This paper was published to be given out during the 'Woods' lecture on 30 April 2007,
during the annual Association of Professional Piercers (APP) convention.
This is not intended as an instructional guide to how to make your own wood plugs.
Please do not contact us for advice about making body jewelry;
all of the information we have decided to make public is already posted to this website.
If you see any errors in the data, please let me know. THANKS!
Basic Wood Anatomy and Behavior
by Erica Nicole Skadsen
We may use wood with intelligence only if we understand it.
-Frank Lloyd Wright
This paper discusses the special case of wood, as it is becoming a more popular material for body piercings, it is a complex substance and thus a vast subject, its characteristics have been largely misunderstood or ignored, and it has been specifically singled out as a body jewelry material that cannot be gotten wet. Stainless steel and titanium for use in body piercings have been presented down to their molecular structure for piercers to understand the materials that they are using more deeply, and it is long overdue for wood to be understood and appreciated on a similar level.
Overall Structure of Trees and Wood
Trees are divided into two broad categories:
• Gymnosperms are conifers/needle-bearing trees with unenclosed seeds such as pine, cedar, spruce, and fir (and the extinct trees that produced amber). Most are evergreen, but larch looses its needles every year. Wood from gymnosperms is referred to as being softwood. However, this term does not necessarily correspond to actual hardness; for example, yew is a hard softwood. Gymnosperms have longer fiber lengths than angiosperms.
• Angiosperms are broad-leafed plants whose seeds are enclosed in fruit. They can be either monocotyledons (one seed leaf and parallel leaf veins, such as orchids, grasses like bamboo, or coconut palm trees), or dicotyledons (two seed leaves and branching leaf veins). Trees such as birch, maple, oak, walnut, and cherry are all dicot angiosperms, and wood from these is referred to as hardwood. This is again a misnomer, as some hardwoods are actually quite soft, such as balsa. Most are deciduous, but holly, for example, and some tropical angiosperms may not loose their leaves seasonally. Vascular tissue from the monocot angiosperms (bamboo and coconutwood) has a different structure, and is not considered wood.
The central part of a tree is a small pith, surrounded by primarily dead xylem tissue, which is the wood, divided into the central heartwood and the exterior sapwood. Water moves upwards through the sapwood in the tree. Older sapwood cells become filled with extractives and stop transporting water, eventually becoming heartwood. The proportion of sapwood to hardwood in a tree varies by species and growing conditions. The sapwood is surrounded by a very thin layer of live cambium cells near the surface of the tree, where cell division takes place, growing both new sapwood xylem inwards, and new phloem outwards. Live phloem cells, sometimes called inner bark, surround the cambium layer and transport water and dissolved nutrients in the form of sap primarily down the tree, but it can move in either direction. The outer bark is composed of dead phloem cells.
Tree rings are comprised of a set of both annual growth rings: the springwood (early wood), which is a wider and lighter area, and the summerwood (latewood), which is darker and more compact. In tropical wood, no discernable rings may be present as growth is continuous, and not seasonal; wood with these conditions is said to be even-grained.
Wood has anisotropic properties, that is, its properties vary by a specimen's directions, which mainly affect very large pieces of wood (timber) and explain various phenomena such as warping when dried too quickly. The three planes of direction, and their corresponding level of dimensional changes are:
• Tangential - most shrinkage:
flatsawn; plainsawn; slash sawn; flat-grained; side grained; circumferentially; wood cut parallel to (on a tangent with) the tree rings; does not pass through the pith; wood cut with growth ring orientations from 0°-45° with the surface.
• Radial - about half the shrinkage:
quartersawn; edge grained; radially from the pith; wood cut parallel to the wood rays; like the radius of a circle; ideally 90°, but any wood cut that has growth ring orientations from 45°-90° with the surface.
• Transverse - very minimal shrinkage (for example, only 0.1-0.2% from green to oven-dry on average, though wood from limbs or juvenile trees may have higher shrinkage, up to 2%):
longitudinal; wood cut parallel to the grain; vertical or lengthwise; end grain; perpendicular to the stem axis; cross-section; as viewed on a stump or at the end of a log.
• The phrase 'perpendicular to the grain' or 'across the grain' is used when referring collectively to both the tangential and radial directions.
Wood is a hygroscopic material, meaning that it can take on and release liquid water and moisture vapor from the air. Wood has been engineered by nature to absorb, release, transport, and store water.
Note that end grain of wood absorbs and lets off moisture at a much higher rate than face grain, which is why rough wood often has its ends sealed with wax, glue, or plastic. Most wood plugs are manufactured so that the transverse section is oriented as the front and back faces (you would be looking at a cross-section of the tree when in use), and thus expansion and shrinkage of the wearing surface is anisotropic, but changes in length are very minimal.
Grain is a misleading term unless specifically defined, as it can have dozens of meanings depending on the context. Common examples include (but are certainly not limited to):
raised grain (surface finish), end grain vs. face grain (surface direction), even grain vs. uneven grain (contrast between earlywood and latewood), straight grain vs. wavy grain (how the longitudinal cells are aligned), closed grain vs. open grain (size of pores), close grain vs. open grain (size of growth rings), and coarse grain vs. fine grain (size of tree rings aka the growth rate).
