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The following information is intended to provide a flavour of the basic science that you can apply to the use of timber/wood. I intend to be adding to it on a regular basis and for those interested in finding out more there is a list of recommended reading although you could also search for 'Wood Science' in your search engine-of-choice.


Wood surrounds most of us although we rarely give that statement much thought. It will be in the fabric of the buildings we live or work in, you may be sat on some as you read this and if you are taking notes you may be writing on a derivative of wood - paper. Most of the time we take wood for granted, after-all it does grow on trees! Unlike other building materials, such as steel, concrete, glass and plastic, timber is a natural renewable product; just plant a sapling and wait!. Being a natural material it displays natural variability. Whilst this results in a material that is aesthetically and texturally appealing this variability can be detrimental where uniformity and consistency are the preferred criteria. Perhaps this is the reason we still use timber so extensively today; to contrast the stale uniformity found in other building materials?

It is easy to assume that timber's diminished role has resulted from advances in the development of other building materials and that it is now only to be used for adornement. Historically, timber was a utilitarian material used for all sorts of construction. This is not surprising as it literally grew all around us (unless you live in the desert!). Population growth and expansion ultimately led to the loss of forests through exploitation and/or other land use so construction and production needs often tended towards the use of other increasingly available materials. Thankfully though, the reliance on timber was only diminished, not negated. In reality there are very few instances where timber is not a suitable choice of material; expediancy has become the norm and today it is more typical to use a plastic bucket that took seconds to produce over a wooden bucket (which would possibly be prohibibitively expensive to mass produce now?)

When you think about it timber was never intended to be used in the way it is. As a plant material it's sole purpose is to support and elevate the food-producing leaves towards the sun. Mankind (and its' precursors) took advantage of this resource long before we had developed language or the desire to record our own history. Human ingenuity developed the usability of this material and eventually 'experience' led to a greater appreciation and understanding of the technical aspects of this resource. We think of concrete, steel, glass etc as being 'engineered' materials that demand a greater degree of understanding; science. Whislt this is certainly true it is often a surprise to many that the same scientific principles used to study other materials have long been utilised in the study of timber, from how it grows to where it can be used.

Unlike other materials, timber is readily used by keen amateurs, skilled craftspeople and technically trained professionals alike. Whilst this reflects the utilitarian nature of the material it can be a double-edged sword as timber is a material that is often quite easily mis-used, because of a lack of understanding of it's technical properties, or over-used (and subsequently discarded) because of the whims of 'fashion'. A little understanding goes a long way and the following information is for those who wish to gain a better insight to where this material comes from (yes, trees), why it isn't all the same, why it works and also why it fails (most often because we use it where we shouldn't!). Also, it may just be curiosity, or it could be a source of useful information but sometimes it is necessary to know not just that it 'is' wood but 'what' wood it is.

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A note on terminology

Wood is a cellular composite material that is principally made up of individual fibres. The phrases 'grain','texture' and 'figure' are often used interchangably when referring to wood but, technically, describe different aspects of the characteristics and appearance of wood.

Grain refers to the orientation of the fibrous component of the wood and is indicative of the working properties of the wood; a wood that is 'straight-grained' will be easily worked (with tools) whereas a wood that has 'interlocking grain' will be more dificult to work.

Texture refers to the relative sizes (or different sizes) of the fibrous component of the wood; wood with a 'coarse texture' (such as oak) has a broad range of sizes of fibrous material within the wood and can be coarse to the touch. Wood with a 'fine texture' (such as beech) has more uniformity to the fibrous component and will feel smooth to the touch.

Figure is a reference to the orientation, arrangement and abundance of individual cell components of the wood and is a method of describing the visual appearance of a piece of wood. Whilst there are defined descriptive categories used to classify figure it can be used subjectively to describe any difference from the norm that makes a piece of wood more visually appealing to the end-user.

