Although many methods of drying timber have been tried over the years only a few of these enable drying to be carried out at a reasonable cost and with minimal damage to the timber.
The most common method of drying is to extract moisture in the form of water vapour. To do this, heat must be supplied to the wood to provide the latent heat of vaporisation. There are several ways of conveying heat to the wood and removing the evaporated moisture.
Nearly all the world's timber is, in fact, dried in air. This can be carried out at ordinary atmospheric temperatures (air drying), or in a kiln at controlled temperatures raised artificially above atmospheric temperature but not usually above 100°C, the boiling point of water. Air drying and kiln drying are fundamentally the same process because, with both, air is the medium which conveys heat to the wood and carries away the evaporated moisture.
Factors Influencing The Drying of Wood in Air
The factors which will be described are those which affect wood when dried in air (in the open or in a kiln). There are several other ways in which wood can be dried, in chemicals, in a vacuum etc. Under these conditions different factors come into play.
- Vapour Pressure and Relative Humidity
To understand how wood dries in air it is necessary to introduce some terminology. When air holds the maximum possible amount of vapour, the vapour exerts what is called the saturation vapour pressure. If the water vapour present is less than this maximum then the air can take up more moisture. The ratio of actual vapour pressure to the saturation vapour pressure at any given temperature, expressed as a percentage, is called the relative humidity (rh).
When a piece of wet wood is exposed to air which is not already saturated (i.e. its relative humidity is less than 100%), evaporation takes place from its surface. At a given temperature the rate of evaporation is dependent on the vapour pressure difference between the air close to the wood and that of the more mobile air above this zone.
The temperature of a piece of wood and of the air surrounding it will also affect the rate of water evaporation from the wood surface. With kiln drying, warm or hot air is passed over the timber and at the start of the drying process the temperature differential between the air and the wet wood will usually be large. As a result, heat energy will be transferred from the air to the wood surface where it will raise the temperature of both the wood and the water it contains. Water, in the form of vapour, will then be lost from the wood surfaces, provided the surrounding air is not already saturated with moisture. This results in the development of a moisture content gradient from the inside to the outside of the wood. As the temperature is raised this increases not only the steepness of this moisture gradient, but also the rate of moisture movement along the gradient and the rate of loss of water vapour from the surface of the wood.
To illustrate the effect of temperature on drying rate, a piece of wood with a moisture content of 16% at the surface and 40% at the core will generally have moisture gradients at 50°C and 80°C which are respectively four and eight times greater than that at 20° C.
With kiln drying, higher temperatures also increase the capacity of the air for moisture. An advantage of this is that less air needs to be heated and exhausted from the kiln. In addition higher temperatures allow more rapid conditioning of a timber load to a uniform final moisture content.
Unfortunately the considerable benefits obtainable by raising the drying temperature cannot always be fully exploited because there are limits to the drying rates which various wood species will tolerate without degrade.
In the drying of many species, especially medium density and heavy hardwoods, shrinkage and accompanying distortion may increase as the temperature is raised. So with species which are prone to distort it is normal to use comparatively low kiln temperatures. A few species are liable to collapse and/or honeycomb if dried at high temperatures. Many tend to darken appreciably and, in resinous timbers, drying at temperatures above about 50° C causes the resin to exude on to the wood surface, although this may not necessarily be detrimental for all products or uses. Finally, since high temperature drying may cause a slight loss in impact strength, it is not advisable to exceed about 60° C when drying timber for items such as tool handles and sports goods.
- Air Movement
If the air surrounding a piece of wet wood is stagnant and of small volume, it will soon become saturated and evaporation of moisture from the wood will stop. Even when there is a continuous stream of air passing over the wood the layer of air in immediate contact with the wood will move more slowly and have a higher vapour pressure than the main stream. This is known as the 'boundary layer effect'. With increasing air velocity in the main stream this effect decreases and evaporation rates from the wood surface increase, particularly when the air flow is turbulent rather than laminar. An increase in air speed can therefore be regarded as equivalent to a reduction of the humidity barrier near the wood surfaces.
