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Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood before its use. When the drying is done in a kiln, the product is known as kiln-dried timber or lumber, whereas air drying is the more traditional method.

There are two main reasons for drying wood:

Woodworking

When wood is used as a construction material, whether as a structural support in a building or in woodworking objects, it will absorb or expel moisture until it is in equilibrium with its surroundings. Equilibration (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if equilibration occurs too rapidly. The equilibration must be controlled to prevent damage to the wood.^{[citation needed]}

Wood burning

When wood is burned (firewood), it is usually best to dry it first. Damage from shrinkage is not a problem here, as it may be in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimney. If a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100 °C exhaust temperature, about 5% of the energy of the log is wasted through evaporating and heating the water vapour. With condensers, the efficiency can be further increased; but, for the normal stove, the key to burning wet wood is to burn it very hot, perhaps starting fire with dry wood.^{[citation needed]}

For some purposes, wood is not dried at all, and is used green. Often, wood must be in equilibrium with the air outside, as for construction wood, or the air indoors, as for wooden furniture.

Wood is air-dried or dried in a purpose built oven (kiln). Usually the wood is sawn before drying, but sometimes the log is dried whole.

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Case hardening describes lumber or timber that has been dried too rapidly. Wood initially dries from the shell (surface), shrinking the shell and putting the core under compression. When this shell is at a low moisture content it will 'set' and resist shrinkage. The core of the wood is still at a higher moisture content. This core will then begin to dry and shrink. However, any shrinkage is resisted by the already 'set' shell. This leads to reversed stresses; compression stresses on the shell and tension stresses in the core. This results in unrelieved stress called case hardening. Case-hardened may warp considerably and dangerously when the stress is released by sawing.

Types of wood

Wood is divided, according to its botanical origin, into two kinds: softwoods, from coniferous trees, and hardwoods, from broad-leaved trees. Softwoods are lighter and generally simple in structure, whereas hardwoods are harder and more complex. However, in Australia, softwood generally describes rain forest trees, and hardwood describes Sclerophyll species (Eucalyptus spp).

Softwoods such as pine are typically much lighter and easier to process than hardwoods such as fruit tree wood. The density of softwoods ranges from 350 kg/m³ to 700 kg/m³, while hardwoods are 450 kg/m³ to 1250 kg/m³. Once dried, both consist of approximately 12% of moisture (Desch and Dinwoodie, 1996). Because of hardwood's denser and more complex structure, its permeability is much less than that of softwood, making it more difficult to dry. Although there are about a hundred times more species of hardwood trees than softwood trees, the ability to be dried and processed faster and more easily makes softwood the main supply of commercial wood nowadays.

Wood–water relationships

The timber of living trees and fresh logs contains a large amount of water which often constitutes over 50% of the wood's weight. Water has a significant influence on wood. Wood continually exchanges moisture or water with its surroundings, although the rate of exchange is strongly affected by the degree to which wood is sealed.

Wood contains water in three forms:

Free water

The bulk of water contained in the cell lumina is only held by capillary forces. It is not bound chemically and is called free water. Free water is not in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces. Furthermore, free water may contain chemicals, altering the drying characteristics of wood.

Bound or hygroscopic water

Bound water is bound to the wood via hydrogen bonds. The attraction of wood for water arises from the presence of free hydroxyl (OH) groups in the cellulose, hemicelluloses and lignin molecules in the cell wall. The hydroxyl groups are negatively charged. Because water is a polar liquid, the free hydroxyl groups in cellulose attract and hold water by hydrogen bonding.

Vapor

Water in cell lumina in the form of water vapour is normally negligible at normal temperature and humidity.^[1]

Moisture content

The moisture content of wood is calculated as the mass change as a proportion of the dry mass, by the formula (Siau, 1984):

${\displaystyle {\text{moisture content}}={\frac {m_{\text{g}}-m_{\text{od}}}{m_{\text{od}}}}\times 100\%}$

Here, ${\displaystyle m_{\text{g}}}$ is the green mass of the wood, ${\displaystyle m_{\text{od}}}$ is its oven dry mass (the attainment of constant mass generally after drying in an oven set at 103±2 °C (218±4 °F) for 24 hours as mentioned by Walker et al., 1993). The equation can also be expressed as a fraction of the mass of the water and the mass of the oven dry wood rather than a percentage. For example, 0.59 kg/kg (oven dry basis) expresses the same moisture content as 59% (oven dry basis).

