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·Conservation and Restoration in Art and Architecture
Investigation of Historical Glasses Using Natural Born RadioactivityM. Schiekel, A. Meister, M. Seibitz
Technical Univ. of Dresden, Dept. Building Materials, Mommsenstraße 13, 01062 Dresden
Museum of Decorated Arts, Dresden-Pillnitz, Schloß Pillnitz, Wasserpalais
|Nuclide||g-Energy in keV||Half life||Characteristic for|
|k-40||1462||1.27 · 109 a||potassium|
|Table 1: Characteristics of used radiation|
Measurements have been done using a well-shielded HPGe-Detector spectrometer with extreme low background radiation in an energy range from 40 to 1800 keV. Detector resolution is 1.3 keV with respect to the 1.33 MeV Co-60 g-line. Due to the low intensity of natural radioactivity a measuring period of 100 h was necessary to obtain not to big statistical errors.
A typical spectrum from an 18th century glass is shown in fig. 1. Beside the g-lines also a significant peak of Pb-K x-rays has been determined due to excitation of Pb-atoms by the high energy g-lines originating from the sample itself.
|Fig 1a: Complete energy range||Fig 1b: Low energy range.|
|Fig 1: g - Spectrum of a historical glass carafe.|
We investigated glass carafes and some covers made in the 18th century and all used by the Saxon elector's court. The carafes had been classified into four sets based on known or supposed manufacturing works and date. All carafes did not real differ in geometry and mass, so there was no need for expended geometrical corrections.
The characteristics of the sets are listed in table 2. The covers are single pieces.
|Set 1||Set 2||Set 3||Set 4|
|mean height in mm||145.1||145.0||138.0||133.2|
|mean max. diameter in mm||98.8||100.0||100.2||98.2|
|mean wall thickness in mm||3.6||3.8||3.5||3.9|
|mean mass in g||188||204||183||207|
|total number of carafes||19||5||5||3|
|period: first third of 18th century||period: middle of 18th century||period: 1763 - near 1770||period: 18th century|
|engraving: monogram Saxon elector Friedrich August II., King of Poland||engraving: monogram Saxon elector Friedrich August II., King of Poland||engraving: monogram Saxon elector Friedrich August III.||engraving:without|
|manufacturer: Dresden glassworks||manufacturer: Dresden glassworks||manufacturer: supposed Glücksburg/Sa glassworks||manufacturer: Dresden or Glücksburg/Sa glassworks|
|Table 2: Characteristics of investigated glass carafes|
|Set 1||Set 2||Set 3||Set 4|
|Table 3: mean values of g and x-ray yields, normalized to sample mass, in arbitrary units (n.d. = not detectable)|
This low concentrations are supposed due to specially refining procedures of the origin material before or throughout the manufacturing process in the Dresden glass-works. Production of different glassworks shows considerably divergent content of radioactive nuclides, so these concentrations can be used to identify the sources of various products. Inside a set of same origin radioactive concentrations only small fluctuate (see fig. 2).
|Fig 2: Natural born radioactivity and lead peak intensity for set 2 samples|
|Ii ~ ciµi(hv)·I0(hv)Ki·Ai(hv)||(1)|
with elemental concentration ci, mass attenuation coefficient µi, primary intensity I0 and geometrical and absorption factor Ai. The factor Ki includes all atomic constants describing the excited element. Excitation is highly efficient for energies not much about the K-edge. Thus it can be assumed, that only the low energy lines of radioactive sample nuclides have to take into account for Pb-K x-ray excitation. Hence excitation energy range was counted from 90 keV to 360 keV, including the strongest Pb-214 lines. This procedure cannot be used to determine absolute concentrations. Recently produced samples with known lead concentration  have been used as calibration standards. Using similar geometrical arrangements Pb-concentration of historical glasses cPb,glass can be estimated by
cPb,glass = cPb,calib · (IPb,glass·Iexc,calib)/(IPb,calib·Iexc,glass)
with IPb as background free peak yield of Pb-lines and Iexc as total yield in the limited excitation energy range.
Some selected results of Pb-concentration are shown in table 4 with partly considerable amounts. Although reproducibility of values is poor some trends can be deduced. Unfortunately no details about glass mixtures used in the Dresden glassworks had been published. This know-ledge was handled as a personal secret of the glass producers. However, in 1709, 1725 and 1746 in the glassworks inventory beside Spanish soda, antimony, saltpetre,
chalk, potash, borax,tartaric stone, grey and white sand and glass scrap also always white and red lead are listed, which was intended for the above mentioned "English glass mixture" with higher lead concentration. This attribute has been on record since the death of the glass manufacturer Müller in 1734. He knew the English formula as a result of his family relationships and brought this knowledge along to Dresden in 1713 . Comparable to the desired porcelain production at the beginning of the 18th century in approximately the same period also manufacturing of "English glass ware" had been invented by order of elector August the Strong ("... Ihro Majt. des Höchst. seel. Königs . . . Befehl") in a quality, that "there are not in least differences in weight and sound compared to real English glasses". In continuation of Tschirnhaus' experiments Müller succeeded in performing this order. The analyses prove that since the first third of the 18th century (set 2 and set 3) glass production with lead oxide admixtures had been a well mastered procedure. Results also show, that in the Glücksburg glassworks (a part of Dresden glassworks) English glass had been still durably produced up to the end of the 18th century.
|Sample||Sample features||CPbO in %|
|set 1||Dresden, first third 18th cent.||< 0.15|
|set 2||Dresden,middle of 18th cent.||1.58 ± 0.53|
|set 3||Glücksburg, 1763 - near 1770||1.66 ± 0.39|
|set 4||Dresden or Glücksburg, 18th cent.||0.91 ± 0.18|
|goblet||Dresden, before 1720||4.32 ± 0.65|
|cover||Glücksburg, middle of 18th cent.||0.21 ± 0.07|
|cover||Dresden, near 1730||13.8 ± 2.07|
|Cover||Dresden, near 1730||4,80 ± 0.95|
|Table 4: Lead oxide concentration of selected historical glasses|
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