Since air passing through a stack of wet wood gives up heat and takes up moisture it is bound to be cooler and more humid where it emerges than where it enters and the drying rate is therefore slower on the air outlet than on the air inlet side of the stack. The faster the air speed and the narrower the stack, the smaller is the difference between the two sides. For this reason fairly high air speeds are desirable in a drying kiln, particularly when the timber being dried is very wet and loses its moisture readily. In most modern kilns the uniformity of drying is further improved by reversing the direction of air flow through the kiln load at regular intervals.
- Movement of Moisture in the Wood
When water evaporates from the surface of a piece of wet wood the moisture content in the outer zone is lowered and moisture begins to move outwards from the wetter interior. In practical terms this movement of moisture can be accepted as being a combination of capillary flow and moisture diffusion, a process which is resisted by the structure of the wood, particularly in dense hardwood species. If the rate of water loss by evaporation exceeds the rate at which moisture from the wet interior can pass to the surface, the moisture gradient within the wood becomes progressively steeper. As the outer layers dry below the fibre saturation point their tendency to start shrinking is resisted by the wetter interior so that stresses develop. If these stresses become large they can lead to a number of drying defects.
In both air and kiln drying the establishment of a moisture gradient is unavoidable and indeed desirable, for in any particular piece of wood at a given temperature the rate of movement of moisture up to the surface is proportional to the steepness of the gradient. The skill in timber drying lies in controlling the rate of evaporation to match the rate at which moisture is reaching the surface; the aim is to maximise the moisture gradient without damaging the timber.
- Supply of Heat
A supply of heat is required to dry timber. In kiln drying, sensible heat is needed to raise the temperature of the wood and the water it contains to the required drying temperature, and latent heat is needed to evaporate the water. To give an impression of the amounts of energy involved, the drying of 15 cubic meters of timber from green to 20% moisture content by electricity could require a minimum of 3000kWh. Energy is a major element in the running costs of conventional kilns which vent warm, moist air to the outside and therefore recover none of the drying energy input. As the relative cost of energy rises, efficient heat utilisation has become a more significant factor and much attention has been given to techniques which might reduce expenditure on energy. These range from simple upgrading of kiln insulation to the development of heat pump kilns.
Methods of Drying Wood in Air
- Air Drying
With air drying there is virtually no control of the temperature, relative humidity or speed of the air passing through the timber stacks. The rate of drying is therefore dependent on all the vagaries of the local climate and can vary between practically zero on a calm, damp day to quite fast enough to cause surface checking during dry, windy weather.
Because of the comparatively high humidity conditions in ASEAN countries, air drying is a slow process. Times taken to reach 20-25% moisture content vary from 2 or 3 months to 1 or 2 years, depending upon the species and size of the timber. With air drying, wood cannot be dried below its equilibrium moisture content and this will vary depending on the atmospheric conditions. So, except in unusually hot and dry weather, the lowest moisture content obtainable is around 16-17%; air drying alone is not sufficient for timber intended for most interior uses in Europe, Japan or N. America where a moisture content of between 8 and 12% is required. In air conditioned buildings moisture contents of about 12% should be anticipated.
- Kiln Drying
In contrast to air drying a modern conventional drying kiln provides temperature control and a steady and adequate flow of air over the timber surface. The air flow rate and direction is controlled by fans and the temperature and relative humidity of the air can be adjusted to suit the species and sizes of timber being dried. It is thus possible to make full use of the increase in drying rate which can be achieved by raising the temperature to the maximum value which a particular timber species can tolerate without excessive degrade. At the same time, the relative humidity can be controlled so that the moisture gradients in the wood are not steep enough to cause surface checking. The same principles apply to the use of heat pump kilns except these recover and re-use a proportion of the energy which in conventional kilns is lost during the drying process when the warm, damp air is vented.
In addition to the advantages of more rapid drying and limitation of degrade, the ability to control drying conditions in a kiln means that it is possible to achieve timber moisture contents suitable for specific uses.
The direct costs of kiln drying are much higher than those of air drying for they include the capital costs of the equipment and the cost of fuel, electricity and supervision. These costs are partially or wholly offset by the reduction in stock level.
- Air Drying Followed by Kiln Drying
Kiln drying tends to become uneconomical when the species and size of timber being dried require long kilning times. Therefore, with material taking more than about 4 or 5 weeks to kiln dry from green it will often be more economical to air dry the timber to about 25-30% moisture content before completing the drying in a kiln.
