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Published in Cutting Edge, The Journal of The Cascade Chainsaw Sculptors Guild
Butts, Stress, and Relief
One of the most common complaints I hear is about a carving that cracked, split, checked, broke or fell apart.
Youve all heard the same thing.. "My neighbor bought one of those and it cracked right in two.." Its usually said when there is a bunch of possible buyers trying to make up its mind about which carving to take home and you suddenly go from the center of attention to a flubbing hacker trying to explain why it is that your carvings dont do that.
Or do they?
Do your carvings crack? Split? Break? Check? Fall apart?
Its a pretty common thing for wood to do and if left to itself, what it probably would do. So, I have to ask, what can be done to keep your carving in one piece and not let it become "one of those"?
From experience, I can tell you that nothing you do will guarantee that a piece of wood wont split. Its been a hard lesson. Ive sunk a lot of time and money into trying to find a way to keep that wood together, but nothing has worked every time. However, I have developed an effective litany to tell people why it is that my carvings are not likely to crack, and if they do there is a warranty that I will fix them at no charge. Thats a pretty hefty promise and not one that a lot of woodcarvers would care to make. However, in the interest of improving the quality of the product, Id like to pass along a few tips on what you can do to keep your own work in one piece.
Lets start with BUTTS. Big butts, little butts and all the in between butts. Its the part of the tree thats closest to the ground. The strongest part of the tree. The part that doesnt break in wind storms and can pull roots out of the ground. The part loggers often leave behind because of the swell, and firewood cutters dont want because its too tough to split. Its the gnarliest, strongest, toughest part of the tree and its usually the cheapest because it has no commercial value.
Read the wood for indications of STRESS. Everyone knows what drying cracks look like on the outside of a log, but to find the main stress line you must look at the heart. Every species of tree Ive ever used has shown me its heart and its stress line. Its very personal, but Ill tell you about it anyway. Look at the heart of your log and youll see that it has a stress line too. It will appear soon after the log is cut and grows over time until it splits the log from heart to cambium. Keep this in mind while we talk about relief cuts because youll need to align your carving in such a way as to minimize any damage that the stress-line crack can do to a finished piece.
The reason a log will crack is because of moisture loss. There is a long-winded explanation of the mechanical process by which the log does this, involving (among other things) the structure of xylem tissue with all it tracheids and parenchmya cells, but Im sure you would rather eat bark than hear about it. I know I would.
Stress relief cuts can go a long way toward keeping you carving together by robbing the log of its ability to crack as a unified mass. Relief cuts break up the grain's unity. It cant crack as one piece. Think of an uncut pizza left out in the sun for a few days. The whole thing shrinks and warps into a hideous organic frisbee that wont fly. The whole crust works together to do this.
If you cut the pizza into wedges, you relieve much of the stress. it will dry as a number of individual pieces and stay fairiy flat in the pan. (This is simply a metaphor for a drying log. If you actually perform this experiment, be sure to soak the dried pizza in water for a few hours before trying to eat it).
The question now is where to put these stress relief cuts, how deep they should be, and how many should be made. For the answer to that one, be sure to read the next article.