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Wood as a material

wood axes Wood is a ubiquitous material and is used in a multitude of ways. A chair, for example, can be made from any type of wood, as can a table and the building it would sit in. In reality though, whilst it is desirable to have furniture made from a hardwood such as oak, it is likely to be restrictive as a general building material on the basis of cost, unless of course you are intent on building an oak barn-type dwelling. Cost (and availability) is often an important consideration when deciding on a particular type of wood for a specific use but there are often more technical reasons for using one type of wood over another; saunas typically use pine for the walls but spruce for the ceilings as it contains less resin which could fall on you in a high temperature environment. Fence post made from pine will give many years of service because of the natural durability of the heartwood whereas posts made from spruce will have a very short life as their heartwood is perishable (sapwood of any tree is perishable so would require treatment with a wood preservative. However, the sapwood of many trees is untreatable, especially in spruce) . Wine casks made from white oak will retain the wine but would leak if it was made from red oak – but they are both oak? English archers required yew from a certain part of the tree to make the best bows etc.

Many of these examples are the result of empirical knowledge. If you relied on wood for your livelihood (or defense) it was important to know that wood from a specific tree was best suited for a specific purpose and this information passed down the generations. It was only relatively recent (within the last century) that we discovered the reasons behind these choices through the scientific study of wood as a material. With such knowledge it is possible to utilise a wider range of timbers for specific uses. This can be beneficial economically as it can reduce the possibility of over-exploiting a specific resource and also make use of previously under-utilised resources. Additionally, a technical understanding of how timber performs can facilitate it's modification for use in areas beyond it's historical usage. Despite the utopian dream of the paperless office one of the main uses of timber throughout the world (next to fuel) is in the production of pulp for paper. Not all trees are suitable stock for pulp production and fibre yields are low per tree yet it may be possible to increase both problems within a short period of time given recent advances in other fields of study; it could be feasible to determine the way a tree grows so it produces timber with 'designed' characteristics that are for specific uses, such as papermaking.

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What's in a name - would you know your wood

When we talk about wood it is normal to use a reference system of 'common', or 'vernacular' names, such as oak, ash, elm etc. Historically, craftsman would only have access to trees that were local and such names would suffice. When timber from further afield became available to craftsmen wood names took on regional associations; English oak, French oak etc. With the spread of colonialism exotic and far-flung places provided ready access to new resources and in order to make this new wood saleable it was often given a 'name' that accurately reflected it's origin, but not necessarily it's real identification, eg African oak, Tasmanian oak etc. Such timbers may have taken on similar vernacular names because their working properties, or their colour or hardness were similar to timbers that were previously known or used. International trade often relied on the use of these vernacular names when selling timber but unfortunately this often resulted in a wealth of mis-information, which was sometimes intentionally deceptive. The familiarity that came with the use of vernacular names often came at the cost of 'caveat emptor'!

Botanical nomenclature

Written clasification systems have been around since the early Greeks and today we still base identifications on the readily available examination of flower and leaf structure, although the recent addition of genetic information is being used to refine these classifications. The term 'species' is readily understood to define, broadly, a member of a unique biological group [a precise definition of a 'species' is open to debate]. In everyday language we may think of examples such as cats, dogs, oaks, elms etc. However, in scientific terms the word 'species' takes on a more defined meaning. Although the species concept was in general use, in 1753 a Swedish botanist by the name of Linnaeus published 'Species Plantarum' which would form the basis for botanical nomenclature and would be adopted for the classification of all living organisms. Species Plantarum built upon his earlier work which introduced an artificial system of classification which grouped living organisms on the basis of observable similarities, such as number of petals or stamens (rather than, eg, colour). He proposed a system of nomenclature that remains in use today; the binomial system.

Nomenclature is simply an organised format for recording information and is often represented as a tree, which is apt. At the root level of classification, living organisms will have a unique two-part name (hence 'binomial'); the 'generic' name (the genus) and a seperate 'specific' epithet (the species) to identify it as unique within the group. Related groupings of 'genera' (plural for genus) can be organised into 'Families', similar Families can be grouped into 'Orders', Orders combine to form 'Classes', Classes combine to give 'Divisions' and Divisions group into 'Kingdoms'. We are familiar with terms such as 'Plant Kingdom' or 'Animal Kingdom' but less familiar with the intermediary classifications. To take an example, eg, oak;

    Kingdom: Plantae

    Division: Magnoliophyta

    Class: Magnoliopsida

    Order: Fagales

    Family: Fagaceae

    Genus: Quercus

    Species: robur

In general use it is normal to use just the genus and species epithet when referring to the scientific name of a species. It is always written in Latin and the genus is always capitalised, but not the species, and usually written in italics (or underlined). So, for example, English oak is described botanically as; Quercus robur. Quite often it is only necessary to refer to a general group rather than a single species so this will be written, eg, Quercus spp, where spp indicates that there are multiple species within the genus.