The economic advantage of this approach may be lost, however, if the layout or lack of handling facilities necessitates dismantling the air dried stack and repiling for the kiln drying phase. Also with some species, the amount of splitting and checking which occurs during air drying in the dry season can be excessive.
Moisture Content of Timber The amount of water in a piece of wood is known as its moisture content. Because this is expressed as a percentage of the dry weight of the piece, not of the total weight, it is possible to have moisture contents of well over 100%.
The moisture content of green wood varies greatly from one species to another. Moisture content can vary between apparently similar pieces of the same species and in addition there may be differences, between and within species, in the rates at which moisture is lost from timber during drying.
These inherent differences in timber mean that it is important during the drying process to be able to monitor moisture content and check that the drying process is proceeding correctly. The following sections describe the methods which can be used to determine timber moisture content.
- Moisture Content Determination by The Oven Drying Method The oven drying method is the standard way of determining wood moisture content. With this method a piece of wood is initially weighed and then dried in an oven at 103°C ± 2°C. Drying is continued until the piece is completely dry (when no further weight loss occurs) and this oven dry weight recorded. The loss in weight during drying indicates how much water was originally present in the piece and the moisture content can be calculated simply, as follows:
|Initial moisture content (%) ||= Initial (wet) weight - Dry weight |
|x 100 |
For example, if the initial weight of the piece was 30.51g and its dry weight 22.60g, then the difference of 7.91g is the weight of moisture initially in the piece and its initial moisture content would be:
(30.51 - 22.60)/22.60 x 100 = 7.91/22.60 x 100 = 35.0%
Alternatively the formula can be written:
Moisture content (%) = [(Initial weight/Dry weight) - 1 ] x 100
So that only the division sum needs to be carried out:
[(30.51/22.60) - 1] x 100 = 0.35 x 100 = 35.0 %
When a particular piece of wood is oven dried, the moisture content value obtained (as above) is an average moisture content for that piece and it is important to emphasise that actual moisture contents at different locations within this piece may vary considerably from the average value.
Pieces used for oven drying will normally be unsuitable for further use and therefore with this method it would be wasteful to use large pieces. Instead small pieces of wood are removed from selected planks or boards and used as follows to estimate moisture content. First, cut off and reject a length of at least 230mm from the end of the board or plank; the more rapid drying through the end grain will usually cause this portion to be drier than the rest of the piece. A full cross-section piece (test section) about 15mm wide is then cut from the newly sawn end and used for moisture content determination.
This should be free from knots as these tend to falsify the result.
The initial weighing of the test section should be made as quickly as possible after it has been cut, after removal of sawdust and any loose slivers of wood. Errors, due to unavoidable delay before weighing, can be minimised by placing the sections in polythene bags immediately after cutting. The sections should be placed in an oven until they cease to lose weight, weighings being made at intervals until drying is complete. Because wood will absorb moisture from the atmosphere, weighings must be made immediately after removal from the oven. With any particular oven and test section, experience will soon indicate the approximate time taken to complete drying and so periodic weighing becomes unnecessary. The time will actually depend on the moisture content, the size and the species of the piece, and on the number of other sections packed into the oven. It may vary from about 6 to 18 hours, but results to within 1 or 2% of the true moisture content can be obtained in much shorter times. If a quick result is required on relatively dry wood a close approximation can be obtained by cutting a number of 3mm thick sections, immediately weighing them all together and spacing them out for about half an hour in an oven containing no other test sections, and then reweighing.
With oven drying, fresh sections should not be loaded into an oven shortly before taking others out for final weighing, since the drier pieces will temporarily gain weight by absorbing moisture released from the wetter ones.
Oven drying requires a well ventilated oven which can control the temperature to between 101 and 105°C and a balance for weighing the test samples. The balance should have a capacity of about 200g and be capable of detecting differences of 0.005g, an automatic type is recommended as these give an instantaneous reading.
Infrared ovens are available for rapid drying. In some of these the heating lamps are directed on to the test section on the pan of a balance (incorporated in the equipment). Drying takes from about 3 to 10 minutes according to species and moisture content. However only one piece can be dried at a time and experience is needed to avoid overheating which can cause inaccurate results.
- Interpretation of Moisture Content Determinations Using the Oven Drying Method
The oven drying method is usually an accurate way of estimating the moisture content of a test section of wood. Indeed recommended moisture contents for most uses are based on values obtained by the oven drying of timber which has been allowed to equilibrate to particular service conditions.
