Japanese bita-sen copying Chinese Yuan Feng

[bgriff99]

Genho Tsuho in Japanese.    They are kaizo-sen, having newly carved patterns, as opposed to directly copied by making impressions in a mold, or directly copied from a modified original.   If these come out in the right order, the first two are nowhere near any original Chinese pattern, while the third is quite similar.

"Bita-sen" means bad metal coin.    In the case of these the bad is a lot of arsenic and iron.    Bright metal looks like iron, they rust and are magnetic unless (in some) non-magnetic compounds of arsenic and iron form using all the iron present.    So some of these test out at 15% iron but are not magnetic.   Just copper, iron and lead will show magnetism at about 1% iron.

These were all found in central Vietnam, the territory of Dang Trong, the Nguyen state which colonized the former Champa.    Dang Trong had to import all metal.    From 1608 through the mid 1650's, all the previous, foreign, private and broken cash circulating in Japan were culled and exported, either for money use or scrap metal.    But also new bita-sen were cast for export, particularly to Dang Trong in the 1630's.     Nagasaki trade cash starting in 1659 was a more controlled system to do that, and supposed to be the sole outlet.    Chinese Yuan Feng Tong Bao cash were cast 1078-85AD.

[THCoins]

May be called bad metal, but it produces lovely variegated colors on these coins ! Is that also a consequence of the alloy, or just a bit patchy oxidation ? And are these surfaces stable or do you need to make precautions for storing ?

[bgriff99]

The surfaces are stable, and not what Japanese collectors prefer.    Arsenic seems to generate a light ochre colored matte plaque, perhaps seen on the third piece, in the fields.    It gives contrast.    It looks even better streaked with azurite.   The first two coins are finger rubbed to some extent to bring out contrast to make scans.   What looks like rust is rust.

Years ago when I was preparing rubbings for various articles and booklets, everything had to be cleaned.   Fortunately those were always common coins, not needing minerals on them to prove genuineness.   Even zinc coins, once they are covered with scale of oxide and carbonate are stable.   
Cleaned or blue-black zinc cash have to be watched, and greased occasionally.   

[THCoins]

Thanks for your explanation ! As i live in a quite humid region i have found some of my base metal alloy coins do need some extra care while others, as you say, are remarkeably stable over time.

[Figleaf]

I am wondeering why the arsenic was added in the first place. I can think of two explanations, but there may be others.

• It came with another metal. Arsenic is often mined in combination with copper. Even today, China is the global top ranking producer of arsenic.
• It was added on purpose to harden the metal (arsenical bronze) or to improve the metal colour, making it look closer to gold when new.

Peter

[bgriff99]

Peter, arsenic was added as ore not bullion, in my opinion, although the means to separately smelt it was available.    The iron probably indicates poor quality copper ore and single stage smelting.    The questions about this are not answered.   We have barely recognized the range of compositions and where they were used.  We don't know if the newly cast cash were yellow or white color.   

Many of the bita-sen test out at or near 18% arsenic.    For the whole range of the copper + arsenic mixture, that ratio has the lowest melting point.   It is possible they were purchased with a specification for low melting point, for recasting.    That much arsenic ruins the alloy for tensile strength, shock strength and ability to be worked or forged into anything useful.    In other words it could be recast into shot, arrowheads or other expendable castings.    The Nguyen state needed to procure copper to make cannons, and this material wasn't it.   

[Figleaf]

Pure speculation, but here goes. Since Roman times, it was possible to make spear and arrow tips that were sharp enough to penetrate light armour, yet they would deform on impact. The importance of the deformation was that the enemy could not pick up a spear and throw it back. From your description, arsenic copper looks like a candidate for that technique.

Peter

[bgriff99]

A completely alternate idea is that the coins are still found in hoards because they were rejected.    Whole shiploads of them were ordered by the Nguyen kings in the 1630's, for their metal.    Bita-sen were the cheapest form of copper, such as it was.    Cash coins were sorted according to type, for export, when old kinds had been demonetized in Japan.    The highest price was paid for Yong-le, the Ming cash of the early 1400's which had been Japan's primary coin for 150 years.   When it was the VOC doing the shipping, detailed records of the amounts of each type of cash, and the prices for them, are still extant and published.   

Slightly later, when Nagasaki trade cash were exported, they were for use as money, and brought a higher price by weight than the 99% pure copper bars Japan had on offer.