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When is oak not oak

When we talk about wood it is normal to use a reference system of common names, such as oak, ash, elm etc. We can also add more specific information, such as English oak, American oak etc but problems can arise when common names become the basis for trading timbers from exotic places, eg African oak, Tasmanian oak etc. Place an order for these four oaks and you will recieve four different looking pieces of timber. Two of them will look broadly similar as they are botanically related but the other two will bear little resemblance as they are botanically unrelated.

Now, being familiar with the binomial system, our four 'oaks' are, in fact, the following species;

    English oak (Quercus robur)

    American oak (Quercus alba)

    African oak (Tectona grandis)

    Tasmanian oak (Eucalyptus gigantea)

As you can see, the genus Quercus (the real oaks) only accounts for two of the samples whilst Tectona is commonly known as teak and Eucalyptus is a diverse group of plants generally known as eucalypts. When using a botanical name it is important to get the genus correct and for general purposes it suffices to use the specific epithet spp where the actual species identity is not known or not relevant, eg oaks are generally referred to as Quercus spp whereas English oak specifically refers to Quercus robur.

In reality it would be a very unlikely that you would be sold teak if you asked for oak but if you are unfamiliar with a timber that is being sold on the basis of a vernacular or trade name it is wise to enquire about it's botanical name first. When it comes to harvesting trees in the forest it would be impractical to identify each and every tree so often timber can belong to a number of species of tree and may be traded as such, eg white oak or red oak. Whilst it is possible to identify the tree to a unique species the timber produced by a number of very related species can be indistinguishable. The examination of flowers/leaves/fruit/bark etc is required to derive a species level identification. Examination of a sample of wood (secondary xylem) can be used to identify the tree that produced it but it is generally only possible to identify it to the level of sub-family or genus, eg white oak group (Quercus spp), ash (Fraxinus spp) etc. There rarely exists sufficient information in the xylem structure to detect species level identification (unless the genus only has one species, eg Gingko biloba).

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When is softwood not soft

Just as common names can give rise to confusion, so too can the common terms 'softwood' and 'hardwood'. Balsa (Ochroma pyrimidales) is a 'hardwood' but it is very soft (and light) whilst Yew (Taxus bacatta) is a 'softwood' that is very hard (and heavy)? In general terms the use of 'soft' and 'hard' do give some indication to a timbers characteristics but when you closely examine balsa and yew you can see very distinct differences in the cell types that make up the woods. This is not unexpected as they are the product of the evolution of two botanically distinct groups of vascular plants that seperated and subsequently evolved different forms. Going back to our system of classification, above, plants which produce wood belong to three different 'Divisions' of the seed-bearing land plants in the Plant Kingdom.;

Angiosperms are subdivided into the dicotyledons (dicots) and the monocotyledons (monocots) but only the dicots produce wood which is [technically] 'hardwood'. Timber producing flowering plants (Angiosperms) are a very diverse group, the oldest of which date back to around 120 Mya (Magnolia spp) whilst the oaks are relative new-comers (~ 60MYa).

'Softwoods' belong to the Gymnosperms (so-called because they have a naked seed) and have been in existence for around 250 million years (MYa). As they are the evolutionary predecessors of the angiosperms their wood (secondary xylem) is of a simpler form and is more uniform in appearance than that found in angiosperm wood, which has a greater diversity of form.

Whilst not very common, the Gingko tree , of which there is only one species known to exist (Gingko biloba), can produce large quantities of timber.

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What's in a tree?

Wood - you'd think it grew on trees

Seed plants produce a variety of growth forms of two general types; herbaceous (annuals, biennials and perennials) and non-herbaceous (woody) Growth originates from meristematic tissue, which are sites where new cells can divide and multiply. Initial growth from apical meristems (tips of root and shoot) produce three primary meristems; protoderm (which develops into the epidermis in leaves roots and shoots), ground meristem (which develops into parenchyma, collenchyma and sclerenchyma), procambium (which develops into primary xylem, primary phloem and vascular cambium) and the deposition of each type will determine the growth form of the plant.

