The amount of moisture in wood is one of those hidden aspects of woodworking, where it is usually assumed that everything is fine until something goes wrong. That is why it is important to measure the moisture content of the wood you are working with in order to make sure that it is at an acceptable level. But the relative humidity of the workshop and the final location of the piece also need to be taken into consideration.
One of the most important aspects of working with wood is to let the wood acclimatize to its surroundings. Wood is usually best worked with a moisture content of between six to eight percent. However, the moisture content of the wood will only change when the relative humidity of the surrounding air changes. When wood is brought from one area to another, for example a lumber yard to the shop, the relative humidity of the two locations is most likely different. Especially if the wood at the yard was outdoors. After the the wood is brought to the shop, it must be left alone for at least a day before it can be machined.
Throughout the entire process from lumber to the finished product, the moisture content of the wood should not change more than two percent. This means that the relative humidity of the shop should not be changing by large amounts either. Why would the relative humidity of the workshop change you might ask? The main culprit would probably be due to seasonal changes.
When air is heated, the relative humidity drops unless moisture is added. This is because when air is heated it expands. The expansion of the air is caused by the molecules moving faster and basically making more room for themselves. So, the amount of water molecules that are in a given area in cold air, when that air is heated up, are occupying a larger area.
This means that the percentage of water molecules per area is lower in warm air than in cold air. The effect is that in winter, the amount of moisture that the air can hold is very low.
Here are a few numbers for you:
|30 F||100 % RH||28 % MC|
|40 F||68 % RH||13 % MC|
|50 F||47 % RH||9 % MC|
|60 F||34 % RH||7 % MC|
|70 F||24 % RH||5 % MC|
Without adding moisture to the air in the winter time, a normally heated shop at about 70 degrees will have a moisture content that is extremely low. The relative humidity of a normal indoor area will range in the 35% to 50% area. As you want your wood to become acclimatized to a relative humidity that will be close to the the customer's, you will need to add some moisture to your shop's air.
Adding humidity to a shop is crucial in the winter in order to keep your relative humidity within normal ranges and to keep your wood from releasing too much moisture to the surrounding air. Not only are you to check the moisture content of the wood, but keeping track of your air humidity levels in the shop is also very important.
If all your moisture content levels and humidity levels were within standard ranges and the you are delivering a piece that will also be placed in an area that will have a similar relative humidity then all should be fine. Once the piece is finished (varnished, lacquered, etc), this will act as a retarder of moisture movement into and out of the piece.
Of course the woodworking piece needs to be constructed in such away to have some adjustment of wood movement. This should reassure you that you will not get a call six months into the future when the season has changed that you piece as cracked or split as the wood expanded or contracted.
Lastly, in order to ensure that the humidity levels of the wood is within an appropriate working levels it is important to check the wood with a good quality digital wood moisture meter. Like it seems everything in woodworking can be done in numerous ways, there are various methods of measuring wood humidity, but that would be the topic for another article.