Types of Wood Cells
Wood cells have a primary wall and three secondary walls of varying thickness, bound at middle lamella.
The cellular voids are the hollow centers of the cells, and are also known as lumens or cell cavities.
Lignification starts at the lamella and proceeds inwards, eventually filling all the voids, and the cell dies.
Living nutrient storage and distribution cells in the phloem and sapwood are called parenchyma. These thin-walled rectangular cells contain protoplasm inside when they are still living. Hardwoods have a greater parenchyma cell proportion than softwoods. Lateral cells called rays conduct sap horizontally. Most of the parenchyma in softwoods are in its rays. Rays represent a weaker area in wood and can be an area more prone to checking or splitting, but can also provide interesting and distinctive figure in wood.
Heartwood xylem tissue contains non-living cells called prosenchyma, which primarily provide support and are longitudinally oriented. Types of prosenchyma include:
• Tracheids - long empty tubular conduction cells; these have larger diameters and thinner walls in springwood, but smaller diameters and thicker walls in summerwood. 90% of softwood tissue is composed of tracheids which transport water up the trunk and also impart mechanical strength. The relative diameter of the tracheids determines texture in softwoods. Some ray cells are tracheids. Small pits (or pores) in the walls of tracheids let sap to flow from one cell to another. Only some hardwoods contain small diameter tracheids, where they primarily give support instead of transporting water.
• Fibers - small diameter, elongated cells that have closed and pointed ends. They are darker due to thicker cell walls and provide strength. In sapwood, some still contain and move sap (water with dissolved nutrients). Softwood fibers are generally much longer than hardwood fibers.
• Vessels - large diameter cells with thin walls that transport water, found in hardwoods only. When cut transversely, they are called pores. The pores form long tubes with open cell wall ends. Their relative diameters impart the aspect of woods termed coarse or fine textured (sometimes called open or closed grained). Vessel lines (also known as surface voids) are when these vessel cells can be seen cut open lengthwise with the naked eye, as in walnut, zebrawood, paduak, or mahogany, and may be large enough that you can actually stick your fingernail into!
A tangential surface of zebrawood enlarged 3X, showing long open vessels.
Some of these pores are filled with whitish grains of sawdust.
Wood has extremely complex chemistry that can vary widely from species to species. Wood has been described as a "biopolymer composite" (Rowell 2005). Hardwood cell walls are composed of several basic types of matter, the percentages of which vary by type of wood, location, and other factors, but averages (for oven-dry wood) taken from several different sources seem to fall within these generalized ranges:
• 40-50% Cellulose
• 20-35% Hemicellulose
• 15-30% Lignin
• 1-30% Extractives (usually under 10% in domestic woods)
• <1- 2% Ash (inorganic minerals and other miscellaneous compounds)
Cellulose is a carbohydrate that is the primary ingredient of plant cell walls; it is "the most abundant organic substance on Earth." It is made of carbon, hydrogen, and oxygen, with a chemical formula of (C6H1005)n (for reference, C6H1206 is basic sugar). Cellulose is a polysaccharide composed of up to thousands of glucose molecules (monosaccharide monomer carbohydrate units) linked end-to-end, forming a long linear chain called a polymer. These cellulose polymers form twisted strands called microfibrils by hydrogen bonding to each other. An imperfect crystalline matrix forms, with the non-crystallized amorphous areas being weaker, hydrophilic, and more prone to degradation.
Hemicelluloses are also biological polymers formed from polysaccharides, but they form shorter polymer chains, they are made up of several different monosaccharide/carbohydrates (such as pentose and hexose sugars) instead of just one, their form is branched, and they are amorphous (not crystalline). Hemicelluloses include xylan, glucomannan, xyloglucan, mannan, galactomannan, galactoglucomannan, galactan, arabinoxylan, arabinogalactan, glucuronoxylans, and glucuronoarabinoxyalans. Hemicelluloses are more hygroscopic than either cellulose or lignin.
Lignin is another type of three-dimensional biological polymer, called an aromatic (containing benzene rings) or phenylpropanol polymer. Lignin bonds adjacent cells of cellulose and hemicellulose together and creates strength. Lignins are difficult to break down and remove from wood and are undesirable to the paper pulping industry as a result. They are brown, which explains the color of most woods and unbleached paper.
Pectins are carbohydrates made up of several heterogeneous complex polysaccharides classified as homogalacturonans, rhamnogalacturonans, and substituted galacturonans. Pectins are largely in the middle lamella of wood cells (though they are also in the cell walls, more so in other non-wood plant tissues) and serve to bind the cells together and regulate water dispersion. Pectin has been called a hydrogel.
Extractives include deposits of a huge variety of organic substances classed as secondary metabolites; instead of providing growth, structure, or nutrients, they provide protection from insect, pathogenic, or fungal attacks, and also give wood species their characteristic color, taste, or scent. The irregular content throughout heartwood of pigmented extractives, along with unique anatomical characteristics, is what gives wood its distinctive figure.
