Continuing with moisture and wood

I have met many woodworkers who feel temperature is a big factor in the movement of wood. I haven’t been able to find anything to support that idea. It has been proven that wood will not change in size or dimension unless the (MC) moister content changes. It’s important for you to have an idea of how the woods dimension and form will change in relation to the MC. Those facts become key elements in designing, assembling and especially warranting your product. My opinion is this, a " professional" woodworker, who is planning to invest hours of labor and money in a project, should consider the risk involved in not knowing the MC of the lumber. Not knowing is a bit like Russian Roulette. Later I’ll talk about measuring moisture in lumber.

Wood will change dimension the most across the face or across the grain of the board. There is very little movement along its length. You will notice a change on the thickness if the wood is exposed to high moisture conditions. You can really see this in a wood planked floor, especially if a latex finish is applied to it. With many floors you only have to look closely at the joint of the individual wood strips. If you see a uneven face, from one board to the next you can be pretty much assured latex finish was applied. I will give some insights on finish later.

Many different moisture content scenarios can develop with your lumber and or components. Lets start with the wood containing a too high MC right from the start. Assuming the wood is consistent in its MC, lets just use 10% MC. Unless your able to maintain a relative humidity of around 55% the wood you are using is going to give up moister and the net result will be shrinkage, again primarily across the grain. The wider the board \ component the more the net change in dimension. Sometimes a "even" loss of moister can be dealt with. The real problem comes when the wood or component gives up moister in an uneven ratio. A cabinet end panel or counter top for instance will give up moisture as the piece seeks to reach its EMC. Just imagine the dry winter air circulating freely over the out side exposed surfaces while the insides of the components have very little airflow over them. The exposed surfaces will dry and shrink at a rate different then its opposing side. This will result in cupping and warping. Either will normally ruin your day, not to mention your customers outlook.. And should you be working for your spouse, well you get the picture. In these scenarios it is my opinion that you as the cabinetmaker, would be at fault. You might be able to pass it on to your lumber supplier but your customer won’t really care at this point. Remember with this last point the reverse is true should you start with extremely dry wood and allow it to seek its EMC within a damp location.

Another factor to consider is that a woods end grain will absorb or give up, moisture approximately five times faster than the side grain. This results in splits on the ends of boards and panels.. Consider the panel of a raided panel door or possiblly a solid wood tabletop. Both have allot of end grain exposure. Consider our slightly damp wood with a MC of 10%, in a glued up panel which has begun the drying process. What would happen is this, the center of the panel would be stable compared to it’s MC at the time is was glued up, while the wood towards the end of the panel would now be shrinking across its grain as it gives up its moisture quicker the center.. If you could plot the stress on the panel it would look hourglass in its nature. If you’ve ever had a joint open completely and took the time to sight down it you can see the curve of each board as it is narrower towards the end and wider in the middle. With today’s glues the glue joint is usually stronger then the wood so its common you will see the split develop some where in the width of the board.

Hopefully I will draw this together more in the next post and give you some ideas of how to deal with some of the stresses moister causes.