Seasonal Transitions Impact More than Kiln Drying Schedules
For lumber mills in Canada, seasonal transitions play an enormous role in the treatment of the log resource to be dried, and make it necessary to modify kiln-drying schedules. A deep understanding of the characteristics of each species, including sap movement (changes from one season to another), is essential whether the mill is processing hardwood or softwood.
In addition, the climatic conditions present in a given season or during seasonal transitions are quite different from region to region.
Sap Movement and Other Ambient Conditions
If the outside temperature is below 32⁰F, there is no discernible movement in sap (water). As the temperature begins to rise, the sap will start to slowly move up to the surface.
As summertime approaches, factors such as temperature, relative humidity and wind speed will come into play. Therefore, it is necessary for mill personnel to plan to protect the rough sawn lumber from defects such as discoloration and mold or insect damage before it ever reaches the kiln.
In regions where the relative humidity is low and greater wind speeds are the norm, mill personnel should be concerned about damage to the wood while it is on stickers waiting to be kiln-dried. The damage could be in the form of checks or cracking on the ends or faces of the boards.
In other areas of the country where higher temperatures, low airflow, and high humidity are present during the summer season, fungal growth and staining are of prime concern. Unless lumber is stored for a prolonged period of time, insect infestation is less of a worry because insects and any unhatched eggs will not survive the kiln.
Effect of Seasonal Transitions on Kiln Scheduling
Most mills have a set of summer and winter schedules for drying. In summer months, summer schedules are followed. During the seasonal transition from summer to fall, kiln personnel will often begin to see charges of lumber coming out of the kiln with average moisture content readings exceeding their target MC. This will trigger modification of the kiln-drying schedule to accommodate the transition to the colder winter months.
When it is springtime or fall temperatures are generally cooler and it may be wet. The sap is just starting to begin to flow. However, where it is much warmer, the sap flow is much faster. Kiln operators may have to make very quick scheduling adjustments because of rapidly changing climatic conditions, especially in recent years.
Protect Lumber in the Yard before Kiln Drying
Every precaution that can be taken out in the yard will help kiln operators develop an effective schedule that will produce the highest-graded lumber possible. In the end, if a mill continually turns out premium quality lumber, then customer satisfaction will remain high, and of course, that maximize profit for the mill.
Mill employees have several methods/treatments at their disposal to help prevent lumber degrade (especially end splits, discoloration and damage due to insect infestation) while it awaits the kiln:
1. End Coating Lumber
Hardwood mills will generally apply a wax-like product such as the well-known Anchorseal® or another compound to the ends of logs/lumber. It is most often sprayed on the ends of boards after they have been sawed into dimensional stock in the sawmill.
Other mills coat the logs upon arrival, especially if for economic reasons the mill has chosen to stockpile logs to be sawn at a later period of time. In addition, a lot of mills spray a fungicide on the ends of logs before the sealer is applied.
Softwood dimension mills typically do not end coat lumber due to the fact that commodity softwood lumber typically sells for substantially less than hardwood species. Hardwood mills can better absorb the cost of protecting their lumber from end splits and the development of mold and inner wood fungal attack.
In addition to end coating, hardwood mills may choose either to air dry lumber for some time or to implement a pre-dry process before kiln drying.
Lumber is placed in a pre-dry building and fans continuously blow across the stacks of wood. This step is taken before subjecting the boards to a more aggressive drying environment (the kiln).
2. Sprinkling Logs with Water
Sprinkling log decks with water is done in various parts of the country to help protect lumber from developing defects before it is kiln-dried. A large lawn sprinkler-type unit is used by lumber mills to keep stacks of lumber wet, especially during hotter times of the year. Water evaporation helps keep the logs slightly cooler, slowing down the instance of fungal growth. When logs are very wet, it is more difficult for fungus to get established because they need oxygen for growth; it is also harder for insects to lay eggs.
In the West, some pine-producing mills will also use this method to retard blue staining, which commonly can be seen on lumber in the summer months.
3. A Regional Strategy to Protect Lumber from Discoloration
At some eastern mills, birch, maple, and pine logs that are to be sawn into boards earmarked as specialty products are frozen to ensure white color. The short logs from winter-cut timber are placed in ground depressions and sprayed with water to form a coating of ice. The frozen log decks are covered with sawdust, wood shavings, or other available insulating material so that the wood remains frozen well into the summer months when further processing will commence.
A Special Note to Mill Personnel
Hats off to all mill personnel who work very hard to complete the pre-kiln processing of lumber, meet the challenges of seasonal changes and use all methods economically possible to protect boards against developing defects (end splits and discoloration, to name a few). Not to mention using proven strategies to stop mold and fungal growth and insect infestation from invading good quality lumber.
Readers now have a sample of the challenges and complex decision-making processes awaiting a kiln operator. However, when all of the necessary tools are used and critical regimens followed, the efforts of mill personnel will be rewarded by producing the best grade lumber possible.
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