Today I was working on the layout for “Honest Labour” and had to revisit the 1936 volume of The Woodworker magazines. I stumbled on this delightful and ingenious way to explain and demonstrate how wood twists as it dries. Read the original text below and check out the illustration.
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— Christopher Schwarz
Every woodworker knows that a certain shrinkage in wood is inevitable, and most know (to their cost) that a board will sometimes twist. Probably the majority connect the two phenomena, and say that a board twists because it shrinks. But this is only a half truth. It is true that the twisting would not take place if the wood did not shrink, but it is quite possible for a board to shrink without twisting. In fact, every well-seasoned board does so. Shrinkage has to be accepted as inevitable, and the fact that a board has remained flat goes to prove that the shrinkage can take place without twisting.
To revert to our subject, however, assuming that a board has twisted, that is become hollow, who can explain why this has taken place? An excellent practical demonstration of what happens is shown in the accompanying photographs. First a piece of paper about 6″ wide and 2′ long is folded up across its width in a series of folds, rather like a fan. The whole thing is then opened out at one side so that a circle is formed (like a double fan) as in Fig. 1, and the joining edges are glued together.
Across the face of this a series of lines is drawn with a brush and black ink. The lines at A are meant to represent the cuts that would be made in a log to produce plain (flatsawn) oak. That at B is a solid square of timber, whilst the C boards represent figured boards (quartersawn) cut radially from the centre.
Now shrinkage takes place around the annual rings, and it is obvious that if a log were never converted it would have to split, because the shrinkage would mean that the length of its circumference was becoming less. In the demonstration it is assumed that the splits have taken place at the two sides, and consequently two cuts are made at these two points. The spring of the folded paper will cause the whole to assume the shape shown in Fig. 2, and this is precisely the shape a split log would assume.
The originally straight lines of the conversion of the plain (flatsawn) boards A are now all curved, the square at B has shrunk badly at one side, whereas the figured (quartersawn) boards, C, remain straight. Thus we can see why plain oak is so much more liable to twist than figured oak, and why the boards always twist with their edges away from the heart. Thus in a twisted board it is always safe to say that the rounded side is the heart side. Furthermore, by an examination of the end grain is is always possible to say which is the heart side, and which way it is liable to twist if at all.
Source: lostartpress.com