The name shibuichi (one quarter) refers to the silver content of 25%, the remainder being copper. It's a generic term though and in reality the alloy is used in a wide range of compositions, from as little as 10% silver to more than 60%. There are also, occasionally, other trace elements present, such as gold and lead. Shibuichi is also called Ro-gin, misty silver (in Japanese), a much more poetic name, that was more common in Edo period Japan (1603 ~ 1867).
It's taken me about a week of experimenting, with various compositions, to eventually get a shibuichi alloy that matches both the colour and fine grain structure of the sample tsuba I'm working from. What I realised right at the start was that even with an accurate technical analysis of the tsuba we have, all I would know was the percentages of the various elements in the metal. The thing about shibuichi that makes it is interesting is the wide range of hues of grey colour and grain size that it can exhibit. These essential characteristics are a consequence, not only of the composition but also the exact times the molten metal is held at liquidus, when the silver is added to the copper and how long it has to diffuse throughout the copper matrix.
Over the past 18 years I've built up a database of alloy compositions based on analysis' made by my good friend, Mark Froneman. We were at school together and he eventually became a forensic scientist. He's a doctor and professor too! I could have become one too...if I could have read his notes, awful hand-writing!
Anyway, By comparing the tsuba with the sample plates I've made up I had a fairly good idea of the rough make up of the metal. I then had to use my instinct and experience to fine tune the mix and the precise timings to get the result I was after.
At this point I have 4 tsuba sized ingots, all cast into boiling water, and have worked 2 down to rough blanks. One of them is exhibiting some cracking on the edges. This is not uncommon in this alloy, it's quite an unyielding material. I'll abandon that one for now and concentrate on the other, which so far is flawless.
This is one of the ingots as cast, this is what it looks like straight out of the water and before it's pickled in acid. As you can see, it has a beautiful surface and no oxide layer at all.
This image shows a small ingot I cast earlier while working out the right mix. I polished the edge to test the colour. Notice how smooth the upper surface is. That's as cast and untouched by me the whole ingot coloured when I did the test patina too, which surprised me as "as cast surfaces tend to resist colouring. I haven't been able to get that degree of smoothness on anything over 250 grams. Not sure why the larger ingots develop that area of crust but I'll figure it out one day.
Alongside it is the ingot I am now finishing. You can see the dished upper surface where I've scraped away the rough patch of porosity and I've done a preliminary colour test on that one too. Also in the picture is the earlier blank, the one that started to show cracks on the edge. It will still be usable but in this case I don't want any nasty surprises some time down the road.
It was my birthday on Monday, Lorenzo and Karl sent me this photo of the 2 of them in a pub in Berlin, drinking to my health. It just made me envious....but I appreciate the sentiment and the good wishes. Thanks fellas, Prost! That's Lorenzo on the right, with the facial hair, Karl is the handsome one ;-)
photo by Makiko, Lorenzo's lovely wife.