The process is pretty easy, and there are lots of videos online to get you started--just burn, brush, wash and oil. As nearly all of the tutorials online focus on burning cedar siding, I thought I would share some insights into how this technique might translate to smaller birch bowls.
As I understand it (and I am no chemist!), charring wood alters the surface of the remaining material. The cell walls of wood are made up primarily of cellulose, which is hydrophilic and helps draw water through the tree. Lignin takes up much of the space within the cell walls and helps give strength to the plant. Lignin is hard and hydrophobic. When you burn wood, the cellulose vaporizes first as it burns at a relatively low temperature, leaving behind lignin, which burns at a higher temperature. Thus, charring creates a hard, hydrophobic skin of lignin and carbon on the wooden bowl. While I don't care about the other benefits of shou sugi ban (termite and fire resistance, for example), I sure like the idea of my bowls resisting wear and water!
Most videos online show people using a roofing torch on cedar planks, but that is overkill for a little bowl. I use a simple Bernzomatic propane torch, which costs about $30.
Obviously, it is preferable to work outside on a non-combustible surface. If you are going to burn the outside of the bowl, I recommend propping it on top of an empty tin can. No matter how dry your bowl is, as you burn the outside moisture will be driven from the inside, and it needs a place to go. Be prepared for the bowl to warp a bit, even if it is "dry." I have not had one crack yet, but you can see it change shape through the process. I try to keep the flame moving over the whole bowl for a bit, bringing the entire piece up to temperature before really working on one spot.
Heat bounces. As you approach a foot, bead or decorative cut, you will find that the heat does not make it into the valley of the cut. Instead, it will project out and burn the surface at a right angle. This means you can cook the foot pretty good trying to get heat into the valley between the bowl and the foot. I have learned to embrace these lighter areas.
|Here you can see the light line around the base of the foot, yet heavy burning on the rim of the foot itself. Heat bounces and does not penetrate into valleys very well. Embrace it.|
|Here the bead acted as a "wall" and helped protect the rim band from burning and proving a sharp transition.|
|Inscribe first, and then kiss the surface with the torch for a nice contrast.|
If you do burn right up the the rim, be careful of thin lips. They will catch fire faster than the rest of the bowl, and if an ember forms it can burn a notch in the lip pretty quickly.
|Here you can see both a decorative cut spared from burning, which is a nice bright line, and a rim burned too much, where an ember formed and burned down into the bowl.|
|Here the bottom half of the bowl has been brushed with a nylon brush. The top has not. No worries--that brown post-brushing color will turn black with oiling.|
I then recommend rinsing the bowl, again being careful not to let the loose soot stain other parts of the bowl. Some people blow the dust away with an air compressor.
Once dry, oiling is pretty straight forward, though be careful not to rub excess oil from the dark areas onto the lighter areas. Here you can see the oiled wood returning to a nice, dark shade.
|Here the top half has not been oiled yet, but the bottom has. Nice, luscious black tone.|
So, there you go. Hope that helps. Enjoy and be safe.