Tuesday, January 2, 2018

Burning bowls

While I love the effect of milk paint, especially the way layered colors gain character through use, I have been experimenting recently with shou sugi ban.  Shou sugi ban (焼き杉), literally "burnt cedar board" or "grilled cedar," is an ancient Japanese technique of preserving wood by charring.  The burnt surface is dark, rich and textured,  especially after oiled.  It is also bug, fire and water resistant, allowing some structures to last for hundreds of years.  You can get lots of different effects with different burn times and treatments.
https://goo.gl/fepi8s
When used on bowls, this technique can create some really striking effects.  The surface is velvety smooth and inviting to the touch.  Visually, the matte black sections seem to suck in all light, such that if you char only the bottom of the bowl, it makes the upper rim appear to float in space...




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.
The other reality that comes from this bouncing effect is that a bead can be used as a "wall" to separate a burned section from a non-burned section.  
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.
I can also envision how one might add a design by using masks to protect some areas from burning.

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.
Once the burning is done and the bowl is cool, move on to brushing.  I recommend a nylon brush to remove the excess carbon.  If you don't remove the excess carbon now, the bowl will leave marks on surfaces later.  Just like when you sand milk paint, watch out that excess soot does not stain the rest of the bowl.  I am often surprised by how much of the luscious black comes off with brushing.  Don't worry, once oiled that black color will come right back.  


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.    
The reason I like the nylon brush over a rag is that rags often leave behind fibers, especially if you rub against the grain.




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.   
After oiling.
I am now experimenting with chip carving burnt bowls.  The charring is not just a patina, it goes down a ways, so only the bottom of a chip reveals the unburnt wood.  But I like the effect.  I was always hesitant to chip carve painted surfaces, as the pigments dull the knife edge.  With shou sugi ban there is no such concern.




So, there you go. Hope that helps.  Enjoy and be safe.


焼け  
  "burnt bowl"




7 comments:

  1. Wonderful post, Eric. Filled with details that will really help to avoid pitfalls and encourage others to try this, including me. And beautiful bowls as well! Stunners.

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  2. Wonderful post Eric! LOVE your blog. Such a tremendous resource for others. Thank you for taking the time to add to it.

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    1. Thanks Chuck. Hope someone finds it helpful.

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  3. I finally found your blog! Great stuff, your contented wonder in the making bleeds through the words. Great to read a post or two before heading out to my own shed for a bit of making of a morn.

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    1. Glad you enjoyed it. Would love to see your own work some time!

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  4. thanks for a brilliant post. that was super illuminating. can't wait to give it a go.

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