Extractives are named as such since these substances can be extracted (solubilized) from wood with organic solvents like water, acetone, alcohol, benzene, and ether, without affecting the remaining structure of wood (the cellulose, hemicellulose, and lignin constituents). Extractives are non-structural substances that have infiltrated the wood; they are not chemically attached to the cell walls of wood, though they may be within the walls.
Extractives can provide chemicals beneficial to humans (for example, taxol, the powerful cancer-fighting substance from yew), but may also cause health problems (such as allergic reactions) as they are essentially toxic biocides made to defend the wood from attack and decay.
Extractive content varies by wood species, season the tree was felled, and the conditions under which it grew or was seasoned (dried). Besides decay resistance, extractive content also affects properties such as hardness, density, compression strength, flammability, and level of hygroscopicity (liquid permeability). The amount of extractives, especially wax, in heartwood can affect how quickly a wood gains or losses moisture.
Heartwood extractives can in and of themselves make wood appear darker than softwood, some can darken further upon exposure to light, but others can contribute to fading of a wood's color in light instead. These effects might be seen within minutes (for example, katalox wood is rather purple when it is first cut, but quickly becomes dark brown once cut), or years to become apparent.
Two samples of chakte kok wood from the same board:
the left shows a transverse surface that has just been cut and sanded;
the right shows a rough tangential surface left untouched for years that exhibits considerable fading.
Some extractives that lend color may be water-soluble, a common example being tannins, which are red-brown. This attribute may cause wood to darken in a moist environment as the extractives move towards the surface of the wood, and may leave a stain on a surface they have come into contact with after being wetted.
Extractives include, but are not limited to:
aromatic and coloring agents, oils, essential oils, fats, resins, waxes, gums, fatty acids, resin acids, salts, alkaloids, antioxidants, proteins, starches, sugars, glycosides, phenols, flavonoids, ellagitannins, tannins, other polyphenols, ethers, alcohols, ketones, fatty acid esters, sterols, steroids, stilbenes, stilbenoids, pinosylvin, rosin, terpenes, turpentine, carboxylic acids, aldehydes, polycyclic compounds, and non-volatile hydrocarbons.
Tyloses are cellular ingrowths that can plug up the vessels in hardwoods, blocking liquid water from flowing through these pores. This affect a wood's abilities to transport moisture, making them more difficult to treat with preservatives, but are beneficial in that hardwood with many tyloses can be made into water-tight barrels (such as for wine-making). Tyloses are mainly found in ash, hickory, and oak (especially white oak), and are also found in chestnut, black locust, and osage orange.
Inorganic elemental minerals found in wood include: calcium, potassium, and magnesium, with lesser or trace amounts of manganese, silicon, phosphorus, sodium, iron, copper, zinc, and possibly trace amounts of others. These are called ash-forming minerals because they are left when wood has been burned.
Wood and water
Moisture content (MC)
Moisture content is the total amount of water in wood, found as either:
• Free water in the cell cavities, plus
• Bound water within the (capillaries of) cell walls.
The maximum moisture content a wood can have is when both the cell cavities and the cell walls are completely saturated. Moisture content can be over 100% because wood can contain more water by weight than its own weight alone due to the amount of free (unbound) water in the wood. Wood that has been recently cut from a tree is called green wood. Sometimes any wood with a moisture content above fiber saturation point (discussed later) is also called green wood, even if that wood had been previously dried. Initial moisture content in heartwood can vary drastically from species to species. For example, the initial moisture content of heartwood taken from osage orange can be a low 31%, while that of black cottonwood can be 162%. It can also vary in wood from different parts of the tree. Usually heartwood has a much lower moisture content than sapwood, but occasionally the reverse is true (as with birch, cottonwood, oak, walnut).
Technically wood cannot ever have zero moisture content, as there will always be a minute amount of water that is bound within the walls of the wood so greatly that it can never by removed. However, oven-dry is a term applied to wood without any measurable amount of water remaining. Wood is considered oven-dry once the weight of the wood when being heated at 101°C-105°C (214°F-221°F) remains unchanged. This can take around a day for even very small (1" or so) sections of wood.
Moisture content is the weight of water in wood divided by the weight of that wood when oven-dry.
(green or initial weight of wood) - (oven dry weight) X 100%
Moisture content = _____________________________________________
oven dry weight
Officially, several techniques for measuring moisture content exist, and are described in ASTM D4442. Moisture content can also be quickly measured with varying degrees of accuracy with electric moisture meters, which usually work best for moisture contents of 30% or less.
Two main methods of seasoning wood are used to slowly bring the moisture content of wood down from its initial moisture content when green, to a moisture content that is close to that of the environment in which it will be used. These include kiln drying and air drying, which can take months to complete.
Hardwood to be used indoors is usually brought down to a moisture content of around 8%, but this can vary from around 6% for arid regions like the Southwest of the US, up to 12% for humid regions like the South. Hardwoods used in outdoor applications can be air dried to 12-20%, with an average for the States of 12-15%. The maximum moisture content usually considered dry is 19%. Kiln drying usually goes down to around 10%. However, wood might be technically kiln-dried for just a short period of time to reduce water content and in doing so decrease the weight of large batches of wood before shipment. Consequently this kiln-dried wood might not even have a moisture content much less than the fiber saturation point!
