A question I get asked more often by beginners in woodworking is why there is a need to dry wood before usage. Of course, the answer to this question is not straightforward because you first need to understand the natural properties of wood. One primary property of wood is its hygroscopic property. The term “hygroscopic” might seem hard to understand, yet, it simply means anything that absorbs and gives off moisture to reach equilibrium.
Because of this property, wood changes its dimensions depending on the ambient humidity. Seasoning or drying wood is necessary because of this property. Drying helps wood gain dimensional stability by minimizing warping and dimensional changes. Besides, you can kiln, shed, or air-dried wood.
Wood Drying Methods and Drying Time
Wood will change dimensions as it absorbs and desorbs moisture. Thus, it has to undergo a drying process to minimize changes in its dimensions when used for projects. Below is a short description of these three common drying methods and their specific drying times:
1) Air Drying
Air-drying wood is the natural way of drying wood. This process was employed long before kiln-drying got introduced. Timber workers usually bring logs of wood to an open area to let them dry from the heat of the sun. Besides, they let the logs in the open area ventilate or allow air to circulate around the logs.
The rule of thumb you need to follow when air drying is to let every inch of the thickness of the wood dry for a year. This rough estimate of the air-drying time is a good starting point if it is your first time engaging in air drying of wood.
You can stack lumber on stickers and let the winds dry the pile of logs. Slow drying due to slow airflow and high moisture might result in wood staining. On the other hand, fast drying due to low humidity and high airflow might result in the splitting or cracking of logs.
Air drying, however, may not suffice if you use lumber to make furniture and many finished products that require around six to eight percent moisture content. Nevertheless, you can utilize air drying as a primary step to prepare lumber for furniture. Then, complement your seasoning method by kiln drying the lumber for the final step.
Air drying is straightforward to do. Yet, the lumber might lose some of its sterling quality when you solely rely on air drying, especially if the weather condition in your place is extreme. Hence, if you can’t afford to let your lumber lose some sterling qualities, it will be best to consider the other option—shed drying.
Air drying, of course, is fraught with many problems and issues. It can lead to wood degradation and damage, which is sometimes more costly than engaging in other methods of drying wood.
Furthermore, air drying takes time. Depending on the weather conditions and the size and lumber species, it can take many months or even a year. Besides, when you air-dry lumber, you must stack them correctly to ensure that air circulates adequately around the wood. Moreover, it will be best to test the wood frequently for moisture content.
You can employ several controls to develop a more efficient air-drying method. Such controls include laying out the wood stack, orienting the wood stack properly, and putting a cover on the stack of wood.
2) Kiln Drying
One of the wood production standard practices is kiln drying. This method of drying wood is very efficient in bringing the moisture levels of green lumber down to the acceptable or workable range. When kiln drying, the lumber is positioned inside a chamber characterized by controlled airflow, humidity, and temperature levels. Thus, the lumber’s drying process is facilitated and accelerated without causing increased defects in the lumber.
Kiln drying allows the wood to reduce its moisture content to an acceptable level without causing drying defects on the wood. Kilns come in different types, yet the most common kilns are the dehumidification kilns and the conventional kilns. You will also find solar and vacuum kilns used for specific conditions and applications.
Kiln drying is perfect for seasoning green wood that will be transformed into boards. It gets also used specifically for drying thicker lumber. Using a kiln, you can have excellent control of the drying process.
As mentioned above, kilns come in different types. Yet, they all operate using the same principle. Kilns usually have a vast, insulated room or chamber wherein temperature, humidity, and airflow are maintained at a safe and efficient level to dry wood.
Thus, within that chamber or room, the ambient conditions are highly controlled, leading to even drying of lumber. The inner core of the wood, as well as the outer core of the wood, dries evenly. This uniform drying of wood also helps avoid wood deformities and defects due to uneven and rapid drying.
