Rusty Tosho Tsuba

I recently bought what appeared to be a rusty tosho tsuba on the internet. As the tsuba was large, probably old (pre-1600s) and cheap, I was not unduly worried about the rust. However, when I received the tsuba I found the 'rust' to be facinating, it is evenly distributed over both faces and resembles same (fish skin) used on sword handles. I've seen many tsuba in my time and I've never seen 'rust' like this, it is a hard stable patina, glossy in most parts. There are some patches of light red rust, but not the regular corrosion patern one would expect. My conclusion is that the surface is deliberate. I've looked through all my books and sale catalogues and can find no similar examples. Anyone got any ideas?

The tsuba has a simple , symmetrical, pierced design, either of four axe heads in negative silhouette, or four circles around a central circle. Four circles may represent a family mon. The hitsu ana is plugged, probably with lead, or possible a tarnished silver metal (pewter or silver).

Overall, the tsuba is quite large, indicating that it was made before about 1550. Although the design is simple, the working of the iron is skilled, the convex surfaces being very even. The convex surface shape is known as go-ishi gata (go stone shape). All indications are that this tsuba was made by a swordsmith (tosho)

Height: 9.2 cm, Width:9.2 cm, Thickness at rim: 0.2 cm, Thickness at Seppa Dai: 0.5 cm.

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