Density and Specific Gravity
Density is a measurement of mass over volume. While mass is technically the amount of matter and energy, wood density is usually measured as weight over volume, technically called weight density.
A wood's specific gravity is a measurement of relative density: it is the ratio of the weight of the specimen to the weight of an equal volume of water; in other words, the ratio of the density of the specimen to the density of water. The density of pure water is set at one gram per cubic centimeter (1.0g/cm³ at 4.4°C or 40°F, which converts to 1000 kg/m³ or 62.4 lb/ft³). Therefore, wood with specific gravities higher than 1 will sink in water, and wood with specific gravities less than 1 will float in water. The spectrum of wood density ranges from miniscule in the case of balsawood, which has a specific gravity of around 0.11-0.19, to various extremely dense woods from several genera referred to as ironwoods, with specific gravities approaching 1.4. Since wood cell walls have a specific gravity of around 1.54, this represents a number that cannot be exceeded.
Since specific gravity is a ratio, it is a relative, unitless measurement, which eliminates confusion as it will be the same number regardless of the measuring system used to derive it. There are only minute differences between density and specific gravity, which can be ignored by woodworkers and the two can be used interchangeably. Technically, the absolute density of an object in a vacuum will be slightly greater than that object's density measured under the atmospheric pressure of air (since the air lends a tiny amount of buoyancy). Specific gravity eliminates these nuances, since the object being measured is in the same atmospheric conditions of pressure and temperature as the water it is being compared to.
The density of wood will vary based on its moisture content at the time of measurement. Density measurements are usually based on the weight of wood that is oven-dry, and the volume measured when the wood is green. However, the moisture content used to derive the density should be specified, as volume when oven-dry, an average, or some other moisture content below fiber saturation point can also be used. The amount of heartwood or sapwood in the sample will also affect the measured density, and there can be around a 10% variation in density within different samples of the same species of wood.
Density gives a measure of how much actual substance is inside a specific sample of wood. Since a greater density means there is more structural material, denser woods are stronger, harder woods. Denser woods also have a greater capacity to shrink and swell due their greater content of cell wall material which can hold more bound water. However, there will be smaller cellular cavities (lumens) in woods with high densities, so the maximum green moisture content possible at saturation will be smaller in these woods as there is not as much space available for free water within the cavities.
Fiber Saturation Point (FSP)
Fiber saturation point is the moisture content when cell walls are fully saturated with water (they are at their elastic limit), but there is no free water in the cell cavities. The dimensions of wood remain unchanged as green wood loses moisture until it reaches the FSP, which in most species occurs at around 25-30% moisture content. "In species having a high extractive content (for example, redwood and mahogany), the FSP will be noticeably lower, around 22% to 24%. For those low in extractives, such as birch, the FSP might range as high as 35%" (Hoadley 2000: 113). Once the wood becomes less moist than the FSP, the volume of the wood begins to decrease (it shrinks). Conversely, the volume of dry wood increases (it swells) in the presence of moisture until the cells reach their FSP. So, wood only shrinks or swells below its FSP. Added moisture after the FSP is breached is considered 'free water' (not bound) and exists in the cellular cavities, and does not cause any more increase in volume. The change in volume "is dependent on the density of the wood."
A good analogy of how wood holds water is to imagine it like a sponge. A dry sponge is like wood that is oven-dry. Place it in a plate with a small amount of water, and the moisture is wicked through the walls, just like bound water, and the sponge grows in size until the walls hold as much water as they possibly can (FSP). Immerse it in a bucket, and the spaces within the sponge also become saturated with water (like free water), but the sponge does not grow any further in size.
The size of the cellular cavities stays around the same in green versus dry wood, though cell wall thicknesses will vary once moisture drops below the FSP. Once the moisture content exceeds the FSP, free water is in the wood. The free water (in the cellular voids) above FSP does not cause swelling or shrinkage (changes in volume or area) of wood when wet, only the fluctuations of the bound water in the cell walls below FSP do. Interestingly, it is only in free water that fungi can start to deteriorate wood.
An interesting phenomenon occurs when drying thick pieces of wood below FSP. The center core of thick wood dries much more slowly than the outside area (the shell). Thus the shell cannot shrink as it normally would below FSP and is set larger than it should be. As the core becomes dry, it attempts to shrink, however, the shell prevents it from doing so. If the tension exceeds the strength of the wood, small cracks called checks will develop to relieve the stress these actions cause. Checks are separation of wood cells which usually follow the plane of the rays; in contrast, shakes are oriented parallel to the growth rings and are more rare. In rare cases, the forces of compression can exceed the strength of the wood, and internal buckling within the core portion of the wood may occur, called collapse, causing severe distortion.
The uneven rate of shrinkage means that technically wood can begin to shrink even though the average moisture content for the piece of wood as a whole is not yet below FSP. Luckily, small pieces of wood are far less susceptible to the problems encountered in drying than larger, thicker pieces of wood, which need far greater time periods to dry.
Uneven shrinkage during drying caused by the anisotropic properties of wood can also cause checks; wood shrinks about twice as much in the tangential direction than the radial direction. Longitudinal shrinkage is minimal enough to not be problematic and is not considered a factor when dealing with the tricky art of drying wood without damage.
