Wood is either classified hardwood or softwood. Hardwood comes from {flowering} broadleaf trees. Softwood comes from {conifers} cone-bearing trees. There are exceptions to this. For instance Balsa and Basswood are hardwoods even though they are extremely soft. The southern pines are softwoods but are moderately hard and much harder than Balsa or Basswood.
The scientific difference is, in hardwoods the pores transport water through the trunk and branches to the leaves. In softwoods these pores are not present the water moves by way of fibrous like cells which conduct water.
As stated above softwood has no pores. Hardwood is either closed or open pored. Open pored means that if you look at it very closely it has fissure like lines. I teach that it is a sin to fill these pores with filler. It takes away part of the woods character and distinction. It would be the same as filling a person's fingerprint, which is one of their distinctive characteristics.
Wood absorbs water from the air during high humidity causing it to expand, during low humidity the water evaporates from it contracting it. After a project is finished this process is called movement in service.
All species of lumber fall into one of four categories of movement: minute, little, average or great. The greatest amount of movement occurs across the grain {width.} Thickness has a lot less movement and the movement lengthwise is inconsequential. The thinner and more narrow the wood the less movement it will have. In fact wood that is less than 2 inches wide and 1/4 inch thick, the movement is minute or even inconsequential.
Coating the wood with a finish will not stop the movement it just slows it down! The finish you choose to use, will determine how long it will take to reach EMC. For instance wood coated with marine spar varnish will take a lot longer than wood coated with tung oil to reach EMC.
The process of reaching {EMC} equilibrium moisture content to be in balance with the surrounding air {not gaining or losing moisture} is a slow process that eventually brings the piece in balance with its surrounding environment.
There are a lot of variables that affect this process. Whether your house is air conditioned with refrigerated air or evaporative {swamp cooler} or a combination of both. If it is heated with gas, electric heat or a fireplace, how well insulated it is and how {tight} losing air to the outside by way of cracks around windows, doors and other openings. If you use a humidifier or dehumidifier system and how much direct sunlight a room receives will also impact the movement of the wood. Even keeping the door closed to a room that has a separate humidifier or dehumidifier will make a difference in the EMC of that room from the rest of house. All houses are different. In North America a lot of houses have an EMC of 6%-13%.
Wood that is kiln dried to between 6%-12% moisture content is extremely stable, 99.9 % of wood defects such as bows, warps, shakes, twists, honeycomb checks and end splits are caused by improper kiln drying. Make sure that your wood is between 6% and 12% and does not contain one of the above-mentioned defects when you obtain it. This will decrease the problems that are caused by using improperly dried wood. If the wood is above 12% do not use the wood for your project until it has reached acceptable moisture content. You can dry small pieces of wood in your gas or electric oven (Watching It Carefully.) Do Not Use A Microwave!
In this day and age owning a good moisture meter is essential to assure that the wood does not have an extreme change after your project has been assembled possibly ruining a future heirloom and costing you a lot of wasted time and money. Another tip to remember is quartersawn lumber is more stable than plainsawn or riftsawn.