One of the challenges that novice woodworkers soon discover it that working with wood means you need to understand it's properties, and one of the most rudimentary properties is the moisture content of wood.
Wood has the unique ability to absorb and release moisture. This is due to the cellular makeup of all types of wood, although different species of wood will absorb and release moisture at different rates. It is the coming and going of moisture that accounts for wood movement. If you were to grasp a handful of drinking straws in your hand, this is very similar to the structure wood. It is comprised of long microscopic tubes all bound together. It is these microscopic tubes that exchange nutrients up and down the tree as it is growing, and primarily moisture is gathered from the roots and distributed through the growth rings of the tree and on up into the leaves.
When a living tree is cut down, no matter what time of the year, there is always large moisture content in the tree, and a much higher one in the spring and summer. Depending on how the tree is milled, and how the wood is dried will also affect, to a degree, how that wood absorbs and releases moisture.
Small operation mills will often fell trees, then immediately coat the ends of the logs with thick, waxy sealer. This is to slow the release of moisture through the ends of the log, or timbers and to make the wood release moisture slowly through the walls of the cells. This slow release makes the wood more stable to work with, often reduces twisting and "honey-combing" or cell collapse ... but it does challenge the patience of the woodworker because air drying wood can often take up to 3 years.
Once the wood is dry enough to work with, whether it was kiln dried or air dried that wood still has the ability to exchange moisture, and if the ends of the wood are not sealed, the wood will want to exchange moisture through the cellular ends first because it is the quickest and easiest way for moisture exchange for the wood. It is this moisture exchange that makes wood move.
Now dried wooden board, with grain running the length of the board will not get longer or shorter with moisture exchange, but it will growth thicker and thinner on all sides as it exchanges moisture, and it wil do it more quickly when the end grain is not sealed.
If you were making a table top out of natural wood by gluing planks together, you can be assured the the width of the table will bet wider and shorter, by small amounts, over the seasons as the wood absorbs and releases moisture. The table top will also get slightly thicker and thinner, but in this case that would not be a concern. What would be a concern is the table top movement and how it would affect the connection of the legs. If the apron did not have some form of sliding attachment to the table top the top could either buckle or warp, or it could even split if it is not connected properly,
Wood movement affects ALL forms of working with wood, sometimes it can be used to enhance a project too. Wood turners will often turn wood that is "green" then allow it to slowly dry so the piece does not crack. When the piece is dry (and hopefully it did not crack) it can twist into some interesting shapes.
Working with wood is never an exacting science, as much as we wish it was. The best way to know what the wood is going to do that you are working with, is to know what the moisture content of that wood is, and for that you will need a moisture meter. There are basically 2 types, the pin type and the surface read type. The pin type is designed to to have 2 pins poked into the wood and a reading is taken. This system works well, especially if you are testing wood that may be slightly dry on top, but could be quite wet inside, like wood at a mill that is stored outside or in open out door facilities. The direct read type, reads the top structure of the wood for moisture content and is equally accurate in the right conditions. This type would be more useful for woodworkers who are checking wood at a lumber store or for wood that has been kiln dried and is now stored and sold from an indoor facility.
Either way ... knowing the moisture content of your wood is key to know what it is going to do in the future and how the wood will move. Knowing this will help you plan your projects and avoid situations where mood movement will cause you repair problems in the future
Copyright - Colin Knecht