Balsa Grain Classification

Balsa grows in a circle arrayed around a central point called pith (center of the tree) It is very weak and squishy like weak foam or a rice cake.  Next are the rings arrayed in a circle around the pith.  Lines can be seen arrayed directly from the center of the tree and are called rays.  These rays are very resistant to flexing and are called C-grain balsa sheets.  They are more brittle but are good for items you don't want to flex.  If you cut at a 90 degree angle from the rays the balsa sheet would be called A-grain which is very flexible.  The last grain classification is the B-grain classification which has to a small degree both A-grain and C-grain characteristics and is the classification of what the majority of balsa sheets end up.

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C-grain is similar to pieces of paper with straws in between.  Having extra thin segments with tiny straws keeps the section rigid with one "paper section" in compression and the other side in tension.  The straws keep the C-grain segment from collapsing when stressed.  A-grain does not have the rays to hold it together so it is very flexible.

Next is a comparison with A, B, and C-grain side by side.  Notice that the C-grain balsa sheet has a distinct checkerboard pattern and that A and B-grain look the same.  You have to look at the narrow edge of the balsa sheet to determine whether it is A or B-grain.  For A-grain you look at the narrow edge and it will have a checkerboard pattern.  B-grain will look the same on the large flat side and the narrow edge.

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