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end grain balsa
End grain balsa is more or less what it sounds like. Essentially they take balsa wood logs and saw of the ends of the logs to create uniform thickness blocks of balsa wood with the pores of the grain on the top and bottom.
This does a lot of things. Balsa wood is very light weight and has a lot of compressive strength parallel to its grain and a lot of sheer resistance perpendicular to its grain which are exactly the loads that are sustained by a core material.
Balsa is a reasonably rot prone wood but the rot spreads parallel with the grain. In theory using balsa with the end grain up and down allows the resin to soak into the open pores of the Balsa sealing the pores and making a better bond. In principle these sealed pores can''t rot because air and moisture are sealed out. That said, core problems can occur where fastenings enter the core allowing water into the core or it can occur where poor workmanship or a trauma to the hull has separated the skin from the core. In minor cases where the core is dry you can inject the area with epoxy and re-establish a seal and a bond.
In its most common form end grain balsa is used in manufactured sheets that have small blocks of end grain balsa adhered on one side to a scrim (a fabric that looks like cheese cloth) and is glue against the molded fiberglass on one side and then the inner skin is laminated directly over the core.
You can find more information by visiting the Baltek website (www.Baltek.com).