Moisture in Wood
Structure of Wood
The cellular structure of wood explains how water is held in wood and determines what happens as wood dries with shrinks and distorts while drying.
Trees do not grow by pushing the trunk upward out of the soil, although as the trunk's girth increases soil is pushed aside so that it looks as if the tree has pushed upward through the soil. Trees grow tall and their crowns spread when new living cells are formed at the tips of the trunk and branches. The trunk and branches increase in girth when other cells are formed in a layer just beneath the outer bark. Some of these cells will make cell walls that become wood and others will become the inner layer of the bark and function to transport sugars between the leaf and root systems. These inner bark cells will die and slough off with bits and pieces of bark as the tree expands over several years. To illustrate this idea take a balloon and partially inflate it. Coat it with paint and let it dry. Then inflate the balloon further. The paint will crack and be shed as the balloon expands. Bark is shed from trees as they mature for the same reason.
In climates with distinct wet and dry seasons, such as spring and winter in North America, the cells that are added under the bark differ in size, being larger in spring when plenty of water and food from photosynthesis are available, and smaller when it is dry and photosynthesis is reduced due to cooler temperatures or leaf loss. If you look at the end of a log or the stump from which it came, a pattern of concentric circles is seen, reflecting the annual differences in cell sizes. By counting the rings, the age of the tree can be estimated.
The cross section to the left is from a 20 to 25 year old tree. At the very center note the pith, a remnant of the stem from when the tree was a seedling. Surrounding the pith are the concentric growth rings of woody cells. The inner most ones are darker than the outer ones due to the accumulation of what are called extractives over the tree's lifespan. The darker wood is called heartwood and outer lighter colored ones are the sapwood. Heartwood does not conduct any materials from the roots to the leaves, only the outer layers of sapwood do. Heartwood functions primarily in structural support of the tree. The rich colors of heartwoods varies by species; those of walnut, cherry or oak, for example, are prized for furniture making. Extractives deposited in heartwood offer some protection against the invasion of bacteria and fungi through openings in the bark caused by limb shedding or injuries.
Heartwood and sapwood are composed of cell walls, an external skeleton that a living cell secreted around itself. The living cell dies within a few years of making the cell wall, but the wall can persist for thousands of years if a tree continues to live, such as has been documented in redwoods or pinion pines, or if the wood is preserved in some way. Photos taken through a microscope, such as those below, show us the shape of these empty chambers in pine wood.
None of the chambers shown here contains a living cell; all are simply remnant cell walls. Nonetheless, the chambers are usually referred to as cells, a terminology that I will use in the rest of this discussion. The colors seen in these photos are due to dyes that were used to improve the view through a microscope.
Photo A is a cross section of a piece of wood, equivalent to what you look at when you look at the end of a log. This view clearly shows the seasonal differences in cell size, with the cells in between the dark partial rings corresponding to one year's growth.
Photo B is a radial longitudinal section, equivalent to what you would see on the surface of a quarter sawn board. The cells running horizontally in the photo and indicated by arrows are radial ray cells. They are responsible for lateral transport of materials in the trunk from the outer edge inward toward the center.
Photo C is a tangential longitudinal section, equivalent to what you would see on the face of a tangentially sawn board. The long narrow cells would have conducted water and minerals from the roots to the leaves.
If these three views are integrated, one develops the concept that wood is actually a collection of closely packed elongated cells, much like a handful of soda straws held tightly together with a few straws passing radially across the main bunch of packed straws. A pine 2 X 4 would contain billions, if not trillions, of such "straws." When a tree is cut down, the "straws" are filled with water which gradually evaporates as the wood dries.
Figure source: USDA Wood Handbook: wood as an engineering material. 2010. Chapter 3. Structure and Function of Wood.
Eastern White Pine
Modern techniques, such as scanning electron microscopy, illustrate the "packed straws" concept. The photo to the right is a scanning electron microscope image of a small block of eastern white pine cut so that the anatomy of the wood is revealed from the three perspectives found in cut lumber: cross, radial and tangential.
