ko-kinko ?

[IanB]

All,

It has always been my belief that the alloys used by the Japanese patinated of their own accord given time. The fact that they had to be patinated initially by some chemical treatment was simply to obviate the need for the maker to leave it kicking around for years before he could sell it. I also understood that handling these alloys actually speeded up the natural patination process, or perhaps it would be better to say repair it, because of the chemical composition of our sweat.

 

Since few of us have lumps of unpatinated shakudo around to see if this is true, we might take copper as an analogy. When installed, copper pipe fittings are bright raw metal but after a year or so in a dry indoor atmosphere they acquire that lovely foxy red patina we so admire in tosogu.

 

I think one of the complicating factors in all of this is the varying nature of the alloys concerned, particularly in the case of shakudo. We have all come across fittings which are hardly black at all and look distinctly coppery. I've no doubt these were made from alloys in which the amount of gold was skimped to save on costs. As a former chemist, long since de-frocked, I have tried to understand the underlying chemistry of shakudo patination but still cannot really understand the role the gold plays. Research at the British Museum has shown that the black patina is copper oxide, so what effect is the gold having? I remember reading that a European spent time at the Japanese Mint in the Meiji period researching alloys that gave the same coloured patina as shakudo but did not contain any gold. Apparently it was for producing medals. He was successful but I cannot now find the reference, although I seem to remember his alloy contained arsenic and antimony. A consiiderable area for study here.

 

Ian

[Ford Hallam]

Hello Ian,

 

I may be one of the few who does in fact have unpatinated shakudo knocking around my studio. In my experience of the material I have found that, generally speaking, shakudo made from relatively well refined copper and gold ( regardless of % ) tends not to colour naturally, certainly not to any sort of black colour, though a tarnish of sorts will eventually develop. The earlier shakudo pieces, say pre; 1600, that show extensive wear and yet have perfectly intact patinae are generally made with unrefined copper ( ie; so called yamagane) and I suspect that the presence of arsenic is responsible in great part for the recolouring, in conjunction with sweaty hands, that is . A contemporary alloy used in Japan, kuromido, which contains 3% As exhibits a marked tendancy to darken quite convincingly simply from handling also.

 

I would say at this point though that I would not advocate the "sweaty hand" solution ( urgh! bad pun! ) for restoring patinae on soft metal fittings. What has happed naturally, and adventageously, on older pieces is as a result of a very gentle process over a long time. I suspect the results of a speedier effort would not be so pleasing. Shibuichi and copper can be coloured very specific tones which natural tarnishing simply does not approach. The development of the alloy palette and the associated, very controlled, colouring processes during the Edo period leads me to believe that the effects of time and handling on these items was not a major consideration of their makers. That metals do further tarnish over time, some much more than others, was understood and in some cases valued but I don't believe that this was factored into the initial patination processes.

 

As to the role of gold in the alloy in affecting a black colour, the most recent research seems to suggest that it may have to do with light absorbtion. The copper oxide formed on shakudo is cuprite ( Cu2O) and is essentially the same as formed on pure copper.( There occurs also a black copper oxide; cupric oxide ( CuO) but this is very fragile and is not what we see on shakudo.) One would therefore expect the colour to be reddish just as on copper. It seems though, that the finely dispersed gold particles in the shakudo alloy absorb the red part of the visible light spectrum and as the rest of the light has been effectively absorbed by the red copper oxide we "see" no colour; and the surface is percieved as black.

 

The person you are referring to who worked at the Imperial mint in the Meiji period was Professor William Gowland. He wrote a very detailed series of research papers on Japanese alloys and metalwork technologies. The gentleman who developed the black alloy for use in making medals was a colleague of his at the mint, a Mr Y Koga.

 

I hope this sheds a little light on the subject, sorry for the ramble

 

regards, Ford