Fibre saturation point

When green wood dries, free water from the cell lumina, held by the capillary forces only, is the first to go. Physical properties, such as strength and shrinkage, are generally not affected by the removal of free water. The fibre saturation point (FSP) is defined as the moisture content at which free water should be completely gone, while the cell walls are saturated with bound water. In most types of woods, the fibre saturation point is at 25 to 30% moisture content. Siau (1984) reported that the fibre saturation point ${\displaystyle X_{\text{fsp}}}$ (kg/kg) is dependent on the temperature T (°C) according to the following equation:

${\displaystyle X_{\text{fsp}}=0.30-(T-20C)/1000K\;}$ (1.2)

Keey et al. (2000) use a different definition of the fibre saturation point (equilibrium moisture content of wood in an environment of 99% relative humidity).

Many properties of wood show considerable change as the wood is dried below the fibre saturation point, including:

1. volume (ideally no shrinkage occurs until some bound water is lost, that is, until wood is dried below FSP);
2. strength (strengths generally increase consistently as the wood is dried below the FSP (Desch and Dinwoodie, 1996), except for impact-bending strength and, in some cases, toughness);
3. electrical resistivity, which increases very rapidly with the loss of bound water when the wood dries below the FSP.

Equilibrium moisture content

Wood is a hygroscopic substance. It has the ability to take in or give off moisture in the form of vapour. Water contained in wood exerts vapour pressure of its own, which is determined by the maximum size of the capillaries filled with water at any time. If water vapour pressure in the ambient space is lower than vapour pressure within wood, desorption takes place. The largest-sized capillaries, which are full of water at the time, empty first. Vapour pressure within the wood falls as water is successively contained in smaller capillaries. A stage is eventually reached when vapour pressure within the wood equals vapour pressure in the ambient space above the wood, and further desorption ceases. The amount of moisture that remains in the wood at this stage is in equilibrium with water vapour pressure in the ambient space, and is termed the equilibrium moisture content or EMC (Siau, 1984). Because of its hygroscopicity, wood tends to reach a moisture content that is in equilibrium with the relative humidity and temperature of the surrounding air.

The EMC of wood varies with the ambient relative humidity (a function of temperature) significantly, to a lesser degree with the temperature. Siau (1984) reported that the EMC also varies very slightly with species, mechanical stress, drying history of wood, density, extractives content and the direction of sorption in which the moisture change takes place (i.e. adsorption or desorption).

Moisture content of wood in service

Wood retains its hygroscopic characteristics after it is put into use. It is then subjected to fluctuating humidity, the dominant factor in determining its EMC. These fluctuations may be more or less cyclical, such as diurnal changes or annual seasonal changes.

To minimize the changes in wood moisture content or the movement of wooden objects in service, wood is usually dried to a moisture content that is close to the average EMC conditions to which it will be exposed. These conditions vary for interior uses compared with exterior uses in a given geographic location. For example, according to the Australian Standard for Timber Drying Quality (AS/NZS 4787, 2001), the EMC is recommended to be 10–12% for the majority of Australian states, although extreme cases are up to 15 to 18% for some places in Queensland, Northern Territory, Western Australia and Tasmania. However, the EMC is as low as 6 to 7% in dry centrally heated houses and offices or in permanently air-conditioned buildings.

Shrinkage and swelling

Shrinkage and swelling may occur in wood when the moisture content is changed (Stamm, 1964). Shrinkage occurs as moisture content decreases, while swelling takes place when it increases. Volume change is not equal in all directions. The greatest dimensional change occurs in a direction tangential to the growth rings. Shrinkage from the pith outwards, or radially, is usually considerably less than tangential shrinkage, while longitudinal (along the grain) shrinkage is so slight as to be usually neglected. The longitudinal shrinkage is 0.1% to 0.3%, in contrast to transverse shrinkages, which is 2% to 10%. Tangential shrinkage is often about twice as great as in the radial direction, although in some species it is as much as five times as great. The shrinkage is about 5% to 10% in the tangential direction and about 2% to 6% in the radial direction (Walker et al., 1993).