Relative Humidity (RH)
Relative humidity is the ratio of moisture (water vapor) currently present in the air to the maximum amount of moisture it is capable of holding if fully saturated at the same temperature:
Current air moisture (water vapor)
RH = ____________________________________________
Maximum amount of moisture the air is capable of holding
(if fully saturated at the same temperature)
For example, if the air could hold twice as much moisture than it currently does, the relative humidity would be said to be 50% (1 representing the current level, divided by twice as much used as the maximum). Any further rise in moisture content beyond 100% (air that is fully saturated) - for example if the temperature is quickly lowered though the absolute humidity level remains the same (making the relative humidity level rise) - will force moisture to collect as condensation on surfaces or fall as precipitation; this is called the dew point. This is demonstrated when moisture droplets form on the outside of a beverage container removed from refrigeration in the Summer.
Warm air can hold more moisture than cool air, and air cannot contain more moisture than its dew point. In Winter, since cold air cannot hold as much moisture, and moisture in air that is cooling condenses and becomes precipitation, absolute humidity is thus low. In heated buildings in the Winter, the temperature of the air goes up. In air that is getting warmer, if absolute humidity stays the same, the relative humidity will go drastically down. That is why many people experience dried sinuses in the Winter. The low relative humidity of heated indoor spaces is one of the main reasons why body jewelry made from wood and other hygroscopic natural materials is more prone to cracking in this season.
The opposite can occur in Summer. Warmer temperatures can hold more moisture. In air conditioned buildings, the air holding a certain level of moisture is cooled, which means it has a lowered capacity to hold that much moisture. Consequently the relative humidity rises dramatically, unless some type of dehumidification system has been installed. Buildings with open doors or windows will have air moisture levels around the same as the outdoors, which will have varying levels of humidity depending on weather, time of day, and region. Climate-controlled environments (heated or air-conditioned rooms) may experience fluctuating levels of temperature and relative humidity if heater or air-conditioner settings are reduced when a business is closed.
It is interesting to note that the relative humidity of the surrounding environment is not the same as the relative humidity at a surface, in this case, a piece of wood jewelry. Also note that relative humidity is not the same as moisture content. As the relative humidity of the surrounding environment changes, it gradually affects the amount of bound water in wood, and thus its moisture content. This is not an instantaneous effect. When wood finally maintains a balanced moisture content at a given relative humidity, it is said to be at equilibrium moisture content (EMC).
Equilibrium Moisture Content (EMC)
The nature of wood is such that it will always strive towards equilibrium with its environment. This is why is takes on and gives off moisture. The point at which wood neither loses nor gains moisture under a constant temperature and relative humidity is called its equilibrium moisture content. Achieving EMC is a gradual process, and can take about two weeks for small pieces of wood, though species naturally containing wax will take much longer. The rate of swelling is quicker in small pieces of wood; however, the end result of the swelling at EMC will be the same. Different species of woods and woods with differing densities will eventually achieve the same EMC over time when kept in the same environment.
"The solid line on the graph represents the curve for white spruce, a typical species with fiber saturation point (FSP) around 30% EMC. It is a fair approximation for most common woods...
(V)ariation due to extractives or temperature will...be most pronounced toward the FSP point end of the curve...
(T)he EMC is about one percentage point lower for every 25°F to 30°F elevation in temperature" (Hoadley 2000: 113).
Note that even with a very high sustained humidity of 90%, the moisture content of wood will be brought up to around 20%, still far below FSP. Water vapor in the form of humidity affects the amount of bound water in the wood only.
Using green wood means that the wood is still loosing moisture, and has not yet reached EMC. The wood can check, crack, split, or warp, as it attempts to reach EMC. These types of defects will have already occurred in wood that has been carefully dried, and can therefore be avoided or worked around; this is why seasoning of wood is so important. The ideal moisture content of a piece of wood should be as close as possible to the EMC of the environment at which it will be used. This can be achieved by kiln drying, or naturally over a longer period of time by air drying (dried to an EMC with its environment).
It is extremely important to note that wood will not remain at one moisture content, even if it has been previously kiln or air dried, if there is any flux in environmental relative humidity, as naturally occurs daily and seasonally. As a result, EMC must be thought of as a short-term state, or calculated only as an average through larger periods of time. The only time that wood can be said to be 100% stable dimensionally is when it has become petrified!
How does wood become wet?
Wood takes on water (water is said to be fixated in the wood) by several methods:
• Absorption is by capillary action in mass flow of liquid water entering the wood's cellular cavities. It becomes free water. Free water can move to adjacent cell cavities in the direction of the fibers due to pits in the cell walls. This capillary action is quicker than the process of diffusion.
• Adsorption is the diffusion of water vapor into cell walls from the air. It becomes bound water. The amount of adsorption that can occur depends on the density of a wood; dense woods have a larger amount of internal surface area, so has a higher level of adsorption.
• Moisture diffuses from the voids into the cell walls where it becomes bound water. The mass flow of water into the voids then into the cell walls is the quickest of these processes in getting water into the cell walls.
• Bound water (moisture chemically held within the cell walls) diffuses by osmosis within the cell walls to other cell walls within the wood that initially had a lower concentration of moisture.