In herbaceous plants (in which the stem can wilt at the end of the growing season) the stem is composed of a core of ground tissue with discrete bundles of vascular tissue around its perimeter and is surrounded by an epidermis which prevents the loss of water and the ingress of pathogens. The vascular bundle comprises two tissue types (primary xylem, primary phloem) which straddle a meristem called the cambium. The xylem tissue is responsible for transport of water from the roots to the leaves and faces towards the pith of the stem. The phloem, just below the epidermis, transports the carbohydrates produced in the leaves via photosynthesis back down the stem where it used for subsequent growth of the plant.

Woody plants are perennial (long-lived) and although their leaves and flowers may not persist throughout the year, their stem does because it is capable of repeatable secondary growth. Like the herbaceous form, a woody perennial initialy lays down discrete vascular bundles but with subsequent growth the seperating cambium layer begins to coalesce and eventually forms a continous band encircling the xylem bundles. As the cambium begins to coalesce, so too does the xylem and phloem. When a new growing period (season) begins it initiates in the cambium which divides laterally producing cells that form new secondary xylem (wood) internally and new secondary phloem towards the bark; predominantly more xylem is laid down than phloem. If growing in a temperate climates, where growth is seasonal, a 'growth ring' of wood is deposited in the tree which increases the girth of the stem (trunk). Because of the increase in circumference of the stem a simple epidermis would continually fracture so a second lateral meristem (cork cambium) produces new cells that develop into cork which produces the required containment of water and nutrients as well as the exclusion of pathogens.

Although monocots are a sub-division of the angiosperms they are not capable of secondary growth. Their vascular tissue takes the form of discrete bundles which are similar to those found in herbaceous plants but are scattered throughout the stem, rather than just the periphery. They will not be discussed further. Subsequent text will focus on the development and form of a specific form of woody perennial; the tree. Many plants form plants form 'wood', which is simply a chemically modified tissue which will be discussed later. Trees are defined as a perennial plant that has a single persistent stem (or trunk) which consists of secondary xylem. Subsequent text will relate to the secondary xylem found in trees only.

The trunk - in support of wood

The stem of a plant fulfills three important roles;

The first role is visibly obvious but the other two roles take place at the level of the individual cells contained within the stem. Secondary xylem contains cells that fulfil these important functions in both types of woody plants. The major difference between the gymnosperms and the angiosperms (strictly dicots) is that the former uses two cell types whilst the latter has evolved a third type of cell that is unique to the angiosperms. A cursory glance at a piece of each type of wood shows there is a difference in the cell types. Gymnosperm wood, which is more uniform in structure, uses a long slender cell called a tracheid to provide both structural support and conduction of water. Angiosperms, which are an evolutionary offshoot from the gymnosperms, have evolved a division of labour between the requirements of support and conduction and have seperate cell types for both; fibres provide support and vessels provide conduction pathways. Because of their evolutionary paths tracheids can be found in many angiosperms although they are relatively sparse in comprison to other cell types. The role of storage is catered for by cells known as parenchyma in both wood types.

Because they are the product of convergent evolution, the xylem of gymnosperms and angiosperms share the three functional requirements the plants need for survival (support, conduction and storage) but they do so in two different ways. The course of evolution can be traced through examination of these tissues and it can be seen, for example, that tracheary elements found in gymnosperms are also found in primitive angiosperms. This essentially means that at a basic level wood is a similar material, whether softwood or hardwood, and that differences exist because of the diversity afforded by evolution, not sky fairies.

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Growth rings

The continual deposition of xylem in a healthy tree is seen in a cross-cut stem as concentric 'growth rings'. Where climate is seasonal the rings indicate periods of growth and dormancy and are often called ‘annual rings’. This can be misleading as trees that grow in a tropical climate can give the appearance of continuous growth without showing discernible ring boundaries. Where microclimates exist the tree may deposit one to many rings in a year depending on the conditions that prevail. In temperate regions, where growth can be assumed to follow an 'annual' cycle, the cambium will start producing xylem (and phloem and cork) once conditions become favourable during early spring and will cease production in the late summer, prior to leaf fall and dormancy for the rest of the year. Deposition of xylem is a result of general growing conditions and will reflect the availability of water, nutrients, light and temperature. These are generally abundant throughout the spring and growth will generally be vigorous whereas towards the end of summer some or all will become limiting and xylem deposition will decrease and eventually cease. The formation of the ring is similar in both gymnosperms and angiosperms from a functional perspective and to the naked eye can look similar in appearance. Under the microscope the only difference will be the respective cell types and form that each utilise to achieve this function.

