Shields, Wood and Grain lines...

Earlier, a discussion on Norse shield construction said something like: the
boards were split, as the Norse didn't saw their wood, implying that the
split wood was stronger as the grain wasn't cut.
..
If so, I'm curious as to how it worked.
Thank you,

Edward

 The following was prepared for the Norsefolk list this morning. May be obvious to those who work with wood - but I had the piece written...

I know those that know me well are likely laughing right now - Him, WOOD?
(First thing I teach my smithing students is ; "Metal Good, Wood EVIL".)

How does it work?
(*Real* woodworkers chime in and correct this!)

This might be a bit hard to explain with out proper diagrams - but bear with the use of symbols.

As has been described, a split plank comes from a clean, straight grained section of log. Wedges are used to break the circular diameter into half, quarter, eight - resulting in narrow 'pie' shapes.
The splits follow the natural lines of the grain - with the right piece of log, straight lines.
If you looked at the top of the piece - you would see a set of slight curved lines - the top edge of the grain.
If you looked at the side of your split plank, the ideal would be the side edges of each of those  grain lines
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This is extremely strong in *compression*.  If you push on the top of such a plank, the wood will bend before it ever actually breaks. Imagine the bow of a ship hitting a wave. The planks will flex rather than shatter.

If you had the technology for making saws (and the Norse could be considered NOT to have this *) you would be cutting a perfectly straight line through the log. You are slicing across a circular 'bulls eye' shape. Depending where the cut is, almost all the boards now have a combination of curved, diagonal cut, very few straight gain lines. This is especially seen on the side of the plank, with grain running pretty much all over the place.
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This is much weaker in compression. If you push on the top of such a plank, the wood can separate along the diagonal lines in the grain. Build that same boat out of equally thin *cut* planks, and when it hits the same wave, the hull will virtually disintegrate.


However, the question was about shields.
Now you are forcing a thin wedge (a blade) against the edges (mainly) of the plank.
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You see that in *either* case, the thin blade attempts to force into the lines of the edge grain. Some argument may be made that with a cut to the edge of a split plank - the board is actually *more* likely to break.
 But remember the shape of your shield - most of those blows are not going to come in exactly at 90 degrees to the end grain - but mostly coming in towards the side of the grain
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With the split board, the force of the cut is directly across the grain line - against its very strongest portion.
With the cut board, the force of the cut is applied to a possible *diagonal* grain line - a point of weakness.

(Since I think attempting to draw this would prove difficult bear with me)
Now cut two very thin planks - and stack them on top of each other. The line of the grain is offset by 90 degrees in the two planks.
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You see what you get with the split planks is that any angle of cut is attempting to slice through the strong side line direction of the grain
With the cut planks, almost any angle of cut is attempting to slice through the much weaker *diagonal* line of the grain.

Clear??

* In the case of the Norse, the metalworking problem of making a wood cutting saw in the first place is actually the limiting factor. The first plank cutting saws are wide, long, straight blades. Such a blade should be thin in cross section - to remove the least wood and so require the least force to move through the piece of timber. It needs to be absolutely dead straight - the smallest variation in thickness or warp (in any plane) will cause the saw to bind in the cut. The metal needs to have some flex in it, to withstand irregular sideways pressures likely during long hours of use. It also needs to be hard enough that the teeth do not instantly wear down to the point they do not cut.
Axes on the other hand, are much smaller cutting surfaces, and considerably more robust in construction. In smaller diameter VA forges, an axe blade can be pretty much all heated at the same time - not at all possible for a long pit saw blade.

(I have made many axes, enough swords - but has never even considered trying to hand forge a pit saw!)