Softwood species such as pine have wood consisting of mostly of one type of cell (called tracheids). What appears to be a large cell on the cross section face is a resin duct made of many smaller cells.
When the wood was formed, a living cell secreted the cellulose and lignin that make up the remnant cell walls seen in the photo. When the wood is in a tree or freshly cut into boards, the space (called the cell lumen) surrounded by the cell walls is filled with liquid water called free water. Closely bound to the cell walls are thin layers of water molecules called bound water. When wood dries, free water evaporates first and the bound water is lost.
Northern Red Oak
The photo to the left is a scanning electron microscope image of a small block of red oak cut so that the anatomy of the wood is revealed from three perspectives found in cut lumber: cross, radial and tangential.
Hardwood species such as red oak have wood that is more complex than softwoods because it is composed of several cell types easily seen in the photo. Most distinct are the large cells (called vessels) joined end to end somewhat analogous to how steel barrels (minus the ends) might be welded together to form a pipe. The smaller cells are tracheids, as are found in soft woods.
Note the horizontally oriented cells below the label "Radial face." These are rays and conduct water radially across the width of the trunk. In several species, especially oaks and sycamores, the rays are well developed and show up as prized figuring in quarter (radial) sawn lumber.
Cell walls are a composite material composed of cellulose fibers embedded in resinous materials called lignin and hemicelluloses. The fibers provide strength when wood flexes and the resinous materials resist compression in the same way that iron bars in concrete provide flex strength and the concrete resists compression. If wood is completely dried so that it contains no free or bound water, 45% of the dry weight is due to cellulose, 20 to 35% is due to hemicelluose, and 15 to 35 % is due to lignin. Less than 1% of the weight is due to minerals such as calcium, magnesium and silica salts left behind when the water evaporated.
Water in wood
When a tree is harvested and freshly cut into lumber, the wood is saturated with water and is often called green wood. Around 40 to 50% of a fresh cut board's weight is due to this water, with the remaining weight representing the cell walls making up the wood fibers. The total water content has two components; free water fills the lumens of the empty cells as liquid and vapor; and bound water (also called hygroscopic water) is chemically bonded by hydrogen bonds to the cellulose and lignin in the cell walls as hydration shells. As a board dries after being cut from a log, free water evaporates first, initially from the cells on the surface of the board and then by diffusion from the inner cells to the board's surfaces. Such evaporation is speeded by low humidity and high temperatures. Loss of free water does not generally affect the strength of wood or cause any shrinkage. When all free water has evaporated, the board is still not dry. Bound water molecules in hydration shells remain because they require a little extra energy to escape their hydrogen bonds. When a board has an approximately a 30% moisture content only the hydration shells remain, and the moisture content is referred to as the fiber saturation point for the wood. Any drying below this moisture content will cause the wood to shrink, more so in some species than in others.
Bound water slowly evaporates until an equilibrium is reached with the amount of water that can be contained in the surrounding air. This capacity of air to hold moisture is called relative humidity and is dependent on temperature. Air that has a relative humidity of 100% at 50o F cannot hold any more moisture at that temperature. If the temperature of that same air is raised say to 70o F, the relative humidity drops and the air can take up more moisture. That is why relative humidity is often higher at night than in the day. The temperature changes, but the amount of water present in the air does not. Outside in natural environments both temperature and relative humidity vary with seasonal and geographic weather conditions. Inside buildings, the climate is usually controlled so that temperatures are usually in the range 60o to 75o F due to heating and air conditioning and relative humidities do not fluctuate as much.
