What quality signs indicate lumber dried properly?
Drying lumber is an art and a science, and there are many factors that affect the process.
The most important thing to remember is that drying lumber too quickly will result in damage while drying it too slowly will allow mold and mildew to grow.
The ideal situation is to dry the lumber slowly and evenly so that the moisture content is reduced gradually. This will result in high-quality lumber that is free of splits, checks, and other damage.
There are a few quality characteristics that show whether the drying process of the lumber was properly done or not.
Absence of Checks and Splits
The absence of checks and splits means that the drying process was done slowly and evenly, allowing the lumber to dry without stress.
As soon as possible after being sawn, all lumber should be treated with an end-coating. Drying of the ends can be slowed down by properly stacking and controlling the length of the lumber.
Low temperatures and moderate airflow are equally crucial for MCs with relative humidity exceeding 40%. For the most part, the drying rate must be tightly controlled to avoid surface drying and prevent end-checking.
Absence of Warp
Warping is caused by the uneven drying of lumber. If the surface of the lumber dries too quickly, while the core remains wet, then the board will try to curl or cup in order to balance the moisture gradient. This can be prevented by slowly and evenly drying the lumber.
All warp is caused by wood factors and sawmilling procedures, with the exception of cupping and warping caused by poor stacking (such as uneven sticker thickness, poor sticker alignment, or non-flat foundations).
Cup is caused by rewetting of the outer surface of lumber that has already dried and shrunk.
So, in order to avoid warp, it is important to dry the lumber slowly and evenly. Also, in order to avoid cupping, it is important to stack the lumber properly and prevent the outer surface from rewetting.
Freedom from Casehardening
Case hardening is caused by the evaporation of water from the surface of lumber while the core remains wet.
When the lumber is warm, the processes for proper stress release or conditioning involve the quick addition of moisture to the timber surface.
The heat of the steam utilized for stress relief frequently raises the kiln temperature above the acceptable level, resulting in inadequate relief. Water should be used to cool the steam or to cool the lumber before steaming.
It is advised to use an air temperature of 82°C (180°F) (also known as the dry-bulb temperature).
It should be noted that if the lumber’s moisture content (MC) is not uniform when stress relief begins, stress relief will be erratic. Solar kilns do not require stress release since the high humidity levels at night prevent casehardening.
Uniform color is an indication of proper and uniform drying. If the drying process is done too quickly or without applying a uniform amount of heat, the lumber may show evidence of color gradients.
Obviously, color uniformity doesn’t solely depend on the drying process, but also on the structural properties of the wood. However, if the drying process is done properly, the lumber should be pretty much in a uniform color.
Low humidity and low dryer temperatures are crucial for high-strength lumber. Because the lumber is dried slowly, the cell walls have time to adjust and the lumber dries without stress.
If the drying process is done too quickly or at too high of temperatures, the lumber will be dried unevenly and will be weaker. So, in order to produce high-strength lumber, it is important to dry the lumber slowly and at low temperatures.
Other elements that will reduce the strength are bacterial and fungal decay, insect damage, and poor sawing practices.
Good machinability is an important factor in the manufacture of lumber. If the wood is too damp, it will fuzz, but if it is too dry, it will chip, split and develop other machining faults such as splinters and cracks.
Therefore, keep an eye on the driest pieces of wood as well as the ones that are still wet in the drier to ensure proper drying.
During machining, don’t underestimate the effect of over-drying. To ensure a proper drying process, avoid temperatures above 71°C (160°F) and extremely low humidity. It is acceptable to condition or heat the resin to 82°C (180°F).
Gluability is the ability of the wood to bond with adhesives. If the wood is wet, the adhesive will not bond properly and if the wood is too dry, the adhesive will not bond properly either.
In order to have good adhesion, it is necessary to have precise final MCs. Check for parts that are either too wet or too dry (MC content less than 5.5 percent), and stay away from drying temperatures higher than 71°C (160°F) throughout the primary schedule. Setting the resin to 82°C (180°F) is appropriate for conditioning.