Relative humidity

The amount of water air can hold depends on the air temperature: the higher it is, the more water it can hold. Given an atmospheric pressure of 1 bar (14 psi), one cubic meter (35.7 cubic feet) of air can hold 4.8g (0.17 ounces) of water at 0 degrees Celsius (32 degrees Fahrenheit), while it can hold 30.3g (1.07 ounces) of water, i.e. about six times as much, at 30 degrees Celsius (86 degrees Fahrenheit).

Relative humidity is the ratio between the actual amount of water contained in the air and the maximum amount it can hold at a given temperature. A relative humidity of 50 per cent in a room or out of doors therefore means that the air holds only half of the amount of water it could hold. As the air temperature rises, the percentage of relative humidity drops: the same amount of water contained in the air as before, which earlier constituted 50 per cent of the maximum possible, now represents a smaller share, say 40 per cent, of the total as the air temperature is higher and the air could now hold more water than before. As the air temperature drops, the percentage expressing relative humidity increases because the amount of moisture in the air is now closer to the maximum possible and the air is more saturated (70 per cent saturation, for instance).

Relative humidity is measured with a hygrometer. Why not use your next shower to set it? When the mirror in your bathroom is beginning to steam up relative humidity is 100 per cent, and you can set your hygrometer accordingly.

moist or wet - and this despite the sawmill employee's assurances that the wood has been kiln-dried. So what has happened? The wood may indeed have been dried; but because it was stored out of doors (or in an open-sided out-of-doors place) after seasoning it has taken up some moisture again. Or it may well never have been dried in the first place. Although a wood moisture meter should, in my opinion, be an absolute must for everybody trading in timber, I have never actually seen one in any of the sawmills I visited for buying wood.

Drying wood

Since there is a direct link between the moisture content of wood and that of the surrounding air, an equilibrium between the two will at some point be reached at which the wood will cease to absorb or lose moisture. This is a very slow and sluggish process that can nevertheless be easily followed by checking the weight of a wood sample.

Due to the direction the transport channels inside the wood run, the loss of moisture is highest at the end grain. For this reason these areas should always be sealed with hot wax or paint. This is an important precautionary measure to take to prevent the development of cracks that can be quite deep and can reduce the amount of usable wood or even make all of it unusable.

Because green wood reacts very strongly to changes that only one side of it is exposed to, it should always be stacked in piles with small spacers between the individual layers to allow air to flow freely around each piece of wood. When you leave a straight board somewhere without putting anything under it, the top side will lose or absorb moisture more quickly than the bottom side and the board will bend. Since the same thing happens when wood is treated on one side only, finishes, for example, should be applied on both sides. And because wood moisture is rarely evenly spread and the center of a piece of wood will always be wetter or drier than its surfaces, any kind of surface treatment such as planing has to be carried out on both sides - if you plane off too much on one side, the two surfaces will become unevenly wet and the wood will be likely to warp.

The main problem with moist wood is the formation of mould. This white, woolly coating develops very quickly unless wood is immediately seasoned and the moisture content reduced to below 18 per cent. Seasoning timber is basically all about finding the right balance between too fast and too slow drying: one can lead to cracks developing and the other to the formation of mould. The seasoning process needs to be closely monitored; it can be slowed, if necessary, by covering the wood with plastic foil. If the boards to be dried are from the same log, they should be placed above each other in the same order in which they were cut from the log. The first board of a pile should be about 30cm (1

foot) above ground and flat wooden spacers should be placed at no more than 40cm (16") distance from each other. To prevent undesired changes of shape occurring, it is also important to place the spacers exactly above each other. 30mm x 40mm roof slats are well-suited for this purpose. The sealed ends of the boards have to be flush and the top board should be protected against drying out too quickly by covering it with a scrap board.

As a result of the loss of water and the associated shrinkage wood warps. Flatsawn timber is particularly affected by this. Wood will, in general, always warp so as to give the impression that the annual rings are trying to become straight. Changes of shape occur because the outer areas of a board dry first and shrink in the process. Only quartersawn wood (i.e. wood with straight, “vertical” annual rings) is not affected by warping; it does none the less shrink. Wood shrinks by different amounts in different directions: radially, perpendicular to the annual rings, shrinkage is less than in tangential direction, parallel to the annual rings. By stacking wood in piles until it is evenly dry its tendencies to change shape and warp are counteracted by the weight of the boards that make up the pile. Putting some additional weight on top of the pile can, however, only be recommended as the top boards will inevitably warp when the wood becomes lighter if there is nothing on top of the pile to replace the lost counter-pressure of the moist, heavier boards. A board containing the core of a log will always crack. For this reason it is advisable to saw out this core bit from the middle board of a log before seasoning it.

All wood moisture content figures can only be averages. Similarly, the equilibrium moisture content will vary according to the climate and the time of year.

In my Central European climate, where average humidity is about 70 per cent, wood stored out of doors will, on average, always have a moisture content of well over 10 per cent. For making furniture and instruments this is still too high a percentage, since the wood will be worked and used in heated indoor rooms where humidity is lower. Wood will therefore continue to dry indoors and often warp or get cracks until it reaches a point

Wood moisture content

The weight of a piece of wood always includes the weight of the water contained in it.The relative wood moisture content is the ratio between the weight of the water contained in wood and the fullyseasoned wood. The approximate wood moisture content can be determined with electronic measuring instruments measuring the electrical conductivity of wood, which is directly linked to the wood moisture content.

Another method is to first weigh a relatively small sample of wood and to dry it in an oven afterwards until it ceases to lose weight, which is when it is fully-seasoned. From the remaining weight of this fullyseasoned sample piece the wood moisture content can be determined.

With the help of the diagram on the right you can estimate what moisture content a piece of wood will have after being stored a sufficiently long time at a particular constant humidity. The resulting wood moisture will lie between the two curves for almost all types of wood.

Wood moisture dependence on humidity