Solder joints and coatings found in bronze artifacts recovered from the Nubian Desert in Egypt show evidence of the decomposition of the lead-tin eutectic structure and solid-state growth of the ε and η intermetallic phases at the solder-bronze interface at ambient temperature. Accelerated aging experiments reproduced the structures observed in the artifacts. The data show that the growth of the intermetallic compounds is diffusion-controlled at low as well as high temperature with an activation energy of 20 kcal/mol.
SOLDER IN THE BRONZE ARTIFACTSThe excavators determined the ages of the artifacts from their knowledge of the burial characteristics and the associated grave goods. Since the only bronze smelting known to have been carried out in Lower Nubia was about 2500 B.C.,9 long before the Meroitic Period, the bronze artifacts are identified as trade goods manufactured within the Roman Empire and traded into Lower Nubia.10 It is possible that Nubian artisans undertook some repairs or modifications before the artifacts were finally placed in graves.INTRODUCTIONThe microstructure of the solder taken from the bottom of bowl B (Figure 7) has a nearly continuous lead phase containing regions of tin phase, and a layer of intermetallic compound that was in contact with the bronze bowl surface. The x-ray maps define the layer of intermetallic compound, showing that the η phase is 6 µm thick and that particles of η are dispersed in the solder. There is no trace of eutectic structure.Metallographic samples were prepared by standard methods. For optical microscopy the lead-tin alloys were etched with a glycerin-acetic acid-nitric acid (84:8:8, respectively) mixture when needed. The specimens were examined in the as-polished condition for microprobe analysis. A JEOL Superprobe 733 operating at 15 kV with a beam current of 20 nA to 50 nA, depending on the element analyzed, was used with wavelength-dispersive spectrometry (WDS) to determine alloy compositions. Energy-dispersive spectrometry (EDS) was used to analyze the coating on bowl PM 262,230. Overall compositions, reported as weight percent in Table II. were determined from the compositions of the individual phases present and the volume fraction of each constituent as found by image analysis.The ladle, D (Figure 3), was made from one piece of 2 mm thick bronze sheet by a complex metal-forming operation that shaped a bowl at the end of a handle that terminates in the figure of a serpent's head. A sample was taken from the solder present in the bottom of the bowl that appears to have been used in an attempt at a repair.The artifacts were recovered by the Pennsylvania-Yale Expedition to Egypt at the sites of two communities in Lower Nubia, Arminna West, and Toshka West, located on the west bank of the Nile between the major archaeological sites of Abu Simbel to the south and Karanog to the north.6,7 The cemeteries at these sites contained more than 250 grave complexes that held iron and bronze artifacts from the Meroitic (300 B.C to A.D. 370) and X-Group (A.D. 370-550) periods.8Two thin-walled bronze bowls (B and C in Table I), have remains of tinning on both interior and exterior surfaces, and bowl B (Figure 2) has an accumulation of solder in the bottom, perhaps from an attempt to repair damage. A sample of this accumulated material was extracted for analysis. The lining material was examined on cross sections taken from the bowl rims.The microstructure of the solder retrieved from ladle D (Figure 8) consists of a continuous lead solid solution (dark in the micrograph), islands of tin solid solution dispersed in the lead constituent, and a few particles of η intermetallic compound. No layer of intermetallic compound detached from the substrate was present in this sample, and there is no trace of eutectic structure.The microstructures of the solders in the artifacts differ from that of a freshly made solder joint in the absence of eutectic structure, the greater thickness of the layer of ε and η intermetallic compounds, the presence of η-phase particles within the solder, and the absence of dendritic form in the primary lead constituent. The composition of the lead and tin phases (Table II) correspond to the equilibrium compositions of the alloy phases at about 50°C. Since in a freshly made solder joint the lead phase contains 3.3% tin in solution and the tin phase, 1.6% lead, it is evident that composition changes in both constituents have occurred in the 1,600-year interval since soldering was done. It is also evident that whatever eutectic structure was present in the solder at the time the joints were made has vanished in the intervening 1,600 years. Recent research has shown that the eutectic structure formed upon cooling; lead-tin solder is not completely stable at room temperature. Coarsening of the eutectic structure can be observed within five years at room temperature, and can be accelerated by creep deformation or stresses induced by temperature changes.15,16 Growth of copper-tin intermetallic compounds at the interface between solder and a copper substrate at room temperature has also been observed.17Microstructures ObservedSoldering with lead-tin alloys was a technique well known and widely used by metalsmiths making bronze and silver products in the ancient world. These artisans also used tinning to decorate bronze and iron objects. In addition, they used it to cover the insides of bronze vessels used as tableware or in the kitchen to protect users from the bad taste that results from direct contact between bronze and food.1-5 Aging of tin-based solders is of particular interest today due to concern about the long-term reliability of soldered connections in electronic devices. This article reports on evidence of microstructural changes in the solder used in bronze artifacts retrieved from the Nubian Desert in Egypt. The solder has aged at ambient desert temperatures for at least 1,600 years.Examples of solder and of tinning were found on a mirror, a ladle, and two bronze bowls in the course of laboratory examination. These artifacts are in the collections of the Peabody Museum at Yale University, and are identified by their numbers in the museum catalog (Table I). The bronze mirror (Figure 1) is from the X-Group Period and is similar to the high-tin bronze mirrors widely used in Etruscan and Roman times, and also in India and China.11-13 A small piece of the solder used to attach the handle was taken for analysis without damaging the artifact.The solder from the mirror (Figure 6) consists of continuous lead solid solution containing regions of tin solid solution, and a layer of intermetallic compound originally in contact with the mirror surface. Dispersed particles of the intermetallic compound η phase are present within the tin phase. Compositions of the constituents are shown in Table II. There is no trace of a eutectic structure in the solder. The composition of the dark constituent in the intermetallic layer is 60% copper and 40% tin, identifying it as the e phase, and the lighter gray constituent has 38% copper and 62% tin, corresponding to the η phase. The average thickness of the e phase is 4 µm, and of the η phase, 12 µm. Since the solder separated from the bronze substrate by fracture through the e phase, the total thickness of the intermetallic compound layer may have been greater than 16 µm.
The microstructures of the solders in the artifacts differ from that of a freshly made solder joint in the absence of eutectic structure, the greater thickness of the layer of ε and η intermetallic compounds, the presence of η-phase particles within the solder, and the absence of dendritic form in the primary lead constituent. The composition of the lead and tin phases (Table II) correspond to the equilibrium compositions of the alloy phases at about 50°C. Since in a freshly made solder joint the lead phase contains 3.3% tin in solution and the tin phase, 1.6% lead, it is evident that composition changes in both constituents have occurred in the 1,600-year interval since soldering was done. It is also evident that whatever eutectic structure was present in the solder at the time the joints were made has vanished in the intervening 1,600 years. Recent research has shown that the eutectic structure formed upon cooling; lead-tin solder is not completely stable at room temperature. Coarsening of the eutectic structure can be observed within five years at room temperature, and can be accelerated by creep deformation or stresses induced by temperature changes.15,16 Growth of copper-tin intermetallic compounds at the interface between solder and a copper substrate at room temperature has also been observed.17
