Every January we get the same two or three emails, almost always about a table bought somewhere else. A split has opened along a glue line, or a panel has cracked clean across. The owner assumes bad glue. It is almost never the glue.
The number that causes it
Wood is dimensionally stable along its length and moves a great deal across its width. Take a 900 mm wide white oak top from an unheated 65% relative humidity house in August to a centrally heated 30% one in January and it wants to shrink by something in the region of 4 mm. That is not an exotic scenario; that is one ordinary winter in one ordinary home.
What has to happen for it to crack
The wood will find that 4 mm somewhere. If the top is free to slide across its frame, it simply gets narrower and nobody notices. If somebody has screwed the top down hard through the apron in six places, the top cannot move, the stress has to go somewhere, and it goes into a split — usually at the weakest point, which is a glue line or a knot.
This is why every top we make is held on with wooden buttons that hook into a slot in the apron and screw up into the underside. They hold it down firmly and let it travel about 8 mm. It is a fifteen-minute job on a table that takes three weeks, and it is the difference between furniture and firewood.
What you can do
Keep the room between 40% and 60% relative humidity. Do not park a piece within 300 mm of a radiator. If you have underfloor heating, put a rug under a case piece. And if a panel has moved a millimetre or two by February, leave it alone — it is going back in April.