Sorption can mean the gaining of water by either process, while desorption is the loss of water (drying).
How wood gets wet can be envisioned the way water moves up a wick. In green wood, water evaporating from the surface creates a capillary force on the free water below the surface, causing it to flow towards the lowered moisture content of the surface. Low relative humidity increases this capillary flow.
The cell wall polymers of wood (hemicellulose and non-crystalline areas of cellulose) are made of saccharides, which are rings of 4-5 carbon atoms and 1 oxygen atom, with attached -OH (hydroxyl) groups. These hydroxyl groups have polarity that easily form hydrogen bonds with ambient moisture because water is also polar in nature. In other words, wood is hydrophilic. Only the amorphous non-crystalline parts of cellulose (around 40%) bond with water. My publication Expanding Inward (available online soon) has a similar discussion of the way horn acts as a material in proximity to water and other chemicals.
The sponge analogy is also useful when thinking of dry versus wet wood strength. A dry sponge will be stiff, and a wet one flexible. Wood is weaker when it has a higher moisture content because its cell wall polymers have formed hydrogen bonds with the water. When it has a lower moisture content, hydrogen bonding takes place between adjacent cell wall polymer chains instead, which imparts strength and rigidity to the wood.
Since drier wood is stronger, it can withstand lower levels of relative humidity and higher drying temperatures during the seasoning process progresses without straining it to the point of checking or cracking. However, since very dry woods are more rigid, they may become brittle as they lose elasticity.
Wood exhibits a complicated phenomenon called a sorption hysteresis loop. It entails a diminished response in going from a dry to a wet state (sorption), compared to going from a wet to the dry state (desorption); the ratio is about .85 between the two achieved equilibrium moisture contents. This effect is more pronounced in the first desorption wood goes through from initial moisture content when green, compared to subsequent desorption cycles in aged wood. The longer wood is kept in an environment (even with regularly changing conditions), the higher the ratio will become - approaching 1. The following is meant to serve as a theoretical example of this concept; stated values are for illustrative purposes only:
Two pieces of wood from the same tree are in a room together at a certain relative humidity, say 70%.
The first piece is green (above FSP), and is experiencing desorption (drying). It will achieve a 15% EMC.
The second piece is oven-dry, and is experiencing sorption (getting wet). It will achieve a 12% EMC.
See the image: Hysteresis loop of wood, given in:
(Garcia Esteban, Gril, de Palacios de Palacios, & Guindeo Casasus 2005: 27).
See also (Wood Handbook 1999: 3-8).
Weathering encompasses the long-term cumulative degrading affects on wood of air, light, water, heat, and mechanical damage (such as dropping the material or abrasion by wind or dirt). These factors normally only come into play when unprotected wood is used continuously in exposed outdoor areas over many years, and will have little to no effect on wood used in indoor applications.
Lignin is photo-degraded by light (especially UV light) quicker than the other cell wall polymers. This decreases its water resistance, and can result in a buildup of yellowish to brown color. The effects of degradation by light in the visible wavelengths usually follows, which cause lignin to become grayish or bleached in appearance over time. This occurs only on the surface of the wood in shallow depths from 0.05-0.50mm. Cell walls on the end grain of wood becoming separated at their middle lamella, along with lowered lignin content, results in loosely attached and hydrophilic cellulose and hemicellulose being more prevalent on the surface of the wood. The decreased degree of crystallization in cellulose mean these amorphous areas bring more available polar hydroxyl groups to surface, therefore making it become more wettable. The cell wall polymers can be broken down by hydrolysis. Hydroperoxide, quinone, carbonyl, and carboxyl groups are formed on the surface of the wood by the oxidation of free radicals created by irradiation. Water-soluble extractives that lent decay resistance and color are leached out of the wood.
Despite all of this, the actual extent of degradation by weathering has been estimated to be around 3mm per hundred years in hardwood. Insect or fungal intrusions being the cause of degradation in wood are far more likely since wood is more vulnerable to these attacks when sustained in moist environments.
Wood and Temperature
How quickly wood gets wet or dries is affected by temperature; warmer temperatures increase the rate of swelling. The rate of chemical reactions that can cause deterioration can rise dramatically with increased temperature and in high relative humidity. The strength of wood increases in lower temperatures, and weakens in higher temperatures: "air-dry wood changes in strength by an average of 2% to 5% for every 10°F change in temperature" (Hoadley 2000: 95). This is normally a reversible process, except when wood is exposed to extremely high temperatures for long lengths of time, such as exposure to hot or boiling water, which can cause permanent structural damage in the form of a large decrease in levels of strength-giving crystallized cellulose and intact hemicellulose, additionally making more cell wall structure accessible to water. The reverse is not true; freezing will cause only temporary loss of strength. Thus, use in extremely hot environments, overheating during the manufacturing process, and attempting sterilization using techniques that involve high heat can be seen to permanently damage the structure of the wood.
