Colouring metals in the Far East

By Agnese Benzonelli

How far can someone go in the name of research? In my case quite a long way. For a month, I loosely taped tiny plates of metal to my hands and woke up every morning with green stains on them. I was investigating craft recipes employed in the far East since the fourteenth Century to create coloured metal and alloys used for beautiful small artefacts. Whereas Western taste preferred the natural colour and the polished lustre of metals, the Eastern preference was to use the process of chemical patination, where different chemicals react with the surface of copper and copper alloys to form new coloured compounds.

Fig.1: Left: a tsuba (handguard of Japanese sword) made of shakudo and gold; BM-TS244. Right: a Chinese gu-tong box; BM-1992,1109.2. Photos made by A. Benzonelli.

The Japanese created a whole class of coloured alloys, called irogane. They added small amounts of different elements to the copper and simmered it in a solution containing copper salts, alum, vinegar and other ingredients, to form coloured patinas on the surface. The most beautiful patina, shakudo, was made with copper and gold and achieved a deep blue-black colour. Combining a variety of alloy compositions and solution ingredients, they were able to create hues ranging from pale brown to black. Inlaying different irogane, gold, silver, and lacquers, they were able to create small artefacts and swords fittings with complex and visually impressive designs and patterns.

Fig.2: Picture of the 13 Japanese solutions created and tested taken from Japanese recipes reported in Eastern books.

Later on, Chinese craftspeople imported the technique from Japan, possibly through travellers or trying to copy the Japanese artefacts, and, from the Ming period (1368-1644), they used it to colour ink or tobacco boxes. In contrast to the Japanese irogane, the Chinese would patinate a gold-silver alloy only, which resulted in a dark colour similar to that of shakudo, which they called gu-tong. In both cases, the precious metal content was 1-3%, a larger amount would not affect the final patina colour.

There’s not a lot known about the two different recipes the Chinese craftsmen employed to produce their patinas. However, we do know that one process they utilised was similar to that used by the Japanese to produce the irogane,butwe have no precise recipes for the solutions. Another involved handling the metal until the perspiration from their hands corroded the surface.

Creating and play with metal colours has always fascinated me. Why using gold and silver to achieve that dark colour, instead of cheaper alternatives? What was the role of the different ingredients used in the recipes? Could sweat really be used as a patinating solution? Why did Japanese recipes tell not add precious metals to bronze (copper alloyed with tin) but only to pure copper? Could it be confirmed through scientific analysis that the technology was really transmitted from the Japanese to the Chinese culture? To find answers, I needed to conduct a systematic experimental research which could explain the reasons behind the technological choices of Japanese and Chinese craftsmen.

 I reproduced a series of alloys with tin, gold and silver, which I then patinated, following the recipes for shakudo and gu-tong from available texts. These are not numerous, as the recipes were mainly kept as secret and passed from father to son. We have city records, information sheets given by dealers and, in Japan, technical texts on metal working. I analysed the resulting patinas with techniques borrowed from modern material science, and used the experimental data as reference for the interpretation of historical patinated artefacts in museum collections.

I found that, for both the Japanese and Chinese recipes, the presence of gold in the alloy did, in fact, change the colour of the final patinas, which resulted in a better, more uniform and darker hue. I also noted how parameters such as the fine grade of surface polishing prior to the patination, the use of tin-free alloys, the selection of copper acetate as a main ingredient and the addition of vinegar are all processes and actions that led to a reduced patina growth. And as it turned out, sweat was indeed an effective patinating solution in the Chinese patination (as it can be noticed by the green corrosion products left on the hands), but the presence of gold in the alloy composition is nonetheless essential to obtain a dark hue, not achieved in gold-free alloys. Sweat is a slightly acidic solution, similar to that of used by Japanese craftsmen, essential to create a coloured copper oxide on the surface in just 48 hours of handling. Similarly, in both technologies cleaning of the alloy before or after the procedure suggested in Japanese texts and the abrasive action of the hands in the Chinese method are both similar and crucial steps in the patination process. I concluded from these findings that a dark patina was not the only feature that the craftsmen of both cultures had been striving for. They also seemed to be after a shiny appearance, given by the metallic lustre coming from the alloy.

Fig. 3: Alloys of different compositions patinated with Chinese patination.

In combining experimental and historical observations I gained an understanding of the intentions behind the technological choices made by the Japanese and Chinese artisans. My analysis confirmed the deliberate nature of these choices made to achieve dark and shiny blue-black colours otherwise impossible. This confirmed the importance of colour as a driver of these artisanal processes, and supports the idea that the knowledge was transmitted between the two cultures.

Was it worth in the end – the green hands and other discomforts suffered in the name of research? It was the first study to provide a scientific basis to our knowledge of the methods and choices made by those people, the ingredients they employed in making dark patinated artefacts, and contributed to our pool of knowledge of the artisanal methods and practices described in historical sources. So yes, it definitely was, and I would definitively do it again.