The development of lumber kilns and the success of the lumber drying process requires a knowledge of moisture behavior of the wood.
The moisture content of timber is expressed as a percentage, and it refers to the mass of water present in the timber divided by the oven-dry mass of the timber. The moisture content of a freshly cut tree is usually 40–200%. In normal use, the moisture content of timber varies between 8–25% of weight depending on the relative humidity (RH) of the air.
Moisture content is not an absolute value but it changes according to the relative humidity of air. This is referred to as the equilibrium moisture content. The equilibrium moisture content of timber is a state in which the moisture content remains the same.
© Puuinfo Oy
The red line demonstrates the principle. When the indoor temperature is +22°C and indoor relative humidity is 50%, the graph shows that the moisture content of the timber is 9.5%.
The fibre saturation point refers to the moisture content at which the cell walls are saturated and the cell cavities have no free water. Timber starts to shrink when its moisture content drops below the saturation point. Similarly, wet timber stops swelling once it has reached the saturation point. The saturation point of the main species in Finland at +20°C is around 30%.
The physical process of wood drying
Hygroscopicality:
Wood is a hygroscopic material, which means it takes on water from the air. The water travels inside the wood either as liquid through lumens using capillary pressure, vapour through lumens or molecular diffusion through the cell wall.
Anisotropicality:
Wood is also an anisotropic material meaning that it shrinks and swells in different ways in the direction of the growth ring radius and tangent and in the direction of the grain i.e. longitudinally. When Nordic timber is dried from the initial moisture content to oven-dry, it shrinks in the direction of the tangent by roughly 8%, in the direction of the radius by roughly 4% and in the direction of the grain only by 0.2–0.4%.
There is variation in the moisture content within one piece of timber. Heartwood is always drier than sapwood, which makes drying timber challenging. The anisotropic nature and internal tensions of timber causes wood to twist and bend during the drying process. Moreover, tangential shrinkage causes checks.
Wood is an anisotropic material meaning that it shrinks and swells in the direction of the radius, tangent and grain in different ways, which affects the drying result. Source: Puuinfo Oy, © Antti Saikkonen
As the density of the wood increases, the shrinkage and swelling caused by changes in moisture usually also increase.
Measuring the moisture
The moisture content of timber can be measured using the oven-dry method and purpose-made moisture meters.
The oven-dry method:
In the oven-dry method, a sample is taken from the timber and the sample is weighed to get its initial weight. The sample is then dried in an oven at +104°C to oven dry and the sample is then weighed again to get its oven-dry weight. The moisture content is determined from the oven-dry weight.
Purpose-made moisture meter:
The moisture content based on the timber’s electrical resistance can be measured using a hand-held meter or continuous measurement meter. The meter’s spiked electrodes are inserted into the wood to measure the wood’s electrical resistance after which it indicates the moisture content taking into consideration the species and temperature.
Measuring the moisture content of a dried load.
© Versowood Oy/Tarmo Valmela
Dry and wet bulb temperature sensor:
The ambient air conditions and relative humidity in a kiln are measured using dry and wet bulb temperature sensors. The dry bulb temperature is measured directly from the air. The wet bulb temperature is a recovery temperature given by a wet sensor subjected to an air flow. The wet bulb temperature is the evaporation temperature of water in its current ambient temperature and humidity.
Continuous measurement moisture meter:
Continuous measurement moisture meters can be connected to an automatic grading system in a dry sorting line. The latest such meters are based on measuring microwave transmission. This means a non-contact continuous measurement. The majority of meters used are based on capacitive sensing where the meter measures the effects of the wood’s moisture on the meter sensor’s electric field.
The effect of moisture on the properties of timber
As timber is dried to the desired moisture content, its strength properties improve when the moisture content falls below the saturation point For example, timber’s resistance to compression and bending is roughly doubled as it dries from freshly cut to 12–15%. Its tenacity is at its highest when the moisture content is 6–12%. When using timber for construction purposes, its moisture content must be taken into consideration as it affects the strength of the timber significantly.
If the moisture content of timber remains over 20% for a longer period, damage will start to occur. In these cases, the relative humidity of air is usually over 80–90%. Mould starts to appear within a few months if the ambient relative humidity remains over 80% during this time. When the temperature is near 0°C, mould requires a higher relative humidity.
When the relative humidity of air exceeds 90%, wood starts to rot slowly if the conditions persist long enough. However, mould and rotting require a temperature between 0 and +40°C.
Blue stain is discolouration caused by the blue stain fungi, and the discolouration can go deep into the wood. Blue stain is not a type of rotting. The blue stain fungi spreads as spores or mycelium and appear especially on stored softwood. The blue stain fungi do not grow in temperatures below +5°C. Blue stain does not significantly affect the strength of the timber.
Source: Puuinfo Oy