So what does the tree need To recap; support and water conduction and each has a different cell type to fulfil theses requirements. Gymnosperm wood is evolutionarily primitive to that of the angiosperms and has predominantly one cell type that performs the dual role of support and conduction; the tracheid. Angiosperms have evolved a ‘division of labour’ and has two basic cell types; the ‘vessel elements’ provides conduction, whilst the ‘fibre’ provides the support. In a tree (plant) it is vital to have cells that provide the essential functions. At the other end of the spectrum - where we make use of the wood that comes from those trees, those cell types become relevant as they can determine where and how a piece of timber can be best utilised, or where it's use should be avoided.

The growing tree

At the onset of growth in the spring, the leaves that develop require water to start producing foodstuffs via photosynthesis. Most of the water that passes up the trunk will be simply lost through transpiration but the basic requirement is for a high volume of water to move upwards in the tree. This is achieved by cells that have a thin cell wall and large internal void space (lumen).

The water-conducting vessels in dicots can be in the order of metres in length in oaks. They are, however, made up of individual, open-ended short vessel elements (generally less than 1mm), that are connected end-to-end in long, continuous structures. Vessels overlap with other vessels along their length within the stem to provide a network of water conduits from the roots to the leaves. Having a network of pathways means the tree can survive the loss of some tissue, such as when lightning strikes or infection breaches the bark.

Conifer tracheids, in comparison, are very different in structure. They are very long compared to their diameter and can be up to 10mm long. Unlike vessel elements, tracheids have closed ends and during formation they overlap and develop a specialised tracheid-tracheid connection (bordered pits) through which water is able to move in the tree.

wood axesIf you look at a cross-section of a piece of wood, at an individual growth ring, with the naked eye you will see a structure that resembles a series of holes. These are the cross-sections of either the vessels (dicots) or tracheids (conifers) and as they resemble ‘pores’, the term ‘porous’ is often descriptively applied to wood (no relation to the capacity for holding water - another story). With dicots, the visible pores are the conductive (vessel) elements of the wood and they will be surrounded by an apparently 'non-porous' tissue which are the non-conducting supporting elements (fibres). Look at the cross section of a conifer and the entire ring looks 'porous'. This is largely true but examine the tracheids further and you will see a difference in the thickness of the cell wall across the ring. This reflects the dual-role nature of the tracheid which functions both as conductive and supporting elements. In a growing season the early-formed tracheids (earlywood or springwood) are thin walled with a large lumen which function predominantly for conduction. As the season progresses, towards dormancy, the cell walls in the tracheids become thicker and hence the lumen becomes smaller and less conductive (latewood or summerwood). In some conifers the transition can be abrupt (Pinus sylvestris) whilst being gradual in other species (Cryptomeria japonica). Trees that grow in tropical climates (dicots and conifers) may appear to lack growth rings. Conversely they may display multiple rings within an annual time period - As the ring merely reflects the periods of growth and dormancy the notion that a ring can be associated with age can sometimes be misleading if dealing with tropical species

wood axes The vessel arrangement in temperate dicots take a slightly different due to the presence of both conducting vessels and non-conducting fibres. Trees from temperate climates will generally form rings in a regular pattern indicating the start of growth and the onset of dormancy, and gives rise to the description of broadly two types of ring structure; ‘ring porous’ and ‘diffuse porous’ (there is an intermediate type, ‘semi ring porous’ but can be subjectively applied so is infrequently used – or is it ‘semi-diffuse porous). Ring porous species such as oak or elm produce very large vessels (visible to the naked eye) in earlywood and very small vessels in the latewood and effectively display a ‘ring of pores’ – hence ‘ring porous’. Not all dicots follow this pattern and in trees, such as beech or maple, there is a more-or-less uniform distribution of vessels throughout the season and the transition from earlywood and latewood is not always apparent. Because of this distribution of pores the wood is referred to as being ‘diffuse porous’

So what about the support In the dicots, the vessels are surrounded by a matrix of fibres. In ring porous trees the proportion of vessel-fibre is high in the earlywood and becomes low in the latewood. With diffuse porous trees the proportion of vessel-fibre is fairly uniform. As mentioned earlier, with the conifers the tracheid performs both roles of conduction and support via the relative size of cell wall-lumen. In earlywood the thin wall provides a large conductive lumen whereas in the latewood the thick wall reduces the lumen volume (conductance) but with the advantage of being mechanically stronger. By this time water is generally a limited resource and the leaves will be falling (in the deciduous species) so there is little need for a large conductive tissue In most trees the ring will contain two regions of growth that may be visible as a gradual or abrupt change in cell structure.

