Seasoning And Moisture Content

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The growing tree which is full of sap can contain anything from 50 per cent free water up to 200 per cent in certain swamp-grown softwoods. A single cubic foot (0.02832 m3) of green oak, for instance, can contain up to 28 pt (16 1) of water which must be reduced to at least 4 pt (2.27 1) before the timber is fit for use. The object of seasoning is, therefore, to accelerate the evaporation of this surplus moisture, rendering the wood lighter, harder, stronger, less susceptible to discoloration and mould attack, and much less liable to shrinkage and distortion.

The water in the tree is contained in the actual fibres which are saturated (bound moisture), and in the open pores or vessels which are filled with a mixture of water and water vapour. Immediately the tree is felled the vessels will start to dry. This process is very slow in whole logs, but can be speeded up by converting the log into plank form, when the water-level will fall steadily until a fibre saturation point is reached (usually 30 per cent moisture content) in which the vessels are empty but the fibres remain saturated. From then on the water loss will be at a slower rate until the board has reached an equilibrium with the surrounding atmosphere. The amount of moisture thus left in the wood is termed the 'equilibrium moisture content' according to the 'relative humidity' of the prevailing atmosphere, and before describing methods of seasoning it is necessary to define these terms.

Relative humidity

Even in the driest of weather the atmosphere will always contain a certain amount of invisible " vater vapour which can vary considerably from hour to hour.The actual degree of moisture vapour present at any one time (relative humidity) is measured with a wet and dry Hygrometer, and is expressed as the percentage of moisture in the air at a given temperature, compared with the maximum amount saturation point) the air would hold at the same temperature without actual precipitation. The humidity rate can fluctuate rapidly, but fortunately wood is a slow conductor, and therefore rapid fluctuations either side of the seasonal average can usually be ignored.

Equilibrium moisture content

As wood is an organic and hydroscopic material susceptible to moisture changes it will always endeavour to reach a state of equilibrium with the surrounding atmosphere, giving up or taking in water like a sponge until the balance is achieved. The term 'equilibrium moisture content' is therefore an evaluation of the prevailing atmospheric humidity in terms of the corresponding actual water content the wood has achieved in that humidity; it is expressed as a percentage of the dry weight of wood substance (see Measurement of moisture, p. 13). The important point to remember is that no matter how old the timber happens to be it will always respond in precisely the same way, and that as its environment changes so will its moisture content.

Methods of seasoning

The whole object of seasoning is to bring the moisture »content of the wood down to acceptable levels, according to the use to which it will be put. as speedily but as gently as possible. The drying out of the actual fibres after the pores are emptied is accomplished by surface evaporation and consequent capillary attraction from the moist interior of the wood. If the evaporation rate is too fast then partial vacuums will be created in the wood substance and the cell walls will collapse, causing widespread splitting and distortion. If, however, the drying-rate is controlled, then the water loss will be replaced by air and the natural strength and elasticity of the wood will always strive to accommodate the strains imposed. In the literal sense, therefore, 'seasoning' means controlled drying, and any wood which has been satisfactorily dried down to an acceptable level is usually regarded as seasoned timber/lumber. There is no doubt, however, that additional storage time over the prescribed minimum does bring certain advantages not measurable in any scientific way. In effect it will have matured like wine—it will be no drier but the moisture will be more evenly distributed, and it will be kinder, more mellow, and more mature, and therefore not so liable to distort. These advantages may be fractional but they do exist and the old craftsmen who matured their oak for 14 years under the shade of apple trees, and surrounded by chest-high nettles, did so not through any mystique but for definite practical reasons.

Two methods of seasoning currently practised are air drying and kiln drying. Generally speaking air drying is slower and cheaper, kiln drying is simpler but more expensive. However, as air drying can never reduce the moisture content to much below 15 per cent, kiln drying is always necessary for lower levels; but as certain timbers do not respond satisfactorily to kiln drying in the green state, preliminary air drying is advisable. Any lingering prejudice against kiln drying—and it still exists—has no basis in actual fact, for both methods are complementary.

8 Brushing sawdust from log prior to next cut

Air drying

Ideally, hardwoods should be sawn and piled for drying when humidity values are high and initial drying relatively slow; but in practice most merchants or dealers cut and pile timber throughout the year, stacking the boards fairly close so that the drying-rate is not too rapid. On no account should green wood be close piled without air spaces between each plank, as this will inevitably invite mould and fungi attack (the same applies to seasoned wood unless it can be adequately protected from the rain). The normal procedure, therefore, is to sweep the sawdust from each plank as it is cut and stack as soon as possible on firm, level foundations in such a way that an uninterruped flow of air is maintained over every surface. The stack itself should be built up on brick or concrete piers or creosoted timber/lumber sleepers, with wood bearers across the piers not less than 9 in (23 cm) above ground level, and the planks or boards piled exactly over each other and separated by stickers of neutral wood—fir, poplar, horse-chestnut, etc. — placed from 18 in (45 cm) to 24 in (60 cm) apart according to the thickness (Figure 9). These stickers can be 1 in square section (25 mm) for thick planks and 3/4 in (19 mm) by 1/2 in (12.5 mm) for thinner boards, the 3/4 in (19 mm) dimension laid flat so that the stickers do not roll over as the boards are positioned. The line of the stickers must correspond with the position of the lower bearers, and they must be exactly over each other throughout the height of the stack, with the ends of the planks or boards well supported and not overhanging each other in random lengths, for the function of the sticker is not only to permit the free passage of air throughout the stack but also to restrain as much as possible any tendency to warping.

