CLAMTEC tool

Ecorr measurements used by corrosion scientists only provide trends in electrochemical behaviour (passivation and corrosion, (see section Electrochemistry / Fundamentals). For the CLAMTEC project, the objective of the Conservation Research Unit at the University of Applied Sciences Arc (UR-Arc CR) was to determine if monitoring of Ecorr versus time could be used to design a portable and non-invasive analytical tool which is easy-to-use. This spot test is based on the comparison of Ecorr versus time plots of an unknown material taken using three electrolytes (KNO3 1% by weight, Henniez mineral water and sodium sesquicarbonate 1% by weight) with materials from a database (Degrigny et al. 2010). The measurement protocol has been standardized to limit error introduced by the operator. One important step, for instance, is the polishing of the metal surface between each measurement. A fairly complete database has been established for copper based alloys. The recent development of software[1] to measure Ecorr versus time and automatically compare the plots (on the basis of the sum of the three distances between the plots of the unknown metal and those of the materials in the database) facilitated the collection of data and its interpretation.

The CLAMTEC tool has been used to determine the composition of a selection of copper based artefacts from the International Museum of Horology in La Chaux-de-Fonds. One application is given below with a bell on a lantern clock dating from the 18th century, which was identified as a quaternary bronze CuSn11Zn1.7Pb2.7NiSi. XRF analysis of the same object confirmed that the metal is a quaternary bronze while the determined composition differed slightly CuSn5Zn8.3Pb2.1S1.8FeNiSb. The CLAMTEC tool is able to recognize the nature of the material. The information obtained with this low-cost analytical tool is sufficient for the conservator to propose conservation strategies.

Lantern clock, 18th century. Using the CLAMTEC tool to analyse the metal of the lower bell.

Comparison of the plots of an unknown metal (solid lines) to those of the materials in the CLAMTEC database (dotted lines). The best matching is obtained by the results on the left. This result enables the operator to suggest that the unknown material has the following composition: CuSn11Zn1.7Pb2.7NiSi.

The application of the CLAMTEC tool to silver-copper alloys has been more challenging. The surface, which is commonly enriched with silver, creates errors in the analysis.

Other measurements have been carried out on tinned copper objects. The results are promising since tin has a more negative potential than copper and its alloys. Therefore the CLAMTEC tool could be used to identify the presence of surface treatments (tinning) on tin bronzes.

In the future we would like to test the CLAMTEC tool on aluminium alloys which are not easy to analyse by XRF.

Text of Christian Degrigny (christian.degrigny@he-arc.ch)

Reference:

Degrigny, C., Guibert, G., Ramseyer, S., Rapp, G., Tarchini, A., Qualitative analysis of historic copper alloy objects by measuring corrosion potential versus time, in proceedings of the ICOM-CC Metal WG interim meeting, METAL 2010, Mardikian, P., Chemello, C., Watters, C. and Hull, P. (eds), Clemson University, Charleston (USA), (2010), 335-341.