However, the moisture content of one test section may not be a typical value for the whole load of timber being dried because moisture content will vary within a piece and between different parts of the same piece and between different pieces of the same species. To obtain a more accurate estimate of the average moisture content for a kiln load, it is necessary to take test sections from a series of sample pieces incorporated in the load. If the moisture contents from all these test sections are then averaged, this should give an accurate estimate of the overall average moisture content of the load and an indication of the range of moisture content in the load, provided the samples have been selected correctly.
- Moisture Content Determination be Electrical Moisture Meters
Moisture meters are available which can give an instant indication of the moisture content of a piece of wood by measuring one of its electrical properties. The electrical resistance of wood increases rapidly with decreasing moisture content once this is below about 25 to 30 %, whilst the capacitance and dielectric loss both decrease with a fall in moisture content at all levels.
Most commercial moisture meters measure the electrical resistance between two electrodes which are driven into the wood. The electrical resistance of the wood is then converted into percentage moisture content and displayed on the meter. The following sections are concerned with the use of this type of meter.
- Good electrical contact with the timber.
To avoid artificially high moisture content readings, it is important to maintain good electrode contact with the wood as the measurements are taken. With short needle or blade electrodes this can usually be achieved by applying a controlled pressure to the electrode holder; with long insulated electrodes, the hammer action, by which they are driven into the wood, ensures good electrode contact.
- Measurable range of moisture content.
With electrical resistance meters physical limitations restrict the range of moisture content which can be measured. The fibre saturation point (usually between 25 and 30 % moisture content) is a practical upper limit because above this the differences in electrical resistance are small and cannot be accurately interpreted. A lower measurable limit of 7% moisture content is imposed by the high electrical resistance of drier wood. Capacitance and dielectric loss type meters can operate at all levels but the density of the test pieces has to be known to obtain an accurate estimation of their moisture content.
During drying the average and core moisture contents in thick pieces will be significantly higher than those in the outer layers. Meters with short electrodes may therefore consistently underestimate the overall moisture content - they may also give inaccurately high results if, for example, the surfaces of the timber under test have been exposed to an unusually damp atmosphere or to rain. With equipment having long insulated probes these problems are largely overcome, and the electrodes can be driven in to provide moisture content readings at considerable depths. It is also possible to obtain a rough indication of the moisture gradient within a piece because the resistance between the exposed electrode tips can be recorded at various depths as the electrode is driven into the piece.
- Variation in electrical resistance.
The electrical resistance at any given moisture content can vary considerably between pieces of the same species and between pieces of different species. Manufacturers can provide meters with multiple scales which provide a corrected reading for the more common commercial species. Alternatively correction factors can be applied to account for this source of error. The resistance differences between pieces of the same species are more difficult to account for and it is normally accepted that moisture content estimations using resistance meters can be inaccurate by up to ± 2%.
- The effect of temperature.
The electrical resistance of wood at any given moisture content decreases as the temperature increases and the effect of temperature is greater the higher the moisture content. When testing timber at moisture contents below or around 15% and when the the temperature of the wood is known, an approximate correction can be made by subtracting 1% for each 8°C below 20°C.
Because it is not easy to measure the temperature of the wood between the tips of the moisture meter electrodes, the accurate estimation of moisture contents is not normally possible during kiln drying when temperatures are high. The moisture contents of samples which have been removed from the kiln and allowed to cool can be determined using meters in the usual way. However, as with the oven-drying method, a number of samples would be needed to take account of variability between pieces in the load.
The presence of certain chemicals in timber, particularly salts from preservative or flame retardant treatments, can cause a marked decrease in electrical resistance. Meter readings from treated wood or wood accidentally wetted by sea water will therefore give moisture content readings which are artificially high. This effect will vary considerably depending on the amount and type of chemical used and its distribution in the wood. At 20% moisture content, for example, the actual moisture content of treated or contaminated wood may be between 1 and 5% lower than the value indicated by the meter; consequently it is rarely possible to provide reliable correction factors for this effect.
