Metallurgical Case Stu di es from the British Museum's Collections of pre-Hispanic Gold S USAN LA NIECE D EPARTMENT OF SCIENTIFIC RESEARCH B RITISH M USEUM Abstract: A program of scientific and technological study of the collection of Pre- Hispanic gold at the British Museum has recently begun , using opti- cal and scanning electron microscopy, radiography and X-ra y fl uorescence analysis. lnteresting case studies, showing a ra nge of goldworking practices, will be illustrated. These include gilding, tlw use of stone m atrices in m ould making and the identifica tion of fakes. Author's address : Bnush Museum, London WC I B 3DG, Resumen: El Museo Británico ha com enzado recientem ente un programa UK de estudio cientifico y tecnológico de su colección de obietos de orfebreria prehispán.ica. Para este programa se usa microscopía óptica y electrónica de Acknowlcdgcments barrido, análisis radiográficos y de fluorescen cia de rayos X. En este artículo Warm est thanks are du c to Professor jcan Thomas, Colm se presentan estudios de caso interesantes que muestra n un rango de McEwan , N 1gcl Mceks, prácticas de orfebrería que incluye dorado, uso de m atrices de piedra para la Shcn dan Bowman and john elaboración de moldes e identificación de fals ificaciones. Mack tlt the Bntish Museum ; lO Clemencia Pla zas, Soma Archlla, Juamta Sáenz, Lu z Alba Gómez, and all thc staff at thc Museo del Oro, Bogota ; tO Warwick Bray, lnsuuue of Archaeology, Umvcrs•ry Col- lcgc, London ; Patncia E.st évcz, he British Museum's coU ection of South American metalwork has Banco Central del Ecuador ; been accumulated over nearly 200 years, but until recently few of Cec1ha Bakula, Banco Central the pieces had been analysed or studied metallurgically. The ob- de Reserva del PerU and Palo - T ma Ca rcedo de Mufarcch jects came from Peru, Ecuador, Colombia, Panama, Costa Rica and up to Mexico and this ongoing study has been set up to provide technological information for the catalogue of che collection. 1 am espcc•ally gratcful to t.he Royal Socte ry of Great Brna1n for a research travcl grane Also Analysis of alloy composition is an obvious avenue for research, but as yet to The H1 storical Mctall urgy Soc1ety for a contri button to there is not a large database of analyses (see Rovira 1994 for a review of conferencc expenses. gold j . As the opportunities increase to have analysis done on pre-Hispanic METALLURCICAL CASE STUDIES FROM THE BRITISH MUSEUM'S metalwork, the data- Photograph 1: Muisca base will become large tunjos (height of largest, enough to allow statis- 1 1 cm). The numbering tically valid compari- corre lates the objects to sons and grouping of the analyscs in tables l and 2, and figure 2. objects now scattered (photo A. Mi l ton, British throughout interna- Museum(. tional collections. ,, Howcver, without comparabili ry in the analytical approach , there are likely to be some problems which . are illustrated by the Muisca tunjos dis- cussed below. l T14 Muisca tunjos Muisca tunjos are minianne representations of arcefacts, animals or (most commonly) human beings, singly or in groups. A few are made from pure copper or gold, but the overwhelming majoriry are cast in gold alloys. They were made as votive offerings by the Muisca of the altiplano of Cundinamarca and Boyacá, central olombia, in the period between the 6th and 16th centuries AD, and are usually found (sometimes severa! together, in ce- ramic offertory vessels) at isolated landmarks such as lakes, peaks, and caves (P lazas, 1975 ; Lleras-Pérez, 1997). They were made by the lost-wax method of casting; the model was cut from wax sheet less than l mm thick and applied 'wires' of wax provided the detail as well as giving the sheet strength and doubling as runners and risers for the casting (p hotographs l and 2). The metal surfaces are conspicuously rough and unfin- ished, presumably because of the vo- tive nan1re of these Photograph 2: Magnificd pieces. Ornaments detail of tunjo 6 showing made by the the modelling of the facc Muisca for wear, and cast stmcture.(width such as the neck- 1 cm). The black patches lace pendants in are residucs of thc photograph 3, are casting mould (photo S. also cast bu t they La Niece, British Museum(. have much smoother surface. 