3) Shed Drying
Shed drying is another alternative way to dry lumber. In this method, you must position the lumber in a shed sans any wall. This way, sunlight will not directly shine on the lumber. Besides, the lumber will be spared from rainfall, allowing adequate airflow around and over the wood.
You can regulate the drying rate of the wood by equipping the shed with mesh curtains made of plastic and pulling these curtains closed during dry and hot weather while drawing them up when the weather is damp and cool.
Wood dried through this process achieve a moisture content of more than 20 percent. You can also use fans to ventilate the lumber instead of relying solely on the natural flow of the wind. Of course, fans can speed up the drying process compared to air drying and simple shed drying. Nevertheless, the operating costs of fans can lead to higher bills.
You can quadruple the number of lumber you can dry if you shed dry them first before you dry them, compared to kiln drying the lumber directly after sawing.
Through shed drying, you can bring down the moisture content of the wood to 25% or less before you subject them to kiln drying. Besides, using sheds lessens the land area you need for drying wood. Pre-drying woods also reduce the defects and discolorations due to air drying.
Primary Reasons Behind Drying Wood
Wood, as mentioned above, is hygroscopic, meaning it releases or gains moisture depending on the changes in the humidity level of the ambient conditions. Thus, if you use wood for construction, whether for structural support or in other woodworking projects, it will expel or absorb moisture depending on the vicissitudes of the ambient moisture level. It does this to achieve equilibrium with the surrounding environment.
If the surrounding area has a high moisture level, the wood will absorb moisture to achieve equilibrium. Moreover, if the ambient area is dry, it will shed moisture. If the absorption or shedding of moisture occurs rapidly, it may cause damage to wood. Thus, the process of achieving equilibrium should be moderated or controlled.
When wood is dried, it gets prepared for use. Hence, excess moisture must be removed from wood to achieve the correct moisture content. If the recommended MC is not achieved, when you use the wood, it will shrink or expand rapidly and excessively, causing deformation and defects in your woodworking projects. Thus, drying wood is necessary to achieve the appropriate amount of moisture for the wood.
Drying or seasoning wood also reduces its weight while increasing its strength. It also gives the wood more resistance to decay and deterioration due to fungi or insect attacks.
Essential Terms to Remember
Aside from learning the different types of wood drying, it will also help if you are cognizant of the following essential terms to remember about wood drying or seasoning:
Equilibrium Moisture Content (EMC)
Equilibrium Moisture Content refers to the moisture content balance that the wood attains in relation to the relative temperature and humidity of the ambient environment or surroundings.
Moisture Content (MC)
Moisture content refers to the moisture level or weight of wood. MC gets often expressed in the percentage form of the over-dry weight of the wood.
Dry Bulb Temperature
Dry bulb temperature refers to the air temperature shown by the thermometer sans any wet cloth cover.
Wet Bulb Temperature
This temperature is usually measured using a second thermometer covered by a clean and smooth water-saturated cloth. The water evaporation coming from the wet cloth lessens the indicated temperature. The evaporation and reduction are interdependent. Besides, they are directly interrelated to the relative humidity of the air.
Relative Humidity (RH)
Relative humidity refers to the ratio between the water amount in the air relative to the maximum water amount the air can handle or hold at a given temperature. RH gets usually expressed in percentage.
The relative humidity is an important indicator because it affects the wood’s drying rate. If the relative humidity goes down, the wood will dry faster. Besides, the ultimate MC of a stack of wood will depend on relative humidity.
Effects of Relative Humidity, Temperature, and Air Circulation on the Drying Process
The external boundary conditions are usually controlled by the temperature, air velocity, and relative humidity, referred to as external drying conditions. These external drying conditions determine the drying rate and the movement rate of internal moisture.
If you would keep the relative humidity at a constant level, for example, you will notice that the higher the level of temperature, the higher the speed of drying. Thus, it is relevant to say that temperature affects the drying rate by raising the holding capacity for moisture in the air while speeding up the moisture’s diffusion rate across the wood.