The pattern of growth rings can yield useful information about timelines and general weather conditions (water availability) over the lifetime of the tree. Where conditions for growth are good a tree will lay down much uniform xylem (wide growth ring) but when conditions are less favourable it will lay down less xylem (narrow growth ring), or even none. This is a specialised field called dendrochronology and there is much information available online and in-print.

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Wood density

Wood, like everything else, has a density - it is simply a ratio of mass/volume (kg/m3). Unlike other common materials, such as steel or concrete which have a narrow density range, a set volume of timber can vary greatly in it's mass (and hence it's density) depending on the species. Even timber of the same species can vary in density depending on factors such as age, position in the tree, growing conditions, water availability etc. Commerciallly available timbers can range in density from < 300 kg/m3 to well over 1000 kg/m3 (at which point they no longer float in water). Density is a valuable indicator of a number of physical properties of wood and though there are exceptions to the rule an increase in density is usually indicative of an increase in hardness, strength, natural durability, treatability, machining properties, drying properties and movement. To give an example of where density is not a good indicator is in the natural durability of wood. Timber is a good material for exterior joinery, if the correct species is chosen. Ash (Fraxinus spp) is more than double the density of Western Red Cedar (Thuja plicata) but would not be suitable for use in an environment where it would be regularly wet whereas Western Red Cedar would perform for decades with good maintainance.

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Moisture content

The wood in the table you may be sitting at is just the same as it was when it was part of a living tree - with one exception. In the tree one of it's functions was to provide conduction pathways for water from the roots to the leaves and hence it was saturated with water. The wood in service is dry, to the touch at least, but will still have a certain amount of water within it; it has a moisture content (mc) which may be ~ 8-10%! When it comes to wood, moisture content is measured relative to the mass of the wood WITHOUT any water - what it weighs if you dry it completely.

Mathematically, mc = 100 x ((W-D)/D)

W = the existing (wet) mass of the sample
D = the oven dry mass of the sample

As the moisture content is based on the DRY mass of the wood (NOT the original mass) it is possible for this value to exceed 100% and whilst the in-service mc of wood indoors is generally below ~15% in a freshly felled tree the mc can be in excess of 400% (typical in oak).

Not everyone has access to an accurate balance and a drying oven and in most instances it it not necessary to determine an exact moisture content. An alternative method is to use a moisture-meter which are readily available and inexpensive. They come in two types; a pin-type where a pair of sharp steel pins are inserted into the wood and the resistance between the pins is correlated to the moisture content of the wood. Alternatively there is a non-contact type of moisture meter that uses the 'Hall-Effect' to determine the resistance, and hence the mc of the wood. They are generally more expensive meters though. Whichever type of meter is used there is generally a modification to be made to the reading depending on the species of wood being tested and are unreliable at high mc's (<30%).

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Wood, water and movement

Wood that has been dried ready for use is considered to be a 'live' material in that it reacts to the conditions it is in; in a humid environment it will swell and in a dry environment it will contract. Atmospheric humidity is what the wood is reacting to but the effects will be driven by temperature as well (which will in turn effect humidity levels). The reactions are generally reversible and gradual, mainly due to the application of surface coatings (which can never prevent the effects happening). Also, depending on the species of wood used, the effects may not be noticeable or problematic and good design will accomodate this aspect of wood; conversely, poor design will soon become apparent when environmental conditions change

Technically, wood is a hygroscopic material (it reacts to water), not a 'live' material (it is devoid of cellular content which are lost during xylem formation and the reason genetic 'fingerprinting' of wood is problematic - very little DNA remains). At a chemical level the chains of cellulose that comprise the bulk of the cell have side chains (hydroxyl groups) that can attach to water (and other) molecules. In a standing (functional) tree the xylem functions as a water transport mechanism so the hydroxyl groups are always chemically bonded (they can also bond to themselves if they are sufficiently close and this condition gives rise to a special form of cellulose known as 'crystalline' cellulose).

