1. Gold in Ruby Glass: A 197 Au Mössbauer Study S. Haslbeck 1, K.-P. Martinek 2, L. Stievano 3 and F. E. Wagner 1 1 Physik-Department E15, Technische Universität München, 85748 Garching, Germany. 2 F. X. Nachtmann Bleikristallwerke GmbH, 94566 Riedlhütte, Germany 3 Laboratoire de Réactivité de Surface UMR 7609, Université Pierre et Marie Curie, Paris, France e-mail: fwagner@ph.tum.de fwagner@ph.tum.de

2. 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 late17 th 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 19 th century wine glass together with a beaker (right) made recently at Nachtmann according to an old recipe given by Kunkel.

3. 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% SiO 2, 22% Na 2 O, 8% CaO, 200 ppm Au) with different additives. 1 wt.% Sb No additivess 0.2 wt.% Sn 20 ppm Sn5 wt.% Pb9 ppm Se

4. 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 197 Au 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.

5. The measurements were performed at 4.2 K in a liquid helium bath cryostat with 197 Pt (T 1/2 = 19 h) sources, which were made by neutron irradiation of metallic 196 Pt. 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). Since the glass is still colourless, the metallic gold must form particles that are too big to yield colour. These particles may already be present in the melt because of the low gold solubility.

6. 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% SiO 2, 39% PbO, 6% K 2 O, 4% Na 2 O, 2% As 2 O 3, 1% Sb 2 O 3, 1% SnO 2, 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% SiO 2, 26% Na 2 O, 350 ppm Au) and the bottom one is a soda lime glass (70% SiO 2, 22 % Na 2 O, 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.

7. 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 % SiO 2, 22% Na 2 O, 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 232- 234 (1998) 644). This shows that the tin causes the formation of many very small gold particles.

8. 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/cm 2 ) 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.