140 BounN Musm DEL ORo E.NERO-DICIEMBRf 199!1 SUSAN LA NIECE Severa] of these nmjos have black mould materi al still adhering to them and no. 8 in photograph 1 has the casting spm e, where the metal was poured in at the top of the mou ld, still attached to the foot (these fi gures we re cast head downward s). lt is not at all uncommon to find tunjos where the cast- ing prue and feeders for the mol ten metal have not been trimmed off (see Bray, 1982, nos. 59 and 60). Many also have majar casting faults, for exam- ple the hales in numbers 1 and 2 in photograph l. The problems of analysing nunbaga alloys are well known (Scott, 1995: 504); loss of copper from the surface occurs both by natural corrosion and, in the case of more finished objects, by deliberare treatment to improve the sur- lace colour. Furthermore, the metal microstmCUire of Uimbaga castings is frequently inhomogeneous (see paper by Meeks, this volume). Because of these problems, the choice of where to analyse the object is very importan t. The aim must be to analyse an area which is most Jikely to refl ect the original composition of the alloy at manufacture. The surface metal is least likely todo so, and may not even accurately refl ect the finished appearance of the newly manufacrured object alter it has suffered many years of corro- sion. Any method which analyses tl1e surface, and that includes most so- ca lled non-destmctive methods, can only produce semi-quantitative results at bes t. 'L•ble 1 gives the range of compositions obtained from anal ys is of different areas of two small Muisca nmjos (photographs 4 and S) to illus- trate the variations in cmnposition which can occur. TABLE 1 COMPARISON OF ANALYSES TAKEN AT DIFFERENT POJNTS %A u %Cu %Ag Muisca tunjo 7 (BM Sl323) photograph4 Sur fa ce 72 8 20 Abraded surface 73 8 20 Area at the centre of a cross-section 75 7 18 Muisca tunjo l (BM Sl326) photograph S Surface 56 36 Abraded surface 35 59 Area at the centre of a cross-section 28 67 The surface analyses were done by X-ray fluorescence analysis, the rest by energy dispersive X-ray analysis (in a scanning elecuon microscope). The precision is ± 1% . 8oLETIN M USEO DEL ÜRO No 44-4'>. ENERO·DlCIEMBRE 1998 141 METALLURGICAL CASE STUDIES FROM THE BRITISH MUSEUM'S Photograph 3: Muisca cast necklace pendants {length of largest, 1.2 7 cm) [phoro A.Milton,British Museum). Photograph 4: Detail of tunjo 7[height 4.8 cm) [photo S. La Niece, British Museumj. 142 BoLETlN M U'iEO DEl ÜRO No 44 ENUO·DIOf.MBRE 1998 Photograph 5: Detail of runjo 1 (height 5.5 cm). [photo S. La Niece, British Museumj. Photograph 6: SEM micrograph of a polished section through the brokcn tip of tunjo 7. Note the continuity of the cast structu.rc between the base pi a te of the tunjo, running horizontally across the bottom of the photograph, and the round sectioned "wire" on top of it. (BSE image. Width of section 1.2 mm) [photo S. La Niece, British Museum]. BoLETIN MUSEO Df.l ÜRO No 44·45, ENERO·DlClEMBRE 1998 143 METALLURGICAL CASE STUDIES FRDM THE BRITISH MUSEUM'S The first of these tunjos (no. 7) has a relatively high gold content and low copper content. In this case the compositional difference between the sur- lace analysis and analysis of metal at the centre of the casting is a few percent. The surface analysis ol the tumbaga (copper-rich gold alloy) nmjo (no. l ) is so different from the valu e lor the core metal that it gives a com- pletely false picture of the quantity of copper in the original casting alloy. To all ow comparison of like with like it is therefore important when publish- ing analytical results to state how the analysis was done and on what pan of the object and it has to be emphasised that surlace analysis is worse than useless, it is positively misleading. The problems outlined above are widely recognised and severa! sampling and analytical strategies have been evolved to produce meaningful analy- ses. Where the casting is thick enough, a successful method of sampling the core metal is to dril! a sample using a 1 mm drill bit, discarding the unrepresentative rurnings of the surlace metal. If drilled into d1e base or a broken edge of an object, the resulting damage is minimal. However, the Muisca castings are olten too thin to drill. The usual solution is to scrape away the surface ola small area lar analysis, but the rouglmess of the sur- lace ol the tunjos required a very deep area to be abraded in arder to remove all the surface inhomogeneiry. The solution adopted in this case for the tumbaga runjos was to take a small piece of metal and mount it in cross- section in epoxy resin (photograph 6) lor analysis by energy dispersive X-ray analys is (EDX) in a scanning electron microscope (SEM). TA.BLE 2 : ANALYS IS OF MUISCA METALWORK BM reg. no. %Au %Cu %Ag l.