Chemical Composition: EMP analysis from sample, Bronze:
Cu, 91.67; Sn, 6.82; Pb, 0.20; Zn, 0.00; Fe, 0.20; Ni, 0.24; Ag, 0.08; Sb, 0.29; As, 0.18
J. Wolfe, June 1998
Technical Observations: The disc fibulae are created out of one piece of metal that has been cast into a general form and continually shaped by cold working. None of the fragments show a complete spring and pin section; however, for 1987.135.22 and 1987.135.19.A-B the existing wire is round and smooth. The wire was most likely cast as part of the initial form and hammered to lengthen and coil a spring. For all of the fragments in the collection, the discs show a variation in sheet thickness. The back of the disc often displays ovoid-shaped hammer marks as seen in a detail of 1987.135.21. The hammering occurs behind the bands that run across the front side and was most likely done to give the disc a slightly concave shape.
The raised lines that decorate the front of the disc are wavy and imperfect. There are recesses on either side of each band with fine striations in the metal, indicating that the metal was chased by hand. The swastikas that occur on three disc fibulae were made using two different techniques. 1987.135.21 and 1987.135.24 have swastikas created by a tremolo line, where a crescent-shaped tool has been rocked back and forth over the metal surface. In slight contrast, 1987.135.19.A-B has a swastika created with a crescent-shaped tool to stamp lines in a regular array. The border of the disc on 1987.135.21 is also inscribed with a pointed tool to create a series of fine lines to make repeating triangles; in the detail images for that fibula, one can see the overall surface of the disc has fine striations running in the same direction as the bands, which is most likely from final sanding or polishing.
An x-radiograph of 1987.135.21 shows more radiodensity in the center of the disc, indicating that the metal is thicker there. The hammering marks can be seen closer to the edges where the metal is thinner. Overall, the metal appears homogeneous in the x-radiograph and does not reveal any unexpected fabrication techniques.
To gain more information about the metal-working techniques, in particular for the large disc section, a sample from one of the small disc fragments (1987.135.25) was taken for metallography. Under a reflected-light microscope, the as-polished section showed natural etching of the crystals by corrosion. The crystals are equiaxed, hexagonal grains showing signs by their distortion of having been worked. Twinning can be seen as double bands within the crystals, which indicates the repetition of cold working and annealing to hammer out the sheet of metal. The cross-section also shows gray, elongated inclusions that turn black under crossed polars. When analyzed with SEM-EDX, only sulfur, copper, and traces of iron were identified in these inclusions, indicating a sulfide impurity.
1987.135.21, 1987.135.22, and 1987.135.25 were tested for alloy composition using EMP analysis (1). Each sample was analyzed twice, first using a small beam to avoid inclusions, while the second test was done with a larger beam that incorporated more inclusions. The analysis identified the metal composition as a copper alloy with 6.5-9% tin. There are traces of arsenic, antimony, iron, sulfur, cobalt, nickel, lead, and silver. Most of the impurities increased in percent with the larger beam, as expected. This alloy composition has been found on other Italic fibulae from the Iron Age (2).
NOTES:
1. See the ancient bronze online resource or one of the three object records for details on the EMP analysis. The sample taken from 1987.135.25 for metallography was also used for the EMP analysis.
2. Compare with the technical observations for Harvard’s serpentine fibulae (for example, 1987.135.48.A-B) and quatrefoil fibulae (for example, 1987.135.9).
Julie Wolfe