Differential transverse shrinkage of wood is related to:

1. the alternation of late wood and early wood increments within the annual ring;
2. the influence of wood rays on the radial direction (Kollmann and Cote, 1968);
3. the features of the cell wall structure such as microfibril angle modifications and pits;
4. the chemical composition of the middle lamella.

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Wood drying may be described as the art of ensuring that gross dimensional changes through shrinkage are confined to the drying process. Ideally, wood is dried to that equilibrium moisture content as will later (in service) be attained by the wood. Thus, further dimensional change will be kept to a minimum.

It is probably impossible to completely eliminate dimensional change in wood, but elimination of change in size may be approximated by chemical modification. For example, wood can be treated with chemicals to replace the hydroxyl groups with other hydrophobic functional groups of modifying agents (Stamm, 1964). Among all the existing processes, wood modification with acetic anhydride has been noted for the high anti-shrink or anti-swell efficiency (ASE) attainable without damage to wood. However, acetylation of wood has been slow to be commercialised due to the cost, corrosion and the entrapment of the acetic acid in wood. There is an extensive volume of literature relating to the chemical modification of wood (Rowell, 1983, 1991; Kumar, 1994; Haque, 1997).

Drying timber is one method of adding value to sawn products from the primary wood processing industries. According to the Australian Forest and Wood Products Research and Development Corporation (FWPRDC), green sawn hardwood, which is sold at about $350 per cubic metre or less, increases in value to $2,000 per cubic metre or more with drying and processing. However, currently used conventional drying processes often result in significant quality problems from cracks, both externally and internally, reducing the value of the product. For example, in Queensland (Anon, 1997), on the assumption that 10% of the dried softwood is devalued by $200 per cubic metre because of drying defects, saw millers are losing about $5 million a year. In Australia, the loss could be $40 million a year for softwood and an equal or higher amount for hardwood. Thus, proper drying under controlled conditions prior to use is of great importance in timber use, in countries where climatic conditions vary considerably at different times of the year. ^{[citation needed]}

Drying, if carried out promptly after felling of trees, also protects timber against primary decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain, generally cannot thrive in timber with a moisture content below 20%. Several, though not all, insect pests can live only in green timber.

In addition to the above advantages of drying timber, the following points are also significant (Walker et al., 1993; Desch and Dinwoodie, 1996):

1. Dried timber is lighter, and the transportation and handling costs are reduced.
2. Dried timber is stronger than green timber in most strength properties.
3. Timbers for impregnation with preservatives have to be properly dried if proper penetration is to be accomplished, particularly in the case of oil-type preservatives.
4. In the field of chemical modification of wood and wood products, the material should be dried to a certain moisture content for the appropriate reactions to occur.
5. Dry wood generally works, machines, finishes and glues better than green timber (although there are exceptions; for instance, green wood is often easier to turn than dry wood). Paints and finishes last longer on dry timber.
6. The electrical and thermal insulation properties of wood are improved by drying.

Prompt drying of wood immediately after felling therefore significantly upgrades and adds value to raw timber. Drying enables substantial long-term economy by rationalizing the use of timber resources. The drying of wood is thus an area for research and development, which concern many researchers and timber companies around the world.

Mechanisms of moisture movement

Water in wood normally moves from zones of higher to zones of lower moisture content (Walker et al., 1993). Drying starts from the exterior of the wood and moves towards the centre, and drying at the outside is also necessary to expel moisture from the inner zones of the wood. Wood subsequently attains equilibrium with the surrounding air in moisture content.

Moisture passageways

The driving force of moisture movement is chemical potential. However, it is not always easy to relate chemical potential in wood to commonly observable variables, such as temperature and moisture content (Keey et al., 2000). Moisture in wood moves within the wood as liquid or vapour through several types of passageways, based on the nature of the driving force, (e.g. pressure or moisture gradient), and variations in wood structure (Langrish and Walker, 1993), as explained in the next section on driving forces for moisture movement. These pathways consist of cavities of the vessels, fibres, ray cells, pit chambers and their pit membrane openings, intercellular spaces and transitory cell wall passageways.

