Comparison of gold leaf thickness in Namban folding screens using X-ray fluorescence
In this work, the thickness of the gold leaf applied in six Japanese folding screens is compared using a nondestructive approach. Four screens belonging to the Momoyama period (~1573–1603) and two screens belonging to the early Edo period (~1603–1868) were analyzed in situ using energy dispersive X-ray fluorescence, and the thickness of the applied gold leaf was evaluated using a methodology based on the attenuation of the different characteristic lines of gold in the gold leaf layer. Considering that the leaf may well not be made of pure gold, we established that, for the purpose of comparing the intensity ratios of the Au lines, layers made with gold leaf of high grade can be considered identical. The gold leaf applied in one of the screens from the Edo period was found to be thinner than the gold leaf applied in the other ones. This is consistent with the development of the beating technology to obtain ever more thin gold leafs.
Gold was one of the first metals manipulated by men because of its remarkable malleability, which makes it easily beaten to form the desirable shape . Its noble metal stability and resistance to tarnish gives durability to the objects on which it is applied to. Its suitability to adorn small or larger areas renders it an excellent material and the finest among metals for use in the decoration of artworks. Two different terms have been employed to define metal sheets of varying thickness: foil and leaf. The former is generally used to describe a sufficiently thick sheet that can support its own weight while the latter, which mainly applies to gold, refers to a very thin sheet that has almost no weight and that can only be handled by specially designed tools.
With modern technology, the gold leaf can be obtained as thin as 0.1 μm ; however, the thickness of the leaf used in ancient artworks could be as thick as the 5–10 μm applied in an Egyptian 50 BC ceramic masque, or as thin as 0.2–0.5 μm used in the gilding of medieval Islamic glazed ceramics . According to Koyano , the Japanese gold leaf is said to be the thinnest in the world.
In Japanese traditional painting, decorating with gold leaf is named Kin-haku, and the finest examples of this craft are the Namban folding screens, or byobu, in paper support, belonging to the late Momoyama (~1573–1603) and early Edo (~1603–1868) periods. Traditionally, a grid was marked slightly across the paper as a guide for laying the hand-beaten gold leaves (~10 cm × 10 cm) in horizontal rows, starting from the top left-hand corner .
As demonstrated by Cesareo , the different lines of an element are attenuated differently as the thickness of the layer increases so the intensity ratio of Lα/Lβ (and Mα/Lα) varies until we can consider the sample infinitely thick and it remains constant.
Similar equations can be written for the Au (Lα/Lβ) intensity ratio.
Equation 1 implies the use of monochromatic incident radiation and curves in Figs. 2 and 3 were plotted considering a 15 keV incident radiation. However, the measurements on the screens were taken using the bremsstrahlung from the X-ray tube, so we cannot use these equations to effectively determine the thickness in our case studies. Nevertheless, using this methodology, we can assess the comparative thicknesses of the gold leafs applied in artworks, as will be demonstrated for the Namban folding screens.
3 Artworks description
4 Experimental setup
The EDXRF equipment used consists on a X-ray generator ECLIPSE II from Amptek (30 kV and 100 µA) with a Ag anode and an Amptek XR-100CR Si-PIN thermoelectrically cooled detector with a 7 mm2 detection area and 300 µm thick, and a 25 µm Be window. The energy resolution is 190 eV at 5.9 keV, and the acquisition system is Amptek PMCA . For collimating the beam, an acrylic support with a 2 mm pinhole in Ta was used and a spot size of 0.5 cm on the sample is obtained. The components are placed in an aluminum structure in 90° geometry and mounted on a tripod with 1.5 m vertical amplitude. Spectrum deconvolution and evaluation was performed using PyMCA software package .
X-ray fluorescence analysis was executed in situ at the museums, and an average of 10 measurements was taken in the gilded area of each screen.
As could be expected, no significant differences were found in the Au (Mα/Lα) and Au (Lα/Lβ) intensity ratios for screens belonging to the same pair (MNAA-Naizen 1 and 2 and MNAA-Domi 1 and 2) as they were most likely executed with the same materials (Figs. 6, 7). Also when comparing these two pairs of screens, signed by different artisans, no significant differences were found in the intensity ratios, hence in the thickness of the leaf. Regarding the screens attributed to the early Edo Period, the obtained distributions for Au (Lα/Lβ) intensity ratios are very similar to the ones obtained for the aforementioned screens, which could mean that the thickness is too small for this different attenuation factors to be taken into account. However, the comparison of the Au (Mα/Lα) intensity ratios rendered more interesting results: while the intensity ratios obtained for the Private-boat screen are in the same range as the MNAA-Naizen and MNAA-Domi, the mean value and range distribution for the intensity ratios of the MO screen are significantly higher, meaning that the Au-Mα line is less attenuated and therefore the layer is thinner.
In this work, we demonstrated that the thickness of the gold leaf used for gilding different artworks can be assessed and compared, in a nondestructive way, by means of X-ray fluorescence spectroscopy. We also established that, for the purpose of comparing the intensity ratios of the Au lines, layers made with gold leaf of high grade (pure Au to 95 % Au 5 % Ag) can be considered identical. The gold leaf applied to six Namban screens was compared, and the leaf applied to the screen kept at Museu Oriente was found to be the thinnest. Actually, a thickness of 100 nm order of magnitude was obtained with a quasi-transversal view with SEM image of a sample collected from this screen . Considering that the thickness of the gold leaf tends to decrease significantly with the advent of gold beating technological development , these results are indicative that this specimen from Museu Oriente is the most recent. This simple comparison allowed establishing a timeline between the manufacture of two pieces attributed to the same period—from 1603 to 1868, proving to be an important asset in the dating of artworks.
Authors would like to thank directors and staff at Museu Nacional de Arte Antiga and Museu Oriente for allowing the study, as well as Salvarte atelier for accommodating us and the screen belonging to the private collector.
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