If a board is placed in an environment at 70o F with a relative humidity of 70%, the board will lose bound water until it comes into moisture equilibrium with that environment, assuming the addition of the water to the environment does not change the relative humidity (which would only occur in small, closed environments). At this equilibrium point every time a water molecule breaks its hydrogen bond with cellulose, it is replaced by a water molecule coming from the air. The water content of the wood at this time is called the equilibrium moisture content (EMC). Such an equilibrium is a two-way street. If the board is now placed in an environment with a lower relative humidity (which can be achieved by raising temperature), the board will lose water until a new equilibrium is reached. Conversely, if the board is placed in an environment with a greater relative humidity, it will gain water from the air and the hydration shells around its cellulose fibers in the cell walls will increase in thickness. This give and take of water molecules goes on for as long as the board exists and can only be prevented by encasing the board with a substance that is not permeable to water, such as certain high quality paints that are properly applied. On average a board kept outdoors at Providence, Rhode Island would have a 14.6% equilibrium moisture content in September. If brought indoors to make a table, it will drop to 6% equilibrium moisture content by the end of the winter heating season only to rise again as doors and windows are open until the next heating season. A plain sawn maple table top that was about 30 inches wide would contract and expand nearly 3/4 of an inch as a result.
Dry lumber is never completely dry and will always have some bound water attached to its wood fibers except under laboratory conditions of 0% relative humidity or temperatures above the boiling point of water (212o F). If 0% moisture content wood were produced and then moved outside or to a living it space, it would take on water from the air until the EMC was achieved with the relative humidity of the air, only to change again as humidity changes. For this reason, wood is sometimes described as "living," although it is not in the real sense of the word.
Some suggest that kiln drying would prevent this. It does not. A board that was dried in a kiln to 6% moisture content, or even less, and then placed outside in Providence would have 14.6% moisture content by September.
The water that is bound to the cellulose and lignin in the cell walls of the wood takes up space. Thus a board that has moisture content of 30% will shrink if its moisture content decreases. In a heated house in winter the relative humidity can be as low as 30% and over several months wood in the house will come into equilibrium with the relative humidity. Cracks may appear in furniture surfaces or in molding joints as the wood shrinks. The same furniture or joints in the summer, when indoor relative humidities may be above 70%, will not have cracks as the wood takes on moisture and expands.
Shrinkage and swelling do not occur equally in all directions as a board dries. Longitudinally, or along the length of the grain, shrinkage is minimal in the range 0.1% to 0.2% of the length and is considered of no consequence by builders and furniture makers. For example, a 10' long board will only change about 0.1" to 0.25" when dried in an oven to 0% moisture content. The greatest dimensional change occurs along the plane of the growth rings or tangentially. Tangential shrinkage from the fiber saturation point to oven dry can be 2 to 10% of the width of a board. This means a plain sawn board 12" wide at 30% moisture content from a species that shrank 10% would only be 10.8" wide at 0% moisture content. If 30% moisture plane sawn boards were laid edge to edge in building a floor, large gaps would develop as the boards came into EMC with the air in the house. Wood also shrinks radially or across the growth rings, but not nearly as much as tangentially. Radial shrinkage is the range 2% to 6% or about half that of tangential shrinkage.
The differences in amount of shrinkage radially and tangentially distorts the shape of wood as it dries. Tangentially cut, quarter sawn or rift sawn boards will distort differently as shown in the diagram below.
Tangentially cut boards tend to cup with the concavity developing on the surface corresponding to the longest annual growth rings.
Rift sawn boards tend to dry out of square.
Quarter sawn boards tend to remain square as they dry except if they include the pith. If high moisture content quarter sawn lumber is used to make round stock such as a dowel, it will dry to become an oval.
Although it seems trivial to mention, a freshly cut board dries from the surface inward. Consequently, cells on the surface may have lost all of their free water and some of their bound water before before the innermost cells have lost any significant amount of free water. This mean that cells on exposed surfaces are shrinking while the internal cells are retaining their original volume, with the resulting forces tending to pull the surface cells apart at weak points. This leads to a condition known as checking. If you ever took a wet board and laid it in the sun to dry, such as when an unpainted deck is saturated with rain and then dries, the checking can be severe. A similar situation is seen when logs are left to dry in the round. Longitudinal surface cracks appear as the outer cells dry and shrink while the inner ones retain their moisture and volume.
Page prepared by Warren D. Dolphin