A Bit of Heat, A Bit of Pressure, and a Lot of Skill

Making mokume-gane is a pretty complex process, and those of us who do it become immersed in the physical science of how metals behave (or don’t) when heated, punched, carved, cut, and handled in dozens of ways. In fact, I write scientific papers about making mokume-gane about once a year, and my ability to get too technical is pretty well known. In this blog though, I’d like to share with you different aspects of making mokume-gane, but in less technical terms for those who don’t wear pocket protectors and stare at phase diagrams for fun! Today, let’s talk about what a billet is (and no, I don’t mean a place where soldiers lodge).

In order to create a mokume-gane piece – whether it’s a ring, a brooch, or decoration for the handle of a katana – you have to create a billet first.

To create a billet, we carefully stack thin sheets of metal, one on top of the other. Which sheets of metal? It depends on the colors we want to use in the item we plan to make and also how the item is intended to be used. Some metals are just better for jewelry than others. But still, we have lots of options, and experimentation in mokume-gane is a big part of the adventure.

Once the metal sheets are stacked, we have to bond them together. The traditional way of doing this is through fusing. Essentially, the metal is heated to the point where some or all of the metals in the stack become partially molten – just hot enough to blend into one another at the edges. Too much heat and your stack of metal becomes a mushy mess. Too little heat and they don’t bond. It takes a lot of skill to do this, and you have to have deep understanding of the melt temperatures and metal behavior of each of the metals in your stack.

There are other methods of bonding the metals. The next is soldering (sometimes referred to as brazing). In that method, instead of heating the individual sheets to the point where they fuse, you use a meltable filler metal in-between them. It’s a bit like using frosting to cement two rounds of cake together to form a stack, only the solder layer has to be extremely thin.The soldering technique is limited by several issues and is not often used for high quality work.

The most modern process for bonding the metal sheets together is called solid-state bonding. This involves a furnace and something very strong to push the sheets of metal together, like a hydraulic press. Solid-state bonding is using a combination of heat and very high pressure to bond the sheets together. It allows us to use lower temperatures, which is useful if we want to use some metals that have very different melt temperatures from one another.

And that’s how we make a billet! Now every time you look at your mokume-gane jewelry you’ll remember that it started its life as a stack of thin sheets of metal, some heat, and a lot of experience and skill.

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