When completed the top of the stack should be weighted down with odd wood slabbing, and well protected from heavy rain and hot sun by a suitable lean-to roof with a generous overhang all round. Rain driving into the sides of the stack is not harmful, always provided the timber has a chance to dry out, and therefore the stack should be kept as narrow as possible and never over 6 ft (1.82 m) wide. Shielding from very hot sun, however, particularly at the ends, is advisable in the early stages. Some form of

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9 Stacking wood for air drying

9 Stacking wood for air drying

iii Wood in stick at Sheffield Park Sawmills, Sussex, England

degrade (splitting, checking, etc.) must be expected on the ends where the moisture is drawn from the cut pores fairly rapidly, but this can be kept within reasonable limits by covering the ends with special paint. Unfortunately, merchants or dealers usually cannot afford these extra precautions for the common run of wood, but some square edge woods imported from tropical countries are end coated in this manner.

As a general rule most hardwoods are air dried out of doors and can remain there indefinitely, although at a later stage the better logs are usually moved into open-sided storage-sheds where they can be fully protected from the rain.

Drying times The usual time allowed for the air drying of timber to the normal equilibrium moisture content of from 15 to 22 per cent, according to the season, is one year for every inch in thickness; but given favourable conditions 1 in (25 mm) hardwoods piled in autumn will in all probability be down to 20 per cent by the following summer, and 2 in (50 mm) planks to the same moisture content a few months later. (Softwoods will dry to about 20 per cent moisture content in three months.) Further drying to 15 per cent is only possible in very dry weather, and in exceptional heatwaves 12 per cent might be reached, but under normal conditions the moisture content will swing backwards and forwards within the higher range, and any further drying time will make no difference beyond distributing the remaining moisture more evenly, and helping to equalize the built-in stains. For lower moisture contents other methods must, therefore, be adopted.

Kiln drying

The use of hot air in drying wood requires specialized knowledge, and therefore the process is described in broad outline only.

The principles are simple enough, but expert handling is required at every stage. In essence, wood is stacked as for air drying in enclosed compartments or buildings through which hot air is passed, either by natural draught or by power-driven fans. As the heated air would induce very rapid moisture evaporation from the wood surfaces, with consequent severe shrinkage and distortion, live steam is injected into the compartment, so that the rate of evaporation is always maintained at a safe level. Constant control is necessary, as the degree of heat and level of humidification must be adjusted so that there is a progressive fall in the relative humidity, until the wood has reached the required moisture content. As woods differ widely in their reactions, specific schedules have been laid down for practically all those in common use; but other factors, i.e. wood quality, intended use, thickness, whether quartered or plain sawn, etc., permissible degrade (defects), must also be taken into account, so that much depends on the skill and judgement of the operator.

The advantages of kiln drying are obvious, for green wood can be satisfactorily dried down to any given moisture content in a matter of weeks only, moreover much lower values are possible than with air drying. While it is true that any tendency to warp or buckle is increased in kiln drying, this can be mitigated by keeping the initial temperature low and thus extending the drying time, or by air drying the wood down to 20 per cent and then kiln drying, and this latter method is probably ideal. As a rough guide, green hardwoods can be kiln dried to a moisture content of 10 per cent in from three to 12 weeks, according to the species and the schedules adopted, and air-dried hardwoods with 20 per cent moisture content down to the same value in one to four weeks. It must not be forgotten, however, that as soon as the timber leaves the kiln it will begin to take up moisture again (moisture pick-up), and will gradually revert to the normal equilibrium content of 15 to 22 per cent. It should, therefore, be placed in a warm store, or otherwise close piled and covered with tarpaulins until used.

Permissible moisture contents

Wood which has been stored in the open, even under the most favourable conditions, is hardly dry enough for use in heated interiors. While little shrinkage takes place in wood seasoned from green to fibre saturation point (30 per cent), from then on the shrinkage is appreciable with maximum movement within the range 20 per cent down to 10 per cent. The maximum moisture content of timber in use should, therefore, not exceed the amounts shown below if excessive shrinkage is to be avoided.

22 to 15 per cent Usual limits of moisture content in thoroughly air dried wood.

20 per cent Dry rot safety line.

16 per cent Outdoor furniture.

15 per cent Joiners' work in new buildings. Normal church-work in intermittently heated interiors.

12 to 14 per cent Bedroom furniture with only occasional heating.

11 to 13 per cent Living-room furniture with normal heating, including reasonable central heating.

9 to 11 per cent Furniture in flats, offices and public buildings, etc. with continuous high degree of central heating.

8 per cent Radiator shelves and casings, etc.

