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Kiln Drying: How Seasonal Movement Of Sap In Lumber Combined With Temperature/Humidity Factors Affects Optimum Dryability

Kiln operators look at many different variables when developing and modifying kiln schedules. A deep understanding of the characteristics of each species including sap movement from one season to another is essential whether the mill is processing hardwood or softwood. Also, in the United States, the climatic conditions present in a given season is quite different from region to region.

Taking A Closer Look At Sap movement In Lumber

If the outside temperature is below 32 degrees F., there is no discernable movement in sap (water) in lumber which has fully equilibrated with its surroundings. As the temperature begins to rise, the sap will start to slowly move up to the surface.

When temperatures start to elevate above around 50 degrees F., the sap runs much faster causing mill personnel to develop/implement plans to protect the rough sawn lumber from defects such as discoloration and mold or insect damage before it ever reaches the kiln.

Now when temperatures start to decline, if this happens quickly, then other strategies are put into action to help prevent end splits or other surface defects on the lumber before it is further dried down in the kiln.

How Sap Movement Affects Kiln Scheduling

Most mills will have a set of summer and winter schedules for drying

They use their summer schedule and when the lumber starts coming out of the kiln too wet they go to a winter schedule. These schedules serve as a platform allowing for modifications that will have to be made based on specific seasonal fluctuations and where the mill is located in the United States.

When it is springtime or fall in the Northwest and the Northeast, temperatures are generally cooler and may be rather wet. Sap is just starting to begin to flow. In the southeast 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.

The basic concept is that if sap is running faster kiln schedules will reflect a longer drying period.

You would think that this scenario would hold true during summer months when the sap should be running very freely. The reality is that drying time could actually be shorter because moisture content is probably lower considering that logs have sat around for awhile in