In the drying kiln, the actual temperature gets referred to as the dry-bulb temperature (Tg). Tg refers to the vapor-gas mixture’s temperature. You can figure out this temperature using a thermometer and a dry bulb.
However, the wet-bulb temperature (TW) refers to the temperature you will get by the evaporation of a minimal amount of liquid in a huge mixture of unsaturated air vapor. It will be best if you keep the sensing element of the thermometer moist using a cloth (porous fabric sleeve) that you keep in a clean water reservoir.
You should keep a two m/s air flow to avoid a stagnating damp air zone around the sleeve. Water evaporates and cools your TW thermometer as air passes over the porous wet sleeve.
The difference between the wet-bulb and dry-bulb temperatures can help you figure out the relative humidity using the standard hygrometric chart. Remember that the higher contrast between these two metrics, the lower the relative humidity is. Thus, if you record a Tg of 100°C and a TW of 60°C, then the obvious relative humidity is 17 percent using the hygrometric chart.
Timber Classifications Relative to Drying
Timber comes in various classifications, for they also come from different tree species. Yet, you can classify them based on their drying rate and susceptibility to drying degradation. Below are the different classes of timber for drying:
High Refractory Woods or Lumber
Woods that are slow and hard to dry get classified as high refractory woods. Woods included in this class are heavy structural timbers characterized by high density. This class includes blackbutt, Ironbark, Brush Box, and Southern Blue Gum. Moreover, these woods necessitate a higher level of care and protection against fast-drying conditions to achieve optimum drying results.
Moderately Refractory Woods
The moderately refractory woods are timber types with moderate tendencies to split or crack during drying. You can dry these woods with less or no defects at all. You can use an 85°C dry-bulb temperature when drying these woods. Excellent examples of these woods include the Sydney Blue Gum. These wood types are ideal for furniture.
Non-refractory Woods
You can quickly season these types of woods, and the results will be defect-free even if you apply high temperatures greater than 100°C. However, if these woods are not dried quickly, they may exhibit discoloration like a blue stain as well as mold on their surface. One example of this wood type is Pinus radiata. Other examples of these woods are softwoods with low density.
Up to What Point Should You Dry Wood?
As mentioned above, woods are classified differently. There are high, moderately, and non-refractory woods. Hence, the drying time for these different woods varies. So, up to what point should you dry lumber? The answer would depend on the wood type you would like to dry.
It will be best if you dry, however, woods up to a specific MC near the expected MC that the lumber would achieve when you use it. This way, you can avoid warping and changes in the dimensions of the wood after you’ve used it.
The wood-in-use MC depends on the RH to which the wood gets exposed. The temperature, however, at this point, is non-relevant. Varying wood species have similar MC-in-use when exposed to similar RH.
Equilibrium Moisture Content (EMC) is the term used to correlate MC and RH. Thus, the air EMC is similar to the wood’s MC when exposed to a particular RH.
As a rule, the final MC of the wood in the kiln must be within 2% MC relative to the expected EMC of wood in use. This way, problems due to moisture can be avoided. Follow this rule to prevent manufacturing losses and loss of customers and sales.
Wood usually shrinks in thickness or width by about 1% for each four percent change in MC. Thus, suppose you have oak lumber around 2-1/2″ wide. If it loses 4% MC, it will shrink by around 1%. So, if you use this wood as a flooring piece of approximately 30 feet wide floor area, the entire floor will lose about 3% MC, leading to a sizeable shrinkage of the total floor area.
This shrinkage will lead to ugly and objectionable cracks. However, the abovementioned rule differs from one wood species to another. For example, hardwood-like teak will only shrink around 1% for every eight percent MC change.
Other Characteristics and Qualities of Good Drying Process
When drying wood, you would want to achieve characteristics that make the wood ideal for use in woodworking projects. Below are several features and qualities that a good drying process should bring in:
Warp-free Result
Warping might be caused by bad wood stacking. Bad stacking includes non-uniform thickness, non-flat support or foundations, and inordinate sticker alignment. Moreover, wrong sawmilling procedures and wood factors cause warping. On the other hand, Cupping is caused by rewetting partially dried lumber.
