When all else fails, lead-210 testing may provide the only concrete evidence that a metal object is not an antique. If the object contains this form of lead it cannot be older than, roughly, 100 years.
Classification of some metal objects, as modern or old, cannot be clearly established based on composition and metallography. These objects require a test for lead-210 (Pb-210, 210Pb), a radioactive isotope of lead. This substance has a half-life of roughly 22.3 years. If Pb-210 is detected in a sample of metal, the material cannot be older than roughly 100 years, maximum.1 The absence of Pb-210 does not necessarily indicate an age older than 100 years, as the metal might never have contained the substance. However, even high-caliber forgeries of Chinese bronzes have often been found to contain this isotope, and lead-210 analysis has successfully removed many exceptional forgeries from the market. In our experience at TK, the objects that have tested positive for Pb-210 were already suspect, based on metallography and/or composition.
Test Facility and Sampling Procedure
Numerous laboratories around the world perform lead-210 analysis on organic samples. However, only one facility is currently known to regularly test metals. The sample size is dependent upon the material's lead content. The higher the percentage of lead, the smaller the sample needed. At TK, the shavings from cutting or drilling of the metal are sealed in a container and sent to Prof. Dr. Ernst Pernicka, of the Technical University of Freiberg (Freiberg, Germany) for analysis. Documentation includes photographs of the overall object and the sample site, object description and purported age.
Sample Analysis
Figure 7·1 Chart of alpha spectrum from Pb-210 test
At left is a typical chart from a lead-210 analysis with positive results, showing two outlined peaks.2 The right one is indicative of the presence of the 210Pb isotope. Prof. Dr. Pernicka also provides a written report that explains the results. In this case, a sample of metal filings, from a purported Warring States tui, was found to contain significant quantities of Pb-210, indicating that it was actually a recent forgery. This same object had been examined by several specialists, all of whom were suspicious but were unable to find conclusive evidence of forgery. The lead-210 test provided the necessary proof and a suite of seven bronze fakes was subsequently prevented from entering the antiquities market.
Conclusion
Though more commonly performed on organic materials, lead-210 testing may be used on samples of metal that contain lead. If a metal sample is found to contain the lead-210 isotope it cannot be more than, roughly, 100 years old. This analytical technique is not used to authenticate antiquities but to detect modern reproductions.
Notes
1 Lead-210 may exist in detectable quantities for roughly 150 years in organic materials.
2 Analysis report 03-07 of a Chinese bronze tui, by Prof. Dr. Pernicka. Technische Universität Bergakademie Freiberg. Chart of alpha spectrum. Item 080502-WS-B-TUI, lab no. FG-030607.
References
Flett, Robert. [date unknown]. Flett research home page [Internet]. Winnipeg (Manitoba): Homepage for Flett Research; [updated 2003 Oct 8; cited 2003 Nov 25]. Understanding the Pb-210 method. Available from: http://www.flettresearch.ca/Webdoc4.htm
[USGS]. [date unknown]. Western lake catchment systems - Bear Lake region. [Internet]. Washington (DC): U.S. Dept of the Interior, USGS; [updated 2003 Jun 12; cited 2003 Nov 25]. Methods used at Bear Lake: Dating: 210Pb (lead 210) dating. Available from: http://climchange.cr.usgs.gov/info/lacs/lead.htm The Bear Lake Homepage is available at: http://climchange.cr.usgs.gov/info/lacs/index.html
