A: This defect is indeed grain separation, and is also called lathe checking or feathering. It results from a fracturing or tearing that occurs when the veneer is manufactured. Such tearing, which occurs on the loose side of the veneer, is normally prevented by using a sharp knife, reasonable feed speed, power, preheating the logs and proper use of a pressure bar on the lathe or slicer (to keep the wood surface from pulling apart).
Definition: The tight side, which will not have lathe checks, in a slicer or rotary lathe will be the side that the knife touches first, or the side furthest from the stock being cut, while the loose side is the side that is touched second or is closer to the core or stock being cut. The loose side is the outer curved side as the veneer is being sliced; this outer curved side is being pulled apart more than the interior side.
There will almost always be a few veneer checks, but they should be quite small and infrequent. Often, when veneer is assembled into a panel, the loose side with the veneer checks is glued onto the core or substrate so that the tight side (free of checks) is exposed or is on the outside. Unless the veneer is sanded heavily (which would sand past the tight side without any checks and into the loose side with the checks), the checks in normal veneering with the loose side glued to the substrate will not be a problem. Of course, if the lathe checks are very deep, then it will be a problem no matter how the panel is assembled.
Once the lathe checks are formed, they are never "healed." Therefore, if a veneer panel with the loose side on the outside that appears to be okay initially is exposed to a drier humidity, the drying that results will cause some shrinkage and opening of these pre-existing checks. If the moisture increases due to higher humidities, the checks will tend to close. This is why you see the checks at your plant (a dry location), but not in a more humid area (such as at the dockside).
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