Casehardening-free
When the wood is still warm, you condition it or relieve it from stress by rapidly adding moisture to the wood surface. Steam heat for relieving stress might increase the temperature of the kiln beyond the required temperature level, resulting to poor relieving of stress.
You can lower the steam temperature using water or you can cool the lumber before steaming. You can use 82°C air temperature (Dry-bulb Temperature) for optimum results.
Remember that you might get erratic stress relief if you don’t have consistent or uniform lumber’s MC when you commence stress relieving.
It will be good to note likewise that solar kilns do not necessitate stress relief because the high humidity of the night can relieve the lumber from casehardening.
Checks and Splits-free
It is advisable to end-coat all lumber quickly after sawing it. You can follow reasonable control of the wood’s length and stacking of lumber to avoid drying the ends fast. A high RH of 40% or above is critical. Besides, moderate air flows, as well as lower temperatures, are also essential.
It will be best if you also control the drying rate within limits. However, the precise drying rate will depend on the lumber’s thickness and species.
Good Color
The resulting color of lumber should be good, for old lumber has around twenty times the risk of exhibiting sticker stains, stain-fungal stains, pinking, browning, greying, and many other color deficiencies. You will need to provide freshly sawn lumber with low temperature, brisk velocities, and humidity levels quickly after stacking while the lumber is still within the 30% MC to enable you to control the stain. Ensure you have partially filled dryers and narrow loads. Remember that rain exposure and poor stacking can increase staining.
High Strength
The goal of drying wood is to provide it with high strength. Low dryer temperatures and humidity can help increase strength. Besides, remember that fungal, bacterial, and other factors can lower strength, and they may be beyond your control.
Good Machinability
It will be good to note that too-wet wood will fuzz. However, too-dry wood under 6% MC will likely split, chip, and exhibit machining defects. Thus, you should ensure that you monitor and inspect the driest wood pieces in your kiln dryer while also noting the wettest ones.
It is wrong to take for granted the impact of over-drying on the machining property of wood. Thus, you should refrain from keeping temperatures beyond 71°C and avoid too low humidity levels. Hence, you should set and condition the resin within the acceptable level of 82°C.
Good Gluing Property
Wood will only exhibit good gluing properties once the drying process results in accurate MCs. Thus, you should check for very wet wood 8% beyond or very dry below 5.5% MC. Besides, you should ensure the temperatures do not exceed 71°C during the drying schedule.
How Does Drying Softwoods and Hardwoods Differ?
This question is a typical question asked by beginners in drying wood. Such a question, however, is very much relevant when drying wood. Remember that the species of the wood has a bearing on the drying process. Each species necessitates varying drying times than other species to get optimum drying results.
For example, you need to dry oak slowly. Otherwise, its properties will degrade. On the other hand, you need to dry pine at a fast rate. Otherwise, mold and stains may develop. So, you need to figure out the correct method for drying every species of wood you would like to use in your woodworking projects.
Will Lumber Remain Dry Once It Has Dried?
Wood, as mentioned above, is hygroscopic, meaning it will absorb and shed moisture. It does this because it wants to achieve equilibrium of MC content. So, the wood will indeed be affected by ambient weather changes. Thus, wood will still change its dimensions even if it is dried.
Conclusion
Reasons abound as to why you should dry wood, and as mentioned above, you can prevent many wood deficiencies if the wood is properly dried. Such defects include staining and decay, dimension changes, and many other flaws. Remember that adequately dried timber is perfect for use in many woodworking projects.
There are three methods of drying wood, yet kiln drying is the best method. However, kiln drying is best complemented by the shed and air drying. As soon as you cut a tree, air drying begins. Thus, if you would use the said cut wood for making furniture, you should ensure you dry it properly to prevent any degradation of the wood.