Understanding Moisture Content in Wood, Part 1
Anyone who works with wood will sooner or later have to learn about the relationship between wood and the moisture that it holds within. We'll be breaking this tutorial up into two parts. This month we'll be focusing on explaining the moisture contained in the wood. Next month, we'll go into detail about how to determine the moisture content of wood and explain what instruments must be used to get accurate measurements.
Why is moisture content important to woodworkers?
Moisture content is extremely important to woodworkers. As water is gained or lost from a piece of wood, the wood will expand and contract. As the wood changes shape, it can create enough force to crack apart boards, break apart glue lines, and take a once flat piece of wood and twist it into all sorts of shapes. Simply put, not paying attention to moisture content in wood and how it will behave will inevitably cause projects to become defective over long periods of time.
What is moisture content?
Moisture content is the ratio of the weight of water in a piece of wood compared to the weight of the wood itself. Mathematically, this is represented as:
Moisture Content = (Total Water Weight/Total Wood Weight) x 100%
Example: If we have a 10 lb block of wood, containing 2 lbs of moisture and 8 lbs of wood, then Moisture Content=(2/8) x 100% = 25%
What moisture content is best for a finished product?
The best moisture content for wood will be one that is similar to the area in which the final product will be used. For outdoor projects, moisture contents of 10%-18% are suitable (except for in very dry, hot environments). For indoor projects, moisture contents of 6%-10% are suitable in most homes and climate controlled buildings. Wood is constantly gaining and losing moisture to remain in equilibrium with its environment. Matching the moisture content of the wood as closely to that of the place where it will be used will yield the best results.
Understanding types of moisture within wood.
There are two types of moisture contained within wood - free water and bound water. Free water is the water that is able to move through the wood's tissue freely. This water moves out relatively quickly, and is usually all removed once the moisture content of a piece of wood approaches about 30%. While free water is being removed, the wood will retain most of its original shape and size. The second type of water contained within wood is bound water. Bound water is actually part of the physical structure of the wood, and is more difficult to remove. As this water is removed, it causes the shape and size of each wood cell to shrink and distort. Most drying defects occur due to the improper removal of this bound water during the final stages of drying.
Understanding the differences between dried and undried wood.
Wood moisture content can essentially be broken up into three categories:
Green/undried: This wood is freshly sawn. Depending upon species, the moisture content can vary anywhere from 30% up to well over 150% in some woods. Green/undried wood will shrink and change shape as it dries. Projects using green wood typically require an intermediate drying stage in order to bring the wood to a suitably low enough moisture content before final processing.
Air dried: This wood has been dried using conditions present in the atmosphere in which it has been placed. For most areas of the country, a piece of wood that is fully air dried (beware, some vendors market woods as "air dried" even when they have not been given enough time to be fully air dried) will range from 10%-18% moisture content. Actual dryness will vary depending upon temperature, humidity, and air flow around the wood in the area in which it has been stored. Air dried wood is suitable for outdoor projects, but will need additional drying to be used for indoor purposes.
Kiln dried: This wood has been dried within a well-controlled environment. Typically speaking, Kiln dried woods will range in moisture from about 6% to 10% in most cases, and are suitable for use in interior projects.
What are the most important things to learn concerning moisture content?
While volumes of books can (and have been) written concerning the relationship between wood and moisture content, there are some very key points to keep in mind.
Wood always shrinks and expands as it absorbs and expels moisture. Learn to match the moisture content of the wood you're using in your project to the area where it will be placed upon completion, to the best of your ability.
Understand how to check moisture content. While experience may help to reduce the need for using proper instruments and methods to check drying, using the proper methods to determine moisture content will improve the quality of your projects over the long term. We'll cover this in next month's newsletter tutorial.
Hopefully, this will cover the basics of understanding moisture content. Additional research will yield a vast amount of other useful information, but is far beyond the scope of what we can hope to accomplish through our monthly newsletter. Just remember, if you ever have specific questions, we're only a phone call or email away!