Other measurements of how wood reacts to temperature include:
• Coefficient of thermal linear expansion or contraction: how much wood will change dimensionally per degree change in temperature. Wood expands when heated and contracts when cooled. This value seems to be species-independent in the longitudinal direction, ranging from 0.0000017 - 0.0000025 per °F change. In other words, even with a drastic 100°F change in temperature, a piece of wood with a 1/2" length will change by about .000085"-.000125" longitudinally (becoming that much shorter or longer in length). Steel length would change by about three times that amount. However, perpendicular to the grain direction (radially or tangentially), the change corresponds to the species' specific gravity, and may be 5.5 - 10.2 times more than the longitudinal coefficient. Still, that is less than a thousandth of an inch (about a .000824" average) difference in diameter in a 100°F temperature change if a 1/2" plug is cut so the transverse section is showing at the ends. Note an extremely minute amount of ovalness will ensue due to differences in radial and tangential coefficients.
However, as temperature changes, the humidity changes that normally occur along with it will make a comparatively much more significant change in its size. In fact, except in very dry woods (with moisture contents of less than around 4%), prolonged heating will cause wood shrinkage due to loss of moisture content, affecting its dimension much greater proportionally than the coefficient of thermal expansion.
• Coefficient of thermal conductivity (K): the rate wood conducts heat from itself to what it is in contact with (air, skin, etc.). K is measured as: "the number of British thermal units (Btu) that will flow through a material per hour, per inch of thickness, per square foot of surface area, per °F difference in temperature from the warmer to the cooler side" (Hoadley 2000: 103)! Woods with larger specific gravities or moisture contents will have greater conductivity.
• Insulating value (R): the inverse of conductivity (1/K), a measurement of resistence to conduction, commonly encountered when purchasing insulating material for walls and ceilings. Measured perpendicular to the grain, oven-dry wood is around 300 times more insulating than steel, and around 5 times more insulating than glass. Wood with a higher moisture content will be less insulating as it contains more water, which is more conductive. Insulating values of wood measured parallel to the grain (longitudinally) will be 1/2 to 1/3 of the perpendicular to grain insulating values, and wood is less insulating in the radial direction than in the tangential direction. In other words, wood is most conductive in the transverse direction, and slightly more conductive radially than tangentially.
• Coefficient of thermal diffusivity: the rate a wood can absorb or release heat for the environment, which is technically "the ratio of thermal conductivity to the product of density and heat capacity... A typical value for wood is 0.161 X 10-6 m²/s (0.00025 in²/s) compared with 12.9 X 10-6 m²/s (0.02 in²/s) for steel" (Wood Handbook 1999: 3-17).
These factors explain why wood feels less hot or cold against the skin than materials like metal. As other species and various materials will all have different values for these coefficients, it is no wonder that disparate inlay materials can easily warp or pop out in changing environmental conditions unless a very strong yet flexible adhesive is utilized, slight gaps are given as clearance to accommodate for these changes, or a mechanical means of holding a setting can be used, such as with a bezel or prongs.
No finish can be 100% effective in preventing moisture transmission in wood. Surface treatments can slow the rate of moisture loss or uptake, but changes in moisture content (and resulting dimensional changes) CANNOT be fully prevented over time, and wood will always strive to reach EMC. There are several approaches to finishes that can be taken:
• Surface treatments (film-forming or coating): varnish, urethane, shellac, lacquer, paint. In wooden body jewelry, the treatment would then be the surface that comes into contact with the skin, not the wood.
• Penetrating finishes (within and beyond the surface): oil, resin, resin-oil blends, wax impregnation, chemicals, preservatives, stains, and water repellents. Some oils must be chemically thinned to penetrate beyond the wood's surface, where they fill the wood's pores and cellular cavities, and then harden to differing degrees. Oils are classed as drying (such as linseed or tung oil - neither of which are normally used for finishing wooden body jewelry) or non-drying (such as vegetable or paraffin/mineral oil). Water and scratch resistance is raised and degree of absorption and wicking is lowered compared to untreated wood. Water repellents are hydrophobic treatments that retard the uptake of liquid water into the cell lumens of the wood by changing the contact angle between liquid and solid at the surface, but they do not have an effect on the uptake of water vapor, which can still wick through the cell walls, so relative humidity is still able to change the moisture content.
• No finishing products: untreated wood.
Applying multiple coats of a finishing product is crucial to its effectiveness. The end grain of wood (transverse surfaces) absorb the most water, and make a critical difference to the resulting quality and longevity of wood, so these are often coated throughout the seasoning process and in finished products to prevent rapid loss or uptake of moisture.
Solvents (other than water) appear as ingredients in many traditional wood finishes; other examples could include ingredients in chemicals used in an attempt to clean or sterilize wood. Solvents can make wood swell more than water due to their ability to soften, plasticize, or make soluble cell wall polymers. Some solvents may remove natural extractives in wood that normally slow wood's rate of change in response to moisture; others may have a greater ability to form hydrogen bonds than water, causing weakened wood. Solvents can also alter the color of wood.
Many other factors should be considered, including effects of a finish on pigmentation, toxicity and environmental concerns, use of products containing ingredients derived from petroleum (such as mineral oil) or animal sources (such as beeswax, buffing compound, or stearate used to lubricate most sandpapers), amount of leaching of a substance applied as a finish into the skin or environment, frequency of re-application, and the durability of a finish (including the effects of mechanical, temperature, and moisture stresses, as well as the breaking down of oil finishes such as olive oil which can go rancid).
