The history of electricity goes back more than two thousand years, to a time when the Ancient Greeks discovered that rubbing fur on amber caused an attraction between the two. Thats ends the history lesson portion of this project. Now, let us continue to the fun stuff.
The principal behind this project is easy. The goal is to pass very high voltage through a piece of wood, as the electricity passes through the wood it burns in a fractal like design. An easy way to generate a high voltage in a typical household is to get your hands on what is frequently abbreviated in the electrical world as a MOT (Microwave Oven Transformer), this is the most important item on your list of things you need. I drove down to my local college campus and perused the alley ways of apartments and found myself a practically new 1000 Watt microwave laying next to a dumpster. MOT's are a rugged piece of equipment and tend to be the last thing to break, so the chances are good that even an old beatup and discarded microwave oven will contain a good MOT. If you are unable to find one for free you could always buy one cheap at a thrift store. Other items you will need are a hammer, some small finishing nails, alligator clips with wire, salt water, and wood (more details on this later).

The MOT is the "muscle" of the microwave oven. The MOT in a microwave serves two purposes. One is to take the 120Volts your house as powered to it, and through the magic of magnetic-flux and copper windings, that 120VAC is transformed into anywhere between 3,000-5,000 Volts DC (depending on the power of your MOT). This large amount of voltage is needed to power the magnetron which generates the radiation thats warms your food. The other purpose, using another coil, is to take that same 120VAC and step it down to a much lower voltage (3-12 Volts) to provide power to the other microwave components such as the light, turntable, fan, and LCD screen.

The MOT achieves these two functions by having a Primary and Secondary winding. Looking at a MOT, the Primary (high voltage) winding is usually on top and is the coil wrapped with small wire. The Seconday (low voltage) winding is usually on bottom and the coil is wrapped with much thicker wire.

I personally found the wood aspect of this project to be the trickiest and most frustrating part of all. Wood seems to be unpredictable. Sometimes part of a board would work really good, but then another section of that same board not so much. I tried and experimented with many different thickness's, lengths, shapes, expensive wood, cheap wood, densities of wood, and this is what I have learned. The thickness of the wood doesn't matter, all the neat stuff is taking place on top of the wood. The length of your wood doesn't matter, what does seem to matter is the distance between the nails placed within the wood. Density played a very important role in all of this. If the wood is really soft and you have a powerful MOT, the burn is going to be too hot and you will be left with big scorch marks and not any fine detail. When the wood is soft it is more susceptible to catch on fire and ruin the wood. On the other end of the spectrum, if the wood is too hard the MOT might not be powerful enough to burn in a large design, if anything at all. In the end, the type of wood that worked best for me was a basic walmanized 1x6 from the hardware store.

Before doing anything else, I got a belt sander to sand off the top dirty and greenish part of wood on all sides. And after deciding how large I wanted my soon-to-be pieces of art, I cut the board into pieces. I didn't think the ends of the boards looked very good cut straight so I cut them at 20° 's; which I felt look a little more appealing to the eye.

Wood is not a very good conductor, so it needs to be encouraged a little. To help it out I a concocted up a mixture of very salty water. With a paint brush I painted on a layer of mixture onto my board. I let the board dry until the top was just bearly wet at all, what remains is a crystaline shimmering layer on the board. I then hammered in two nails.

When you have your microwave, the first step is to take it apart. This is where the danger begins. Microwave ovens have a very powerful capacitor that can hold an electrical charge for an indefinite amount of time, enough power to kill a human. Here is a great instructable written on how to safely take one of these things apart. While reading that page, notice all the other useful parts your microwave can provide to you, take advantage of that. I tested the output of my now removed MOT. My voltmeter is only rated up to 2000 Volts and when I tested it I got an OVERLOAD error, so I know I was getting at least 2000VDC. Inside the case of the microwave was a wiring diagram which noted that the this MOT's output was 4200VDC, so I have to assume this is true. The low output was 3.5VDC.

Once the MOT was removed and tested, I got a scrap piece of 3/4" plywood to mount everything on. Wiring up the unit at this point is very simple, you should be able to look at the picture I provided to figure it out. It is a basic open circuit design, using a household light switch to turn the unit on and off. And for added safety I plugged it into a surge protector so that I could have two on/off switches. I added in a 15amp fuse after the light switch, just in case. I must add that all the wiring, screws, and fuse/holder are all borrowed from the microwave. On the output side of the MOT, one alligator clip attaches to the high output tab, and another to the ground wire.

When you have your piece of wood (details above) that you want to burn, attach the other end of your alligator clips to each nail. Flip the ON switch and behold the mesmerizing electrical dance that unfolds before you. The electricity can easily jump several inches, so keep your distance. This makes a lot of smoke at times, and should probably been done outside. The nails can be moved around on the same board to get other designs. I found that after a little bit the wood starts to dry, when that happens I turn off the machine and with a damp, almost dry, brush of salt water I "paint" the top of board again. I originally tried a spray bottle but it just seemed to get the board too wet.

I came out with six sections of burnt wood I really liked. I used a vibrating sander with a very high grit paper and very carefully sanded the top burned layer. Doing this removed a very thin layer of soot and brought a better contrast between the burns and the wood, it also revealed more small burn paths in the fractal.
It was now time to give the pieces a nice shiny finish. This part was easy just time consuming, waiting for the layers to fully dry. My finish of choice is tung oil, applied with a foam brush. For the first 2 coats applied I used a mixture of 60% tung oil to 40% mineral spirits. When it is thinned the oil is much easier to work with, and the wood can absorb more of the oil easier with a faster drying time. For the next 5-6 coats I used only tung oil, not a mixture. Once the wood was as smooth and shiny as I wanted, I got the vibrating sander again and gently sanded all sides to remove any rough spots and oil runs/inconsistences. I then applied two more final layers of oil.

I am giving these as gifts, so I wrote up some labels to glue onto the sides so people will know what the heck it is they are looking at. You can download a full page of the labels I made here.


References:
Youtube Videos
Tesla Down Under
HighVoltage Wiki