Section 4: Composite artefacts (PDF 20858kb) - National Museum of ...

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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967The thread diameter, width of metal foil and number of coils per 5 mm thread were measuredmanually through digital imaging analysis. All µm measurements were rounded to the nearest 50 due tothe relative imprecision of the measurement technique.All metal thread samples in this study consist of solid metal strips wound around a silk core, which iswhite or light yellow for silver threads and dyed in darker yellow shades for gilt threads. All but onesample exhibit S twist direction of the metal coils and are categorized according to thread diameter andcolour of metal strip in Table 1. The metal strip width and number of coils per unit length were foundto vary with thread diameter. Large diameter threads (≥ 700 µm) generally have wider metal strips andfewer coils than medium (< 650 > 350 µm) or small diameter threads (≤ 350 µm), which show thefinest metal strip width, the most coils per unit length and are also the least corroded, Figure 2. Therelationship between diameter, width and coil number was also observed in the silver threads and thosethat were heavily corroded.Table 1. Categorization of metal threads from five MODHT Renaissance tapestries.GroupNumber of samples in tapestryHCP PNM1 PNM2 PNM5 BXL2 Totalgilt large diameter (≥ 700 µm) 14 - - - - 14gilt medium diameter (< 650 > 350 µm) 25 3 3 5 10 46gilt small diameter (≤ 350 µm) 11 6 6 3 - 26gilt corrosion 19 7 3 - 4 33silver 26 4 4 1 - 35double wrapped 12 4 2 1 - 19triple wrapped - 1 - 3 - 4restoration - - 1 - 7 8no assignment 5 2 - 1 - 8Total number of samples in tapestry 112 27 17 14 21 193Metal strip width in µm650600550500450400350300250200Thread diameter in µm850750700600550-500450400350-3002505 7 9 11 13 15 17 19Number of coils per 5 mmFigure 2. Relationship of metal thread diameter, metal strip widthand number of coils per unit length of metal thread (67 samples).A total of 23 threads with two or even three layers of metal wrapping were found in samples of fourout of the five tapestries studied. Metal threads with multiple wrapping have rarely been mentioned inthe literature (Hacke et al. 2003). Two double wrapped gilt leather substrate metal threads with linenand cotton cores were found in a 14 th century Near Eastern textile (Indictor et al. 1988). Montegutmentions one double wrapped thread from late 15 th century Spain or Italy, possibly Venice (Montegutet al. 1992), while Indictor identified one double wrapped metal thread of Indian origin (Indictor andBlair 1990). Figure 3 shows samples of triple wrapped threads; the abundance of precious metal, theirintricacy and evenness of structure is striking, indicating that these are threads of premium value, usedonly in textiles of the highest quality such as tapestries of the Abraham series commissioned by HenryVIII and the tapestry series of the Fables of Ovid in the Royal Spanish collection.© Published by the National Museum of Australia www.nma.gov.au417
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967Figure 3. Triple wrapped metal threads.Metal threads in tapestries are in close proximity to wool and silk and it has been suggested that thedegradation of textile fibres influences the tarnishing of metals (Hoke and Petrascheck-Heim 1977;Howell et al. 1999). This study investigates the corrosion products on metal coupons tarnished byaccelerated ageing, as well as historical metal threads. The combination of X-ray ElectronSpectroscopy (XPS), Secondary Ion Mass Spectrometry (SIMS) and Secondary Electron Microscopycoupled with Energy Dispersive X-ray analysis (SEM/EDX) have been applied to investigate the bulkand surface metal alloy contents and corrosion products. These techniques have been previously usedto study metal threads and provide a powerful complementary approach to characterise surface andsub-surface composition and morphology (Howell et al. 1999; Hacke et al. 2003).2. Experimental techniques2.1 Accelerated tarnishingSilver and copper coupons, 5 x 10 x 0.25 mm, > 99.9 % purity, were cut and a pin hole punched ineach to allow suspension during the Oddy tests. The coupons were cleaned successively in propanol,hexane, chloroform and methanol for 10 minutes in an ultrasonic cleaner. Silver and copper couponswere incubated under thermal or light Oddy test conditions indicated below with the following wooland silk samples: undyed, dyed with weld, woad, madder or brazil.2.1.1 Thermal Oddy test0.2 g of wool or silk yarn was placed in the bottom of a test tube together with Durham tubes (6.5 x30 mm) filled with deionised water. Silver and copper coupons were suspended from the top of the testtube using nylon filament and the tubes were sealed with a stopper and tape. 12 yarn samples and onecontrol were incubated at 60°C for 4 weeks.2.1.2 Light Oddy testWool and silk fabrics, 4 x 5 cm, were stapled to a cardboard backing and placed in glass cells (2 x 5x 15 cm) along with tubes, 6.5 x 120 mm, filled with deionised water. Silver and copper coupons weresuspended from the top of the cells containing the fabric pieces and one control sample containing onlythe tube with deionised water. The coupons were shielded from direct irradiation with cardboard placedaround the top of the sealed cells. The fabric samples were irradiated for 300 hours using an OSRAM400 W HQ (MB-U) mercury lamp in a Microscal Ltd. Mark V Light Fastness Tester. The cells wereequipped with a circulating water cooling system maintaining the temperature at 27-29 °C.2.2 SEM/EDXThe samples were examined with a Hitachi SEM S-3000N / EDAX system, using the secondaryelectron detector, a medium aperture, approximately 40 spot size, a working distance of ~ 5 mm and a5 keV accelerating voltage were used for all samples. EDX spectra were obtained at a 15 mm workingdistance and a 25kV accelerating voltage. Quantification was performed using background subtractionand ZAF correction.© Published by the National Museum of Australia www.nma.gov.au418
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