A good immediate test of the durability of a finish on a wood plug is to get one piece out of a pair wet. After it dries, compare the quality of the surface to the unmoistened piece. Any discernable change will indicate the quality and appropriateness of the finish for short and long-term use in real-world environments, since fluctuations in humidity levels are unavoidable.
How wood body jewelry is manufactured before a finishing product is even applied can drastically affect the overall quality of the finished piece in terms of smoothness, the ability of a finishing product to adhere or penetrate the wood surface, and the responses of a finished piece in the presence of moisture. Obviously any large scratches, pits, tool marks, or other inconsistencies should not be present. Sanding can cause dust to fill the pores of the wood, especially causing problems when the sanding direction is across the grain as is most often the case with wood body jewelry oriented with the end grain as the faces; successive grits of sandpaper creates larger numbers of infinitely finer minute scratches upon the surface of the wood. Using chisels or gouges on a lathe is a form of scraping, which can result in torn and rolled tissue filling the cellular cavities. The sharper the scraping device, the more completely it can shear the cell wall structures. This could only truly be achieved by a blade with the thickness of a razor. Thus regularly and continuously sharpening tools is critical to a smooth finish and eliminates many of the problems associated with unremoved torn and frayed tissue.
Aspects of wood dependent upon moisture content, presence of extractives, and type and quality of finish:
• Coefficient of friction and sliding: slipperiness. Increased moisture content will cause increased friction, until the fiber saturation point is reached; wood at or above FSP has low friction due to the presence of free water. Wax or oil extractives and finishing products will decrease friction and increase slipperiness.
• The quality of luster (shininess) is the way a surface reflects light. Some woods, such as granadillo, may even reflect light in narrow bands, called chatoyancy after the French words for cat's eye. Though unrelated, it is interesting to note that other materials used in body piercing jewelry, such as tiger's eye, or more rarely horn, may also exhibit chatoyance. Buffing wood is sometimes used to increase the shine on a surface, but not only imparts compound onto the surface and into the pores of the wood, but is all too often used as a quick substitute for proper and careful finishing, and the level of shininess will instantly disappear once the wood has come into contact with moisture.
There are many other factors that come into play when considering wood as a material for use in body piercing jewelry. Some of these include:
• Weight: large plugs or shapes in very dense woods can have considerable weight.
• Appropriateness of overall shape and dimensions, including sizes that are too thin to be durable.
• Use of adhesives, including their bonding ability, moisture resistance, durability, toxicity, and possible use of animal-derived ingredients.
• Possible allergic reactions to certain wood species, which is covered by the "Wood Hazards" paper (also available online at organicjewelry.com/woodhazards.html).
• Endangered wood species (see www.cites.org). This important consideration is also discussed on my website here. As wood goes by many confusing common names, knowledge of Latiny names (scientific names) - genus and species - becomes helpful.
While these aspects are not discussed further here, their importance should not be dismissed.
High relative humidity, high temperatures, and exposure to UV light (including daylight, fluorescent lights, and especially black lights) can all speed up the deterioration of wood. Ideal conditions to decrease the rate of deterioration in wood include stable moderate humidity (around 30-40%), a cooler environment (at room temperature maximum), and limited exposure to light. While controlling all of these factors may be impractical in the realities of daily use, they may be brought into practice when wood is not in use, for example, in a shop or at home, whether on display or being stored.
One way to buffer the rate of humidity change is via the 'isolation' technique, such as keeping the wood in a plastic bag (the polyethylene plastic commonly used in packaging body jewelry has been recommended by woodworkers) or another airtight container such as a glass jar. Coating wood with a finish achieves a similar effect. This concept is something to consider not only when the wood is not in use, but also when one first receives a shipment of new wood; it could be kept in an airtight container to allow it to more slowly adjust to its new environment.
Consider the ambient conditions in which the material is being displayed or stored. Locations close to heating vents, direct sunlight, air conditioning units, leaky windows, de-humidifiers, or other environmental controls might not be the best choice. Be mindful of the materials chosen to display natural body jewelry on in a display case that may absorb too much moisture (such as rice). A hidden tray of water with a large surface area, rather than simply a cup of water inside a display case might help to increase humidity levels. Reapplication of a finish may be necessary upon receipt of natural materials, as shipping through different climates can in and of itself cause stress, and repeated as needed; frequency will be dictated by the environments in which they are kept.
Understanding the nature of wood, including its structure, chemistry, and properties, can help body piercers and their clients to more fully appreciate wooden body jewelry, and to understand some of the complex wood science behind how conditions of manufacturing, storage, and usage can affect wood's stability, condition, and longevity. With careful application of this knowledge, wood can continue to provide one of nature's most valuable and intriguing materials used to create body jewelry for millennia to come.
Helm, Richard F., Ph.D. "Wood/Bark Extractives." Notes part of WOOD 3434: Wood Chemistry, Products and Processes, Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA.
Rowe, John W. and Conner, Anthony H. "Extractives in Eastern Hardwoods-A Review." General Technical Report FPL-18, Madison, WI: Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, 1979.