Ancient Japan is roughly divided into 5 periods: Jyomon(~3 C.B.C.), Yayoi (3 C.B.C. – 3 C.A.D.), Kofun(3 C.A.D. – 7 C.A.D.), Asuka-Fujiwara (7 C.A.D. – 709), and Nara (710 – 784). Metal artifacts appeared in middle Yayoi period (about 1 C.A.D.) under the influence of China and Korea. Bronze and iron have been actually excavated in archaeological sites of Yayoi period. Gold and silver were rare even in Kofun period; but, to get gold-like image, gilding was very popular. Most of ancient metal objects with gold surface, which were excavated in Japan, were fabricated from copper or copper alloys. Their surfaces were treated by gilding, especially by fire gilding with gold amalgam for obtaining brilliant appearance.
Scanning electron microscope (SEM) observation was done to the sample surfaces made experimentally applying traditional gilding method. On the basis of these results, the surface of gilded metal artifacts that were excavated from archaeological sites were characterized. It was revealed that final polishing process is important for obtaining gold brilliance. The precipitate particles of gold amalgam, which are the most prominent feature of fire gilding, remained on the surface even after the final polishing. The trace of polishing was also recognized on the surface. It was found out that the ancient technique of fire gilding can be detected by these remarks on the surface.
As the excavated gilded copper objects are usually covered with patina on the surface, they are greenish or bluish. Patina hides not only gold appearance but also the original surface condition; for example, the polishing process and the trace of the chiseled images.
The mechanism of patina formation on the surface of gilded copper objects was detected with electron probe microanalyser (EPMA) and scanning Auger microscope (SAM). The observation with them tells that the gilding layer, about 10 μm in thickness, may originally have the paths along which a copper element moved from the bottom to the topmost surface. The Auger depth profiles of gold, mercury, copper and oxygen in the patina were measured. It is presumed that cuprite formed on the topmost gilding layer was a forerunner of the patina. The removal method of patina with high water absorbent resin, which was developed in our Institute, makes the surface of the gilding layer appear gradually. The original surface reveals us the feature of the gilding and the ancient technique of chiseling.
The ancient gilded metal objects excavated in Japan may include the objects made in China, Korea and japan. Only from an analytical point of view, it is difficult to say there is the big difference among areas.