Mokume Gane is Japanese and is literally translated as wood eye metal. This implies to a pattern that sometimes resembles woodgrain. In mokume gane different metal slabs are joined together in a similar way the damascus steel is made, using diffusion welding. But different materials require different approach. Mokume can be made using almost any metal but gold, silver, copper and their alloys are most commonly used.
Mokume gane technique was developed in 17th century Japan, an was most commonly used for samurai-sword fittings. At that time mokume was obviously made using very simple tools. Metal slabs were bind together using steel wire and were heated in a charcoal forge judging the temperature only by eye. Nowadays mokume is often made using sophisticated electric- or gas ovens, hydraulic presses etc. Googling 'diffusion welding' one can find quite a bit of scientific data about it because it's variety is used, for example in aeroplane industry to weld aluminium and titanium.
In following I try to enlight the way I make my mokume gane. This is not by any means the only way but only one of many. Please notice that following is not to be used as a tutorial and I don't take any responsibility in it or any possible damage or injury that might happen if one tries to copy this technique.
First I cut equally sized slabs using two or more different alloys. Their surfaces need to be cleaned very thoroughly from oxides, any dirt and grease. After this I pile these pieces in turn, similar to the way that a damascus steel billet is created. The slabs are then tightened snugly in a special clamp.
I use an electric kiln, so the billet needs to be sealed inside a cannister which is loaded with charcoal. Charcoal consumes all oxygen inside the cannister and thus creates an oxygen-free environment.
After this the cannister is placed inside a preheated oven. It's impossible to lay down any kind of unambiguous rules about the temperatures. Temperature is dependent on the metals, possible alloys they might form during heating, their melting points, pressure and time used.
After the joining is completed, I begin forging the billet to -50% thickness by hand, annealing often on the way. Later I'll proceed using a rolling mill. When appropriate thickness is achieved, it's time to do the patterning.
There are two basic principles in patterning. Material is removed selectively from the surface of the billet, for instance with a high speed burr (or hammer and a chisel, as I prefer) to reveal the underneath layers, after which the surface is rolled back even. Other option is patterning the surface with punches or hammers and then grinding the surface until the dents are gone. With imagination the patterning possibilities are somewhat endless.
After patterning the plate needs to be rolled down into desired thickness. Now making of the actual piece can begin.