Eddy current testing methods were used to characterize model gilding layers on bronzes and to make
measurements on gilded bronze objects in the Freer Gallery of Art. Swept frequency eddy current
measurements were able to discern differences between gilding layers of varying thickness, and also
between gilding layers of varying composition, i.e. conductivity, on a series of reference samples.
Measurements made on the Freer Gallery objects were consistent with reference sample results for
mercury amalgam gilding.
The critical depth of penetration of the eddy currents was calculated for gold and bronze for each of
the four frequency ranges used. A simple model was developed to describe the influence of the
different types of gilding layers on the change in probe impedance resulting from the interaction of
the sample and probe. The model treats gilding layers of three different thickness ranges separately.
In the first case, the gilding layer thickness is less than the critical depths for gold or bronze, tAu layer <
tcr(Au). , tcr(bronze). In the second case, the gilding layer thickness is greater than the critical depth for
gold, yet less than the critical depth of the bronze, tcr(Au) < tAu layer < tcr(bronze), and in the third case, the
gilding layer thickness is greater than the critical depth for both gold and bronze, tcr(Au) , tcr(bronze) < tAu
layer.
Variations in the bronze substrate composition and microstructure influence the measurements. Thus
a differential measurement was used in this study for the gilded surfaces, normalizing the raw data by
a measurement on a bare bronze surface. Surface roughness was found to influence results at high
frequencies where the roughness is much less than the eddy current critical depth for the metal.
Experiments using nonconducting Mylar were undertaken to model the effect of surface and
subsurface corrosion, and lift-off. It was concluded that surface corrosion, and similarly lift-off, will
decrease the eddy current penetration depth into the sample. Subsurface corrosion does not affect
eddy current penetration into the gilding layer, but does decrease penetration into the bronze
substrate. In both cases, the relative effect of the substrate and gilding layer on the probe impedance
change due to the presence of the corrosion.
By making measurements at two frequency ranges, one at low frequency and one at high frequency, it
was possible to discriminate between samples gilded using several different gilding techniques,
including electrochemical methods, leaf gilding, mercury amalgam gilding, and foil gilding.
A second eddy current technique that uses a pulsed excitation source was briefly explored. The
technique was able to discriminate between samples with 24K gold leaf, 23.5K gold leaf and 24K foil
gilding.