The upper values in each case are usually safe lough, dependent on the wood species, as there is always some air movement from open doors, windows, etc.

It should not be assumed that because a workshop happens to be continuously heated throughout the winter months wood stored in it will automatically dry down to the lower levels. it might conceivably fall to 6 per cent in exceptional circumstances, but owing to the continual movement, opening of doors, etc. it is more likely to be within the range 10 to 12 per cent and even 14 per cent and it is the average value which counts, for wood moves slowly and is virtually unaffected by day to day changes. some method of conditioning the wood before use may have to be adopted, therefore; but while the old adage that it should be stored for at least six weeks in the room in which the completed furniture is to stand is very sensible, it is hardly practicable. However, it is possible to create a small warm store, or even a very simple kiln, built of insulating materials and with a gentle source of heat, for even electric light bulbs in a closed compartment will give sufficient for small sections, though care must be taken not to overdry and at too rapid a rate. on occasions the writer has even pressed a heated linen-cupboard into service, but a close witch should be kept for splitting and distortion, with the wood sections cut slightly oversize so they can be squared later.

Additionally, it is always wise to protect the completed furniture at the first possible opportunity, for oil-paint (three coats) will give 50 per cent protection, French polish and cellulose (two coats) 30 per cent, and wax-polish 3 per cent against moisture absorption over a period of one month, with much higher values over seven days.

Dehumidifiers

In recent years, the dehumidifier as a wood seasoner has completely revolutionized the kiln drying process. Many small furniture making concerns can now undertake drying economically both for their own use and for resale.

Until this breakthrough, hot air steam kilns were in normal use demanding both specialist expertise and expensive equipment. The dehumidifier, first developed to reduce humidity in store rooms and basement conditions, works on the refrigeration principle, and is actually kinder to the wood in the process as no real heat is involved.

Timber/lumber seasoners using this method come in a variety of sizes and are easy to install and to operate. The small portable de-humidifying unit is also invaluable to the furniture maker for controlling the humidity within the workshops, or, quite frequently, within a small part of the premises as previously described, in the form of an insulated room or chamber with the dehumidifier replacing the light bulbs.

Small kilns or drying areas with these smaller portable units are ideal for the craftsman who does not wish to undertake serious kiln drying, but wishes simply to condition and finish off air dried stock, particularly when humidity is high.

Measurement of moisture

The most accurate method of measuring the moisture content is the oven method, in which thin slices of the wood to be tested are accurately weighed, heated in an oven or drying-chamber, repeatedly reweighed until it has been established beyond question that all the moisture has been driven off, and the

11 Dehumidifiers

HPD30: A compact 3 kW model for use where space is at a premium moisture loss then calculated in accordance with the formula—net weight minus dry weight over dry weight and multiplied by 100, to yield the percentage moisture content. Thus, if the timber sample weighed initially 5 gm, and after reweighing only 4 gm, then the calculation would be:

5 gm 4 gm x 100 = 25 percent moisture content 4gm

Care must be taken not to scorch the wood, and the weighing must be done on an accurate chemical balance, while knots and other defects should not be included in the sample.

A simple oven using one or more 40 watt electric light bulbs is illustrated in Figure 12.

While the results obtained by the oven

PD15: An easily portable model with similar performance to the HPD30

PD75: Can be placed inside or outside the drying chamber with ducting. This 9 kW model is for more serious wood drying.

11 Dehumidifiers

HPD30: A compact 3 kW model for use where space is at a premium

PD15: An easily portable model with similar performance to the HPD30

PD75: Can be placed inside or outside the drying chamber with ducting. This 9 kW model is for more serious wood drying.

method are extremely reliable, they are more applicable to scientific research, and for field-work the electric moisture-meter will give instantaneous readings accurate enough for most practical purposes. These meters work on the principle that, as wood itself is a bad conductor of electricity, any moisture present will facilitate the passage of an electrical current passing between two electrodes spaced apart and inserted into the wood, and the measure of resistance to the passage of the current can be expressed in terms of moisture present as a percentage of the total bulk. One such meter uses a transistorized sensing-plate to record the effect of the material being tested in a high-frequency electrostatic field; but most common forms of meter employ contact, clamp or drivein pin electrodes attached to cable leads, with the meter actuated either direct from the mains. or with standard dry-cell high- and low-tension batteries.

The range of measurement varies according to the type of meter, with a normal coverage of from 6 to 30 per cent, and a margin of error of from 1 to 2 per cent. It should be borne in mind. however, that the presence of mineral salts in the wood may affect the readings, while some woods give large errors, especially in the upper moisture range; but most manufacturers give a table of adjustments covering many of the commonly used woods.

In taking any measurement, whether by oven method or moisture-meter, the readings should not be taken from the end of the plank, but from the thickness of a cut section at least 9 in 228 mm) in from the end and immediately the cut has been made, otherwise surface evaporation may falsify the test. If an approximate moisture content only is required, then the electrodes should be pushed into the surface at several points throughout the length of the plank not less than 3/16 in (4.5 mm) deep, and then the average of the reading used.

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