With certain wood-based panel products, meter moisture content determinations may be artificially high; here a lower electrical resistance will often be caused by the adhesive present. The effect is particularly noticeable with plywoods bonded with phenolic adhesives, where meter measurements of moisture content may be up to twice the real value. Again it is not possible to allow for this effect accurately because the error varies according not only to the type of board but also the type of bond and its history and location within the board. Some meter manufacturers supply general correction tables for use with wood particleboards.
- The manufacturer's instructions.
It is always important to follow precisely the manufacturer's instructions supplied with the meter. For example, electrodes inserted into end grain will not yield accurate moisture content values.
The estimation of timber moisture content is a straightforward operation whether oven drying or a moisture meter is used, provided the limitations of each method are appreciated. In both cases repeated measurement and careful interpretation of values obtained are required to obtain an accurate estimate of moisture content. A useful practical approach is to utilise the advantages of both methods.
- Advantages of the Oven Drying Method.
It can be used to estimate the complete range of moisture content (electrical resistance meters can only measure moisture content accurately between 7 and 25/30 %).
It normally gives a direct and definitive moisture content value for the piece (meter readings are liable to inaccuracies depending on for example, variable resistance and effects of electrolytic contaminants).
It can be used to obtain a definitive measurement of moisture gradient with depth of the piece (meter readings are liable to inaccuracies caused by variation of resistance within the timber). It can be used to estimate moisture content or moisture gradient with depth of a piece during a kiln run. (Moisture meters cannot normally be used to determine moisture content accurately at kiln-operating temperatures).
- Advantages of Moisture Meters.
For many uses, frequent moisture content readings can be taken without loss in value of the timber (oven drying results in a small loss in value).
Many readings can be obtained quickly and with minimal effort (oven drying is a slower process and more labour intensive).
Recommended practice is to monitor moisture content and moisture distribution during drying with the oven method. For each load several samples of wood should be monitored and care is needed to select and position these so that the progression of drying can be monitored and controlled accurately. Once the wood is below fibre saturation point, moisture meters can be used to give estimates of the moisture gradients along a piece and to confirm that the pieces being monitored by the oven method are indeed representative of the load.
A high proportion of the world's timber is either wholly or partly air dried before drying in a kiln. Air drying involves the open piling of fresh-sawn timber out of doors, or in open sheds, so that the wood surfaces are exposed to the surrounding atmosphere. Wind and local convection currents will cause air movement through the stack and this conveys solar heat energy to the wood and carries away evaporated moisture.
The Pre-drying of Timber
The employment of correct techniques can reduce air drying times and keep drying degrade to a minimum.
- Siting and Layout of Yard
Ideally, timber should be stacked well away from trees and buildings on a cleared, level and welldrained site which has been concreted, covered with ashes or treated to prevent the regrowth of vegetation. When this is not practicable, the site should be cleared in the first instance and all reasonable care taken to keep down the growth of vegetation, which tends to interfere with the free circulation of air beneath the stacks. Sawdust and odd pieces of timber should not be left on the ground between the stacks where these could harbour wood-destroying insects and fungi.
In most situations, the orientation of the stacks has little effect on the drying rate and the most important consideration in planning the yard is to arrange stocks and roadways to facilitate handling.
Adjacent stacks should be parallel and oriented with either ends or sides to the roadway depending on the methods used for transport and stacking. Consideration should be given to the possibility of fork-lift trucks or side loaders.
Stacks should be erected on good solid foundations and, in order to permit ample ventilation, the bottom layers of timber should be raised well above the ground. The clearance should certainly be no less than 230mm and should preferably be about 460mm. The most convenient form of foundation, and probably the simplest to erect, consists of a series of timber cross-members (bearers) not less than 100 x 100mm in section, preferably preservative-treated and lifted clear of the ground on brick or concrete piers or on treated timber, e.g. railway sleepers (below, Figure 1). The piers should be placed at intervals of 600mm along the whole length of the stack. Stringers or longitudinal timber members can be used to give added strength and rigidity to the foundations, but they are generally only necessary where special stacking arrangements are involved. It is essential that the bearers should all be in one plane, but it is not critical whether these are level or on a slight slope. In either case, any necessary adjustment can be made by varying the height of the brick piers and inserting wooden packing blocks between the bricks and cross-members where required.
