Wooden products and cracks
First I need to say I have no official education about wood or woodworking. What I write is based solely on my personal experience without any theoretical knowledge.
Wood is a live material and it has to be considered this way. This is what gives it that special charm, even though I have to admit I am sometimes disappointed when I discover a new crack once I've started working on a piece. A newly discovered crack this morning is actually the reason for this article. I discovered two new cracks on a piece of birch wood I started working on yesterday. I thought it was already dry but, as it turns out, not enough.
I left it in my workshop during the night uncovered. And so today two cracks awaited me in an otherwise perfect cube of birch wood with a very nice structure. Normally I always wrap my unfinished pieces with a cloth and put them in a plactic bag, to prevent moisture from escaping too fast. As even if wood seems dry, cutting away material opens up internal places where there might still be some more moisture and this sudden change can cause tensions in the newly formed piece, resulting in new cracks. So my advice is to wrap up your pieces during the "rest period" - according to my last "lesson" today - also those you think are already dry ;)
Cracks appear as the outer layers in a log dry and shrink faster than the core. And when the outer layer shrinks it has to create cracks to compensate for the "lost material". I was able to see this very clearly on one of my products. I made a small jewelry box out of a freshly cut maple tree trunk. I chopped it into two halves and used one for the bottom box and the other for cover. Until I finished them, I kept both parts in a plastic bag and when the form was finalized and sanded I applied some oil. The cover fit nicely all around the jewelry box. But a few days later there was more than 5mm space on both sides. So the cover could swing sideways. Both halves namely got deformed. There were no cracks anywhere, only deformation. As I made the walls quite thin and there were no forces to prevent the shrinking. So the outer edges moved down a bit and on the cover in the opposite direction. So the cover was now touching the bottom part only in the middle, where the core of the trunk was. If the log would not have been divided in two halves, it would definitelly crack somewhere in the middle.
This means you can actually avoid cracks in certain situations. Of course it depends on the shape of the products, wall thickness and wood orientation. Before this I tried it the same way with a large bowl of almost half a meter in diameter, but it was cut out perpendicularly to wood growth and in this case the crack still appeared as the outside diameter shrank and there was "material missing" resulting what resulted in a big gap. I asked a producer of small wood drying devices - Plano, whether this is preventable with a proper drying procedure, and he replied that no. As the core will dry and shrink differently than the outer layers. The only way to prevent this would be to remove the core and dry both individually ;)
So, what can we do with cracks then? Personally I am against covering them up or hiding them, as they cannot be hidden. Unless they are really tiny. And I strongly advise you not to use wood putty as it will only make things worse. It is guranteed to fall out. So just don't do it! I like to use epoxy to fill up the cracks. You can color it with various additives and on the above photo I used a carbon grey pigment. It would also look nice in silver or for darker wood you can use bright yellow, blue or green colors. It actually emphasizes the crack. Depends of course on the size and number of cracks. For a large crack on the outer edge, you could actually use metal wire or leather string to "tie" it together (like shoe laces). And the end result might even look better than without a crack! And no one will know when and or how the crack appeared or why it is there ;)
You might have asked yourself why I would actually need to wrap up my pieces, as they could surely be finished in one day since I am selling and promoting such great tools to do this. A good question! Well, the reason does not lie in the tools. Or does it? Well, without tools which enable me to do complex shapes, I would not make my pieces so complicated, so I'd finish them faster. But since I have this option, I like to complicate things for myself and I create complex shapes which I invent as I go and I need breaks in between stages to decide what my next step should be. It is much easier to remove wood than adding it back ;) Once you cut it away it's gone forever... I will often need to sleep on it before continuing. I am still learning and discovering the possibilities. Often I make something I decide to never repeat, as it makes the sanding process too complicated ;) I have one bowl on my dining table more than two months already. It is almost finished, but lacks some refinements and I still do not know what those are. So, now you know the reason I need to wrap things up ;)
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