When a tree is cut down and converted it is necessary to remove most of the water from the wood. Two methods have been used historically; air drying and kiln drying. Both methods utilise the effect of relative humidity (RH) and, in the latter method, is expedited by the use of elevated temperatures. 'Relative Humidity' (RH) is on often-heard but often-misunderstood phrase; at a certain temperature a given volume of air can hold a maximum amount of water in the form of vapour. If you elevate the temperature the holding capacity increases and, conversely, it decreases with a reduction in temperature. If you measure the actual amount of humidity in a given volume of air and express it as a fraction of what that volume could maximally hold you have a measure of the air's RH. In a hot oven you could measure an RH of nearly 0% and in a sauna the air would have an RH of 100%, ie, the air would be 'saturated'.

When wood is air dried it is kept under cover (to protect from rain) but exposed to the air. Depending on your location the air's relative humidity will generally be well below 100% so will have the capacity to accept more water. As the air moves over the wood it will take the water from the surface and the wood will dry - very slowly - from the outside in. In a kiln elevated temperatures are used to speed up the transfer of water from the wood surface to the air (thermal energy) in a controlled process; water removed from the wood surface results in a moisture gradient developing within the wood (drier surface and wetter core). Although wood is considered to be porous there is a limit to the speed which water can move from the centre of a piece of wood to it's surface. If this gradient becomes too steep (attempting to dry too quickly) the wood can become damaged and unusable so it is vital to kiln dry wood in a controlled manner and for most common species a kiln 'schedule' will be available which will provide the necessary conditions of temperature and humidity, and changes therin, to dry wood and retain it's usability.

Irrespective of the method chosen to dry wood, until it reaches a moisture content of ~30% it's dimension will hardly change. Until this point is reached all the water that is being removed is the water that is contained within the cellular structure of the wood - the 'free water' that would have have been cycling throughout the standing tree. Once all the free water has been removed the only water left is that which is chemically attached to the hydroxyl groups within the cellulose chains - the 'bound water'. When all free water is gone and only the bound water remains wood is said to be at it's 'fibre saturation point' (fsp). FSP is generally assumed to occur at a moisture content of ~30% on the basis that a molecule of cellulose (mol wt 168) can hold three molecules of water (mol wt 54); 54/168 ~ 30% mc. However, due to the presence of, eg crystalline cellulose, the actual value of fsp is generally lower and can vary between and within species. Water is an incompressible molecule and it's presence adds physical bulk to the cell wall. Once it is removed, as with further drying, the cell wall will contract in a predictable manner so as the moisture content is reduced so will the physical dimensions of the wood change. As water is removed the mass of the wood changes but the volume also decreases and proportionately the density of the wood will increase, as will it's stiffness and strength.

If you leave a piece of wood in a (relatively) consistent temperature/humidity environment it will eventually reach a moisture content at which there will be no further transfer of moisture to the air - it will have reached an 'equilibrium moisture content' (emc). If you air dry timber in the UK you can expect to attain an emc of 16-20%, depending on seasonal variations. A generally drier environment (such as a desert) would yield timber with a much lower emc. Since the advent of modern heating and better home insulation the conditions that prevail in a typical household throughout a year will result in a typical emc for wood of ~ 8-10%. This is why it is necessary to 'condition' wooden products, such as flooring, in the environment they will eventually occupy prior to use. A given environment may have a 'mean' stable value but this will generally fluctuate over a given time period. As temp/RH fluctuates so will any timber within that environment and these changes are referred to as 'movement' of the wood and within the general environmental fluctuations found in a typical house it's effect may not be noticed once an emc has been reached. If you have a wooden front door you may notice the tenons are slightly proud in summer and flush in winter?

Not all woods have the same degree of movement and within a single piece the movement will vary according to the orientation of the grain. Humidity fluctuations, at the molecular level, cause wood to 'move' but it is because of the cellular configuration of the wood that the dimensional changes are different according to the way the timber was cut. Wood is an anisotropic material - it behaves differently in different orientations. There are three principal axes in a piece of wood; longitudinal (along the grain), radial and tangential (both across the grain). Longitudinal movement is very small (almost negligible) and, in general, tangential movement is approximately twice that of radial movement.

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