- Size and Spacing of Stacks
In very wide stacks the timber in the centre will dry slowly, and stain and rot are likely to develop. As a general rule, therefore, a stack should not be wider than 2m. The height is limited only by stability and ease of piling. Tall stacks are generally to be preferred, especially in the case of those hardwoods which have a marked tendency to distort. Apart from convenience of handling, there is no advantage in leaving more than about 300mm between adjacent stacks.
- Piling Stickers
Except in the special cases of certain classes of dimension stock, stickers should always be used to separate the layers of timber. The planks themselves should not be used as separators; drying will be retarded at the areas of contact and this could lead to the development of stain or rot. Whenever possible the sticks should be of clean, dry timber. The size most generally used is 19 x19mm but by varying the thickness of piling sticks a degree of control of the drying rate can be achieved.
Besides separating the layers of timber to allow a free circulation of air, piling sticks when correctly placed also help to prevent distortion during drying. Although ideally the spacing between the piling sticks should be varied according to the species and thickness of the timber which is to be dried, such an approach would be impracticable. However, the following generalised suggestions on piling may be useful:
those hardwoods which show little tendency to distort when sawn to thicknesses of 50mm and upwards, should be piled with the vertical columns of sticks spaced at intervals of 1200mm along the stack, but for thinner boards it is advisable to reduce this spacing to 600mm. Hardwoods which tend to distort considerably during drying, should have sticks at 600mm intervals even when sawn to thicknesses of 50mm, and the same spacing could, for convenience, be used without undue risk with boards down to 25mm in thickness. Boards of these species less than 25mm in thickness are best stacked with the sticks spaced only about 300mm apart. With such close spacing it is advisable to incorporate longitudinal stringers in the foundations, so that it is not necessary to incorporate an unduly large number of cross members.
- The Normal Method of Stacking
Whenever possible, different species and thicknesses should be stacked separately. It is an advantage if timber can be sorted to length at the outset, and when a variety of lengths has to be stacked it is convenient to place the longest pieces at the bottom and to reduce the length of the stack as the height increases. Alternatively, if sorting beforehand is not practicable, a stack of uniform length may be built by arranging the timber as shown in Figure 2. This is sometimes referred to as box-piling.
Sticks should be laid on the cross-bearers underneath the bottom layer of timber and a vertical line of sticks should be positioned as near to the ends of the stack as possible. To do this, it may in some cases be necessary to make slight adjustments to the spacing of the foundations at the ends of the stacks. As piling proceeds, care should be taken to place the sticks in each line vertically one above the other, so that the weight of the stack is directly transmitted from each stick to the one below. Distortion will usually be minimised if the piling sticks are arranged correctly; any misalignment will exaggerate distortion.
The end of planks, especially if they are thin, are iiable to distort badly if unsupported. When it is impossible to avoid incorporating a number of long pieces which overhang the ends of the stack, special supports for these should be erected. Within the stack itself, short lengths of stick should be used as supports underneath and over the ends of pieces which do not reach the vertical lines of piling sticks. With air drying it is not advisable to place the wood edges close together, since this restricts the free movement of air within a stack and may cause stain or rot to develop on the edges. A space of about 25mm between adjacent pieces has been found to be sufficient.
- Other Methods of Stacking
In special circumstances various non-standard methods of stacking may be adopted.
When drying dimension stock of convenient sizes such as squares and rails these may be open piled by self crossing. Piling sticks are not needed and the quantity of timber which can be stacked per unit area is increased. The use of pieces much above 50mm wide as separators is not recommended because drying may be retarded enough to cause stain to develop. Furthermore, as the cross pieces distort during drying these will exert less restraint on the layers of timber they are separating.
Piling of boules.
It is often economic and desirable to leave timber unedged until it is air dried and ready for further conversion into shaped pieces, such as curved chair legs. Advantage can then be taken of any natural curves in the log from which they were sawn. There is also a demand for wood cut from the same log, as this can be readily matched for figure and colour.
In both these cases, the unedged pieces may be piled in log form. Use of this method reduces handling to a minimum since the planks can be piled straight off the saw and each boule transported as a unit.
The drying of timber in log form tends to be faster than in normal stacks because each pile is narrow and has plenty of space around it. Therefore for species which are prone to surface check it is normal practice to use sticks only 13 or 19mm thick when piling logs in the dry season.
