Wow, it's been far too long since my last Instructable. How's everyone doing? This Instructable's been on my list of stuff to to for a long time, and i've only just got around to it. Eh, well.
Mokume-Gane is japanese in origin, and is a composite material formed from layers of metal of various compositions fused (welded) together through heat and pressure. High heat means you don't need high pressure, high pressure means you don't need high heat.
The whole point of making Mokume is to make an attractive metal that can be made into jewellery, ornaments, etc.
It's offensively simple to make. The setup i used to make mokume for this Instructable only cost a few pounds. (I already had the gas and the torch. Yes, you have to buy them.)
There's no picture on this page because i didn't get a picture of the finished product. It's pretty though.
A brief legal note. I am not associated with T-cut, or whichever company it is that makes is, etc.
You CAN make Mokume out of coins, but in the UK it's treason, and in the US it's scarily illegal. I'm not saying don't do it, I'm just saying don't get caught.

Step 1: Make the Stack

WE're using cupro-nickel and brass for this. These two metals create a nice contrast and weld fairly easily, and are also readily available ;0
The really important thing in the process is to get good welds, and to stop the hot metal from squirting across the shop. To do this, we make a set of pressure plates, to keep everything locked still.
To make pressure plates, get some thin steel strap or strip, whatever you want to call it, (B+Q for UK readers) and cut about 2.5 inches off the end. Put it back onto the strip of steel, and drill holes, for rivets to go through. I think the pictures explain this best. After you've got the drilled plates, assemble a stack of alternating metals. The metals you use must have similar working properties and melting points.
Put the stack onto the bottom part of the pressure plate, put the top part of the pressure plate on top of the stack, and rivet them together. This is actually qiute tricky. My best advice to you: Be Creative.

Step 2: Weld the Stack

Into the fire with the Stack!
It's a shame to burn the pressure plates, but it's got to be done.
In the pictures, you see my advanced setup. That is: A hollowed-out porous breezeblock (please don't tell me about cement exploding, etcetera. This stuff doesn't explode, it just melts.)
When this one's beyond use, i'll post an instructable on making one. So simple.
For heat, I've got a Bernzomatic blowtorch using propane. Just out of shot is the anvil i'll use: A big hammerhead in a gallon-can of concrete.
Now then, the instructions:
Put the stack in the fire. Make sure that the torch flame is contecting the stack, otherwise it won't get a good, even heat.
Don't run the torch too high, either. Heating the stack too quickly will mean that the inside's still cool, while the outside's melting away quickly.
Once one side of the stack's a good hot orange colour (do this in semidarkness, so you can see the colours better) take the stack out and turn it around, so the other side gets hot. Take the heat up slowly and steadily until you notice the stack starting to sweat tiny little drops of liquid metal. That's when you weld it.
When the stack sweats, QUICKLY AND CAREFULLY take it out of the fire, place it on the anvil, and hit it with the hammer. It doesn't need to be hit hard, just hard enough to bring the layers together. As you notice the stack start to feel more solid under the hammer, then you can hit harder. It's not unusual for little bits of metal to break from the outside of the stack and fly off at this point. These pieces are, of course, very hot. Have a bucket of water ready (you'll need it later anyway) and remove flammable items, and items easily damageable by heat from the welding area.
After it's welded, it should look like picture 2. When you take the now-fused stack out from the pressure plates (use a hacksaw, and recycle as much of the steel as possible, it's cheaper that way) it should look like picture 3.
Now you need to flatten the surfaces of the stack. Heat it to a low orange heat and hammer it. You're looking for smooth, flat, black surfaces, free from dents, bumps, etc.

Step 3: The Pattern

Now you've got a welded stack of Mokume, but if you were to forge it out and polish it, it'd only show one colour of metal. Why? Because you didn't drill down through it and flatten it out.
By drilling through it and flattening it, you're bringing other layers up to the surface, and creating a contrast.
Drill into it, (Don't go all the way through) then heat it up, put it DRILLED SURFACE DOWN on the anvil, and hit it with the hammer until the holes have gone. Make sure the drilled surface is always down, for the best results. Once the holes have disappeared, it's time to polish.

Step 4: Polish It

You've not got Mokume, but it needs more work, as seen in the first picture.
This step's simple enough:
Get some wet/dry sandpaper suitable for metal in 80-grit, 280 grit, 400-grit and 600-grit.
First, sand with the 80-grit until all the black fire scale is gone. Then, sand with the 280-grit until all the 80- grit scratchmarks are gone. Continue until the surface is at 600-grit with no scratchmarks. By now, you should see the pattern. This isn't hard, it just takes a bit of time.
After 600-grit, you can either hit it with a buffer, or use a metal polish like T-cut or Brasso. I used T-cut, simply because it was the first thing to present itself.
Picture 2 shows exactly how much crap came off my Mokume with only a few wipes.

Step 5: Ideas, Stuff Like That

I only made a flat piece, which is fairly boring. It's really, really easy to make this into a necklace or bracelet. It can be made into a ring, too, if you can figure it out.
Other weldable materials:
Alloys of Gold, Silver and Copper. Some of these can give great contrast, some can give great subtlety.
You could also patinate your metals. Different alloys react differently to chemicals, so you could get blood-red on silver, for example, or dark blue on gold.
For the advanced worker: Words.
It would be interesting if someone cut readable words into their Mokume. That'd need a milling machine, though, really.
Mosaics: Make different stacks, weld pieces of these together to make a mosaic pattern. More work, though.
Have fun making you Mokume, and post pictures in the comments by all means!
Edited to say: People in the US CAN stack pennies together and weld. US pennies are already made of copper and cupro-nickel sammiched together.