Movement of water takes place in these passageways in any direction, longitudinally in the cells, as well as laterally from cell to cell until it reaches the lateral drying surfaces of the wood. The higher longitudinal permeability of sapwood of hardwood is generally caused by the presence of vessels. The lateral permeability and transverse flow is often very low in hardwoods. The vessels in hardwoods are sometimes blocked by the presence of tyloses and/or by secreting gums and resins in some other species, as mentioned earlier. The presence of gum veins, the formation of which is often a result of natural protective response of trees to injury, is commonly observed on the surface of sawn boards of most eucalypts. Despite the generally higher volume fraction of rays in hardwoods (typically 15% of wood volume), the rays are not particularly effective in radial flow, nor are the pits on the radial surfaces of fibres effective in tangential flow (Langrish and Walker, 1993).

Moisture movement space

The available space for air and moisture in wood depends on the density and porosity of wood. Porosity is the volume fraction of void space in a solid. The porosity is reported to be 1.2 to 4.6% of dry volume of wood cell wall (Siau, 1984). On the other hand, permeability is a measure of the ease with which fluids are transported through a porous solid under the influence of some driving forces, e.g. capillary pressure gradient or moisture gradient. It is clear that solids must be porous to be permeable, but it does not necessarily follow that all porous bodies are permeable. Permeability can only exist if the void spaces are interconnected by openings. For example, a hardwood may be permeable because there is intervessel pitting with openings in the membranes (Keey et al., 2000). If these membranes are occluded or encrusted, or if the pits are aspirated, the wood assumes a closed-cell structure and may be virtually impermeable. The density is also important for impermeable hardwoods because more cell-wall material is traversed per unit distance, which offers increased resistance to diffusion (Keey et al., 2000). Hence lighter woods, in general, dry more rapidly than do the heavier woods. The transport of fluids is often bulk flow (momentum transfer) for permeable softwoods at high temperature while diffusion occurs for impermeable hardwoods (Siau, 1984). These mechanisms are discussed below.

Driving forces for moisture movement

Three main driving forces used in different version of diffusion models are moisture content, the partial pressure of water vapour, and the chemical potential of water (Skaar, 1988; Keey et al., 2000). These are discussed here, including capillary action, which is a mechanism for free water transport in permeable softwoods. Total pressure difference is the driving force during wood vacuum drying.

Capillary action

Capillary forces determine the movements (or absence of movement) of free water. It is due to both adhesion and cohesion. Adhesion is the attraction between water to other substances and cohesion is the attraction of the molecules in water to each other.

As wood dries, evaporation of water from the surface sets up capillary forces that exert a pull on the free water in the zones of wood beneath the surfaces. When there is no longer any free water in the wood capillary forces are no longer of importance.

Moisture content differences

The chemical potential is explained here since it is the true driving force for the transport of water in both liquid and vapour phases in wood (Siau, 1984). The Gibbs free energy per mole of substance is usually expressed as the chemical potential of that substance (Skaar, 1933). The chemical potential of water in unsaturated air or wood below the fibre saturation point influences the drying of wood. Equilibrium will occur at the equilibrium moisture content (as defined earlier) of wood when the chemical potential of water in the wood becomes equal to that in the surrounding air. The chemical potential of sorbed water is a function of wood moisture content. Therefore, a gradient of wood moisture content (between surface and centre), or more specifically of water activity, is accompanied by a gradient of chemical potential under isothermal conditions. Moisture will redistribute itself throughout the wood until its chemical potential is uniform throughout, resulting in a zero potential gradient at equilibrium (Skaar, 1988). The flux of moisture attempting to achieve the equilibrium state is assumed to be proportional to the difference in its chemical potential, and inversely proportional to the path length over which the potential difference acts (Keey et al., 2000).

The gradient in chemical potential is related to the moisture content gradient as explained in above equations (Keey et al., 2000). The diffusion model using the moisture content gradient as a driving force was applied successfully by Wu (1989) and Doe et al. (1994). Though the agreement between the moisture-content profiles predicted by the diffusion model based on moisture-content gradients is better at lower moisture contents than at higher ones, there is no evidence to suggest that there are significantly different moisture-transport mechanisms operating at higher moisture contents for this timber. Their observations are consistent with a transport process that is driven by the total concentration of water. The diffusion model is used here based on this empirical evidence that the moisture-content gradient is a driving force for drying this type of impermeable timber.