In the drying of certain species, care must be taken if the light colour is to be preserved and if stain and stick marks are to be avoided. One precaution is to dry off the surfaces as rapidly as possible, and a simple way of doing this is to pile the sawn pieces vertically against a wall or some form of rack which keeps them at least 25mm apart. End-racking may not be very effective during damp, cool conditions.
- Protection From the Weather
Ideally, timber should be stacked in large, open-sided sheds but when these are not available some form of roof should be provided to protect the timber from heavy rain and from the full heat of the sun.
Roofs should have a good fall to one side and be large enough to overhang the stack by a generous amount. They can be constructed of corrugated metal or of frames covered with heavy gauge polythene sheet. All roofs need to be firmly secured.
If it is impractical to provide a proper roof, some useful cover and protection from the sun can be obtained by placing rough slabs or boards on top of the stacks. A sheet of polythene spread over the stack and held in position by the last layer of timber is better than nothing as it ensures that the rain runs down the outside and not into the interior of the stack.
One of the commonest forms of degrade which occurs during air drying is end checking and splitting. This may become serious when green timber, especially certain hardwoods is put out to dry in hot weather. The damage occurs because the exposed ends dry out more rapidly and tend to shrink in advance of the rest of the piece. The tendency for this to happen may be considerably reduced by protecting the ends from the sun and air. The best method of doing this is to apply a good moisture resistant coating to the ends.
When end coating would involve too much expense or labour, it may be feasible at least to shade the exposed ends by draping tarpaulins or sacking over the ends of the stack. Wooden cleats fixed to the ends give some protection, but if these are used they should either be nailed in the middle only or should be very thin so that they buckle as the wood shrinks. As noted for kiln drying the use of strong, securely nailed cleats during air drying can restrain natural shrinkage and induce and exaggerate end splitting.
Seasonal Stacking of Timber
Timber is commonly converted and stacked at all times of the year, but provided stacking follows conversion with the minimum of delay and is properly carried out, degrade can be kept within acceptable limits even if the timber has not been stacked during the most suitable season. When a choice of season exists then undoubtedly it is best to stack hardwoods when the drying conditions are mild; many hardwoods are liable to split if dried too rapidly during the early stages.
Air Drying Times
Because the rate of drying will vary markedly with the weather conditions it is impossible to give other than approximate average times taken to air dry various timbers and sizes. The following times are given as a rough guide to assist in planning air drying operations: In Malaysia, where the climate is fairly uniform throughout the year, most timbers achieve an equilibrium moisture content a little below 20%. Light and medium density species take 2-3 months to dry to this level when sawn to a thickness of 13mm, and 4-5 months when sawn 38mm thick. Denser species such as balau and chengal take longer. In other ASEAN countries, where the climate is more seasonal, moisture contents in thin boards may be as low as 12% at the end of the dry period.
Measuring the Progress of Air Drying
It is important to have some knowledge of how air drying proceeds so that timber is not left in the stacks once it has reached a particular moisture content or the lowest moisture content it is likely to attain. Ideally, a few sample pieces should be incorporated in the stack so that they can be withdrawn and weighed at any time. If test sections of timber are taken in the same way as indicated for kiln drying their moisture content can be estimated by oven drying. Alternatively, measurements with a resistance type moisture meter can give an indication of the dryness of the timber once it is below about 30%. Moisture meter readings should not be taken on pieces at the edge or end of a stack as these will not always give a true indication of the overall average moisture content.
Cost of Air Drying The cost of air drying, excluding that of handling, depends on a number of factors:
- The capital cost of establishing the drying yard, including preparation of the site, provision of roads, stack foundations, piling sticks and roofs.
- Value of the land.
- The value of the timber and current rate of interest on capital.
- Overheads including depreciation, maintenance, supervision, and insurance.
- Time taken to dry: a very variable and unpredictable factor depending as it does on the particular weather conditions encountered, as well as on the species and thickness of the timber.
The major component in the cost of air drying is usually the interest on the value of the timber held in stock and the cost per unit volume of timber is often directly proportional to the time taken to air dry. Therefore when air drying is being carried out prior to kilning, it may be economic to transfer the timber to the kiln when at 30-35%, rather than wait the disproportionately long time often needed to air dry down to around 20% moisture content. The provision of good air drying facilities accounts for only a small part of the total cost, especially when valuable timber has to be dried, and there is no doubt that provision of suitable roofing, for example, pays for itself in a relatively short time.