Innovations in eddy current analysis of metals for Heritage Preservation

Curtis Desselles, Mary Striegel, and Jason Church

Abstract

In an era of economic uncertainty research laboratories can rarely afford the latest and best instrumentation. Older technologies can be resurrected and improved upon to meet their needs. The use of “eddy currents”—induced currents in metals produced by a coil—has been a valuable tool in the study of defects in metals for over 50 years. In 1999, the National Center for Preservation Technology and Training (NCPTT) funded a study of gilded objects at the Freer Gallery of Art. The study evaluated the use of eddy currents analysis as a means of non-destructive testing of gilded museum objects.

The Freer study used off-the-shelf probes and instruments, created reference test samples, and tested actual museum objects. Blythe McCarthy, project leader, stated “We are excited by the results we have found so far and plan to continue research in several areas”. Building on these studies, NCPTT set out to evaluate additional uses for eddy currents within historic preservation. This paper will illustrate that a sensitive handheld metal tester can be constructed for less than $200.00 with commercially comparable features. Possible applications include recovery of marks and serial numbers on museum objects and archaeological artifacts, identification of defects in metals, measurement of surface thickness of gilding and plating, and investigation of corrosion properties.

In the summer of 2008, a handheld metal tester was constructed to investigate the feasibility of producing an economical yet sensitive instrument that could measure eddy currents in ferrous and non-ferrous metals. The instrument circuit is based on five functional units. The input signal is a one megahertz square wave. The tank circuit—a coil and a capacitor—resonates at a frequency of one megahertz. A voltage divider is used to filter and lower the voltage to an acceptable level. The resulting voltage (0-5 volts) is converted by an analog to digital converter circuit (ADC). The digital output of the ADC circuit is displayed graphically and referenced to a database of known samples.

Test results indicated that metals with defects and marks could easily be detected. The recovery of serial numbers and hallmarks on corroded museum objects was successful in the field and laboratory. The measurement of gilding and plating was equal to that of commercial eddy current instrumentation. The handheld metal tester holds much promise and it is economically valuable for research laboratories on a tight budget. This paper should challenge the scientific community to reuse old technology and increase basic research and development.