DISTINGUISHING ANCIENT LEADED TIN BRONZE AND MODERN ONES ACCORDING TO THE COLMPOSITION OF THEIRS LEAD INCLUSIONS

S. Shilstein (Weizmann Institute of Science), A. Berner (Technion), Y. Feldman (Weizmann Institute of Science), S.  (Haifa University), Yu .Rosenberg (Tel Aviv University) 

Abstract 

Composition of lead inclusions in modern and ancient leaded tin bronzes dated from 4^th century B.C. up to 14^th century A.D. was studied by X-ray diffraction (XRD) and fluorescence (XRF). 

Lead lattice parameter measured by XRD gives quantitative information about composition of lead inclusions provided that lead and tin form substitutional solid solution.

It was found in particular, that lead inclusions in samples of modern bronze might contain as far as 3 and more atomic % of tin.  This value agrees well with max tin concentration of 2 % derived from generally accepted Pb-Sn phase diagram by extrapolation of high temperature data.  

It was revealed at the same time that in lead inclusions in samples of ancient bronze the tin content is about 1%. It means that the generally accepted Pb-Sn phase diagram does not correspond to truly equilibrium state.  As a result of Sn diffusion out of lead inclusions, the Pb-Sn solid solution gets closer to the equilibrium state in times on the order of centuries. 

Thus, distinguishing of ancient tin leaded bronzes from modern bronzes according to the tin concentration in their lead inclusions is indeed possible. It seems that the described phenomenon can be used also to distinguish real ancient artifacts from modern counterfeits. 

Bronze is an alloy consisting primarily of copper, usually with tin and rather often with lead and zinc admixture.  Bronze is known to mankind since antiquity and might be considered as one of the most studied materials.  The basis of leaded tin bronze is a solid solution of about 10 at % tin (Sn) or also zinc (Zn) in copper (Cu) while lead (Pb) which is immiscible with copper in the solid state is distributed over copper-tin matrix as micron size inclusions.

The positions of diffraction lines from lead inclusions are well measurable by lead content about several weight percent typical for modern bronzes see Figure 1

Fig.1.

XRD patterns for pure lead and for modern bronzes 1 and 2 (data about studied
bronzes are presented in Table).  

The difference of positions between lines from lead metal and lead inclusions in bronzes is some direct indication that inclusions are indeed solid solutions Pb – Sn (Cu and Zn are immiscible with Pb in the solid state).  It means that these inclusions are similar to Pb-based phase in solders, which are also studied in this research for comparison, where the existence of Pb-Sn solid solution is unquestionable.  From measured lattice parameter for inclusions we established for the first time the genuine composition of lead inclusion in bronzes. 

It was found in particular, that lead inclusions in samples of modern bronze might contain as far as 3 and more atomic % of tin (Figure 2).  This value agrees well with max tin concentration of 2% derived from generally accepted Pb-Sn phase diagram by extrapolation of high temperature data.  

One can also see that in solders the lattice parameters of Pb – Sn solid solution correspond to 3.0 ÷ 3.5% at. Sn.  That is to say that the smallest Sn concentration in the lead inclusions in bronze reaches the same value as Sn concentration in Pb-based phase in Sn - Pb eutectic in solders.   

Fig. 2. Lattice parameter values (Ångstrom) for lead standards, for lead rich phase in solders and for lead inclusions in modern bronze samples.

 To perform SEM EDS study on lead inclusions in bronzes 1 and 2 an additional mechanical polishing proved necessary to visualize lead inclusions.  After this standard surface metallographic preparation lead inclusions became well visible, but contrary to possible expectations detected XRF spectrum was completely free from Sn lines. Repeating XRD measurements on these samples the lattice parameter of pure lead for lead inclusions are received (tin is extracted from solid solution by such polishing). Thus we have to conclude that direct determination of composition of individual inclusions by this way by means of SEM EDS is impossible.

Figure 3. Measured lattice parameters for historical and modern bronzes.

 It was revealed at the same time that in lead inclusions in samples of ancient bronze the tin content hardly exceeds 1%. It means that the generally accepted Pb-Sn phase diagram does not correspond to truly equilibrium state.  As a result of Sn diffusion out of lead inclusions, the Pb-Sn solid solution gets closer to the equilibrium state in times on the order of centuries. 

 Thus, distinguishing of ancient tin leaded bronzes from modern bronzes according to the tin concentration in their lead inclusions is indeed possible. It seems that the described phenomenon can be used also to distinguish real ancient artifacts from modern counterfeits. 

 Table. XRD data about measured lattice parameters values of the lead metal standards, Pb-Sn solutions in lead inclusions in the tin-lead bronzes and in the lead reach phase of the solders, together with calculated from lattice parameters tin concentrations in these solutions.