, about
200 ppm Au) melted at about
1450ºC and annealed at 520 ºC for
5 hours. The glass in the middle is a
soda glass (74% SiO2, 26% Na2O,
350 ppm Au) and the bottom one is
a soda lime glass (70% SiO2, 22 %
Na2O, 8 % CaO, 200 ppm Au).
The Mössbauer parameters do not depend much on the composition of the base glass.
The bonding situation of the gold thus seems to be quite independent of the glass
matrix. Since Au(I) usually forms bonds with two ligands in a linear arrangement, one
concludes that the gold dissolved in the glass matrix most probably has two oxygen
ligands in a virtually linear bonding arrangement with two oxygen ligands.
Tin is known to strongly affect the speed with
which the metallic gold clusters form as well as
the number and size of these clusters (J. A.
Williams et al., J. Amer. Ceram. Soc. 64 (1976)
709). The spectra on the left demonstrate the
effect of different tin contents on the formation of
gold nanoparticles in a soda lime glass (70 %
SiO2, 22% Na2O, 8% CaO, 200 ppm Au).
Spectra of the quenched glasses are shown on
the left, spectra after annealing at 590ºC for 10
hrs on the right (S. Haslbeck et al., Hyperfine
Interactions 165 (2005) 89). At low tin
concentrations (20 and 200 ppm) the spectra of
the annealed glasses do not differ much from
those for the glass without any tin. At high tin
concentrations (0.2% and 2%), however, a
broad component near zero velocity occurs,
which is reminiscent of features observed in the
Mössbauer spectra of gold nanoparticles smaller
than a few nanometers embedded into a mylar
matrix (Stievano et al., J. Non-Cryst. Solids 232234 (1998) 644). This shows that the tin causes
the formation of many very small gold particles.
To conclude, we would like to stress that the measurements of the Mössbauer spectra
of gold ruby glass is tedious due to the small gold content of the glasses and the limits
to the absorber thickness (about 3 g/cm2) imposed by photoelectric absorption. The
measurements would not have been possible without the hundreds of sources
irradiated at the Munich Research Reactor (FRM I). We should like to thank the
operators for their help in making these sources.