This paper focuses on Byakudan-nuri, one of the traditional decoration techniques of Japanese Urushi (Urushi natural lacquer) crafts. Works produced by Byakudan-nuri express a unique luster color owing to painting gold or silver leaves along with highly transparent Urushi. The specific problem of Byakudan-nuri is the exfoliation of Urushi off the metal leaf surface, to solve a lot of repair work which is done nowadays. For Byakudan-nuri, silver leaf is preferred to a gold leaf. This paper tries to clarify the influence of the different kinds of metal leaves on the appearance and peel strength of works of Byakudan-nuri and, moreover, to elucidate the reason why silver leaf has been used more frequently for Byakudan-nuri until now. It was found out that the reason for the more frequent use of silver leaf in Byakudan-nuri is that silver does not seem to affect the appearance and peel strength in works produced by this technique.
The gold leaf has a long history and has been already made and used for fashion and decorative objects in Egypt in about 1200 B.C. [
In Japan too, the technique of gold tooling has been used for a long time, as shown in “Urushi kinpakue no ban” as part of the Shoso-in treasures [
However, whereas there is abundant research material on the material properties of gold leaf itself, the same cannot be said about the state of gold leaf after processing of crafts [
In this study, Byakudan-nuri, which is one of the traditional decoration techniques of Japanese Urushi craft, was studied using the results of our past research. Byakudan-nuri work expresses a unique luster color because of painting a gold or silver leaf with highly transparent Urushi. Examples of its use can be found in the works of art produced during the Heian period (794 - 1185 A.D.). It is thought that the technique was affected by Chinese lacquer-ware [
Therefore, we aim to clarify the following two subjects in this paper:
The influence of the different kinds of metal leaves on the appearance and peel strength of Byakudannuri.
The reason why silver leaf is used much more frequently in Byakudan-nuri.
Gold leaves are classified according to their compositional ratio of gold, silver, and copper. Six kinds of Byakudan-nuri specimens were used in this experiment, as shown in
The gold leaf is also classified according to the two kinds of manufacturing methods. One is the traditional method named “Entsuke”, and the other is the modern method called “Tachikiri”. According to Ohgaku’s research that compared Entsuke and Tachikiri, the surface
pattern and thickness of the leaf were somewhat different [
The manufacturing method of silver is the same as that of a gold leaf. However, the thickness of silver is a bit larger than that of a gold leaf. The thickness of silver leaf is about 0.2 μm whereas the thickness of gold leaf is about 0.1 μm. The structural scheme of Byakudan-nuri is shown in
A spectral colorimeter (SQ-2000, NIPPON DENSHOKU INDUSTRIES CO., LTD.) was used for colorimetry in order to measure L*, a*, b*, and C* values. The L* a* b*, or CIELab, color space is an international standard for color measurements, adopted by the Commission Internationale d’Eclairage (CIE) in 1976. L* is the luminance or lightness component, which ranges from 0 to 100, and parameters a* (from green to red) and b* (from blue to yellow) are the two chromatic components, which range from 120 to 120. C* value which is calculated from a* and b* is corresponding to a chroma. A glossimeter (IG-410, HORIBA, Ltd.) was used to measure the glossiness in the measurement range of 0 - 1000.
SAICAS (surface and interfacial cutting analysis system, DN, DAYPLA WINTES CO., LTD.) was used to measure the peel strength. The measurements were performed in a constant speed mode (vertical velocity 5 µm/s, hori-
zontal velocity 0.05 µm/s). A digital microscope (VHX- 900, KEYENCE Japan) was used for the fractographic study of the SAICAS experiments.
It should be noted that Byakudan-nuri uses two adhesion layers with the metal leaf between them, as shown in
The yellow area is gold leaf and the black area is Nakanuri Urushi in
The L*, a*, b*, and C* values of all specimens are shown in
Likewise, the glossiness of the specimens is also not influenced by the variations in the compositional ratios of gold or slight differences in the number and composition of various gold and silver leaves, as evidenced by
Thus, the present results indicate that the appearance of a Byakudan-nuri work is not influenced by differences in the metal leaf, even if a silver leaf is used instead a gold leaf.
The scheme of the peeling test by SAICAS is shown in
Two exfoliation types were observed between SukeUrushi and metal leaves, as shown in Figures 7 and 8. In the case of exfoliation of type (A), exfoliation took place only on the part which was hit by the SAICAS blade. In contrast, exfoliation of type (B) propagated from the blade.
Load–depth curves of exfoliation of type (A) of all investigated samples are shown in
The results are restricted to exfoliation of type (A) because it is impossible to quantitatively compare the peeling strength between non-propagative and propagative exfoliation. Based on the results presented in
The cost of silver leaf is much lower (a fifth) than that of gold leaf. Therefore, it is obvious that a silver leaf is preferred and used much more frequently for Byakudannuri considering its moderate cost and the fact that there was no difference in the appearance and peel strength.
This paper tries to clarify the influence of different kinds
of metal leaves on the appearance and peel strength of Byakudan-nuri works and to find out the reason why silver leaf is preferred to a gold leaf for Byakudan-nuri.
It is shown that differences in the kind of metal leaf do not affect the appearance of Byakudan-nuri because no significant differences in the color and glossiness of Byakudan-nuri could be observed, irrespective of the type of leaf used (gold or silver). In contrast, the color and glossiness of Byakudan-nuri was affected by the Urushi layer. Similarly, the compositional ratio of gold and the kind of metal leaf hardly affected the peel strength. Therefore, it is concluded that the reason why silver leaf is used much more frequently in Byakudannuri is that the appearance and peel strength are not affected.
The results of this paper show new findings regarding the relationship between metal leaf and Urushi. Also, the weakness of the peel strength between metal leaf and Urushi is substantiated. The weakness of the peel strength does not reveal the flimsiness of Byakudan-nuri directly despite its use for decorating objects of daily use, such as Japanese soup bowls and chopsticks. However, sustained study is required to clarify the peeling mechanism between the metal leaf and Urushi. The results may be applied for the repair of cultural assets in the future because exfoliation between the metal leaf and the Urushi layer is the main reason for damage of Byakudan-nuri works.
The authors wish to thank sincerely Prof. H. Hamada of Kyoto Institute of Technology for his cooperation and K. Maeda of Kyoto Institute of Technology for his kind help with the experiments. The authors express their appreciation to the members of Shimode Maki-e studio for their kind supply of samples for our study.