Differences in moisture content between the surface and the centre (gradient, the chemical potential difference between interface and bulk) move the bound water through the small passageways in the cell wall by diffusion. In comparison with capillary movement, diffusion is a slow process. Diffusion is the generally suggested mechanism for the drying of impermeable hardwoods (Keey et al., 2000). Furthermore, moisture migrates slowly due to the fact that extractives plug the small cell wall openings in the heartwood. This is why sapwood generally dries faster than heartwood under the same drying conditions.

Moisture movement directions for diffusion

It is reported that the ratio of the longitudinal to the transverse (radial and tangential) diffusion rates for wood ranges from about 100 at a moisture content of 5%, to 2–4 at a moisture content of 25% (Langrish and Walker, 1993). Radial diffusion is somewhat faster than tangential diffusion. Although longitudinal diffusion is most rapid, it is of practical importance only when short pieces are dried. Generally the timber boards are much longer than in width or thickness. For example, a typical size of a green board used for this research was 6 m long, 250 mm in width and 43 mm in thickness. If the boards are quartersawn, then the width will be in the radial direction whereas the thickness will be in tangential direction, and vice versa for plain-sawn boards. Most of the moisture is removed from wood by lateral movement during drying.

Reasons for splits and cracks during timber drying and their control

The chief difficulty experienced in the drying of timber is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fibre saturation point while the interior is still saturated, stresses (called drying stresses) are set up because the shrinkage of the outer layers is restricted by the wet interior (Keey et al., 2000). Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain (fibre to fibre bonding).

The successful control of drying defects in a drying process consists in maintaining a balance between the rate of evaporation of moisture from the surface and the rate of outward movement of moisture from the interior of the wood. The way in which drying can be controlled will now be explained. One of the most successful ways of wood drying or seasoning would be kiln drying, where the wood is placed into a kiln compartment in stacks and dried by steaming, and releasing the steam slowly.

Influence of temperature, relative humidity and rate of air circulation

The external drying conditions (temperature, relative humidity and air velocity) control the external boundary conditions for drying, and hence the drying rate, as well as affecting the rate of internal moisture movement. The drying rate is affected by external drying conditions (Walker et al., 1993; Keey et al., 2000), as will now be described.

Temperature

If the relative humidity is kept constant, the higher the temperature, the higher the drying rate. Temperature influences the drying rate by increasing the moisture holding capacity of the air, as well as by accelerating the diffusion rate of moisture through the wood.

The actual temperature in a drying kiln is the dry-bulb temperature (usually denoted by Tg), which is the temperature of a vapour-gas mixture determined by inserting a thermometer with a dry bulb. On the other hand, the wet-bulb temperature (TW) is defined as the temperature reached by a small amount of liquid evaporating in a large amount of an unsaturated air-vapour mixture. The temperature sensing element of this thermometer is kept moist with a porous fabric sleeve (cloth) usually put in a reservoir of clean water. A minimum air flow of 2 m/s is needed to prevent a zone of stagnant damp air formation around the sleeve (Walker et al., 1993). Since air passes over the wet sleeve, water is evaporated and cools the wet-bulb thermometer. The difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression, is used to determine the relative humidity from a standard hygrometric chart (Walker et al., 1993). A higher difference between the dry-bulb and wet-bulb temperatures indicates a lower relative humidity. For example, if the dry-bulb temperature is 100 °C and wet-bulb temperature 60 °C, then the relative humidity is read as 17% from a hygrometric chart.

Relative humidity

The relative humidity of air is defined as the partial pressure of water vapour divided by the saturated vapour pressure at the same temperature and total pressure (Siau, 1984). If the temperature is kept constant, lower relative humidities result in higher drying rates due to the increased moisture gradient in wood, resulting from the reduction of the moisture content in the surface layers when the relative humidity of air is reduced. The relative humidity is usually expressed on a percentage basis. For drying, the other essential parameter related to relative humidity is the absolute humidity, which is the mass of water vapour per unit mass of dry air (kg of water per kg of dry air). However, its influenced by the amount of water in the heated air.

Air circulation rate

Drying time and timber quality depend on the air velocity and its uniform circulation. At a constant temperature and relative humidity, the highest possible drying rate is obtained by rapid circulation of air across the surface of wood, giving rapid removal of moisture evaporating from the wood. However, a higher drying rate is not always desirable, particularly for impermeable hardwoods, because higher drying rates develop greater stresses that may cause the timber to crack or distort. At very low fan speeds, less than 1 m/s, the air flow through the stack is often laminar flow, and the heat transfer between the timber surface and the moving air stream is not particularly effective (Walker et al., 1993). The low effectiveness (externally) of heat transfer is not necessarily a problem if internal moisture movement is the key limitation to the movement of moisture, as it is for most hardwoods (Pordage and Langrish, 1999).

Classification of timbers for drying

The timbers are classified as follows according to their ease of drying and their proneness to drying degrade:

Highly refractory woods

These woods are slow and difficult to dry if the final product is to be free from defects, particularly cracks and splits. Examples are heavy structural timbers with high density such as ironbark (Eucalyptus paniculata), blackbutt (E. pilularis), southern blue gum (E. globulus) and brush box (Lophostemon cofertus). They require considerable protection and care against rapid drying conditions for the best results (Bootle, 1994).

Moderately refractory woods

These timbers show a moderate tendency to crack and split during seasoning. They can be seasoned free from defects with moderately rapid drying conditions (i.e. a maximum dry-bulb temperature of 85 °C can be used). Examples are Sydney blue gum (E. saligna) and other timbers of medium density (Bootle, 1994), which are potentially suitable for furniture.

Non-refractory woods

These woods can be rapidly seasoned to be free from defects even by applying high temperatures (dry-bulb temperatures of more than 100 °C) in industrial kilns. If not dried rapidly, they may develop discolouration (blue stain) and mould on the surface. Examples are softwoods and low density timbers such as Pinus radiata.

Model

The rate at which wood dries depends upon a number of factors, the most important of which are the temperature, the dimensions of the wood, and the relative humidity. Simpson and Tschernitz^[2] have developed a simple model of wood drying as a function of these three variables. Although the analysis was done for red oak, the procedure may be applied to any species of wood by adjusting the constant parameters of the model.

Simply put, the model assumes that the rate of change of the moisture content M with respect to time t is proportional to how far the wood sample is from its equilibrium moisture content ${\displaystyle M_{e}}$, which is a function of the temperature T and relative humidity h:

${\displaystyle {\frac {dM}{dt}}=-{\frac {M-M_{e}}{\tau }}}$

where ${\displaystyle \tau }$ is a function of the temperature T and a typical wood dimension L and has units of time. The typical wood dimension is roughly the smallest value of (${\displaystyle L_{r},\,L_{t},\,L_{L}/10}$) which are the radial, tangential and longitudinal dimensions respectively, in inches, with the longitudinal dimension divided by ten because water diffuses about 10 times more rapidly in the longitudinal direction (along the grain) than in the lateral dimensions. The solution to the above equation is:

${\displaystyle {\frac {M-M_{e}}{M_{0}-M_{e}}}=e^{-{\frac {t}{\tau }}}}$

Where ${\displaystyle M_{0}}$ is the initial moisture content. It was found that for red oak lumber, the "time constant" ${\displaystyle \tau }$ was well expressed as:

${\displaystyle \tau ={\frac {L^{n}}{a+bp_{\text{sat}}(T)}}}$

where a, b and n are constants and ${\displaystyle p_{\text{sat}}(T)}$ is the saturation vapor pressure of water at temperature T. For time measured in days, length in inches, and ${\displaystyle p_{\text{sat}}}$ measured in mmHg, the following values of the constants were found for red oak lumber.

a = 0.0575

b = 0.00142

n = 1.52

Solving for the drying time yields:

${\displaystyle t=-<!--\; -\; -->\mathrm{\tau <!--\; \tau \; -->}\ln <!--\; \; -->}$Zdroj:https://en.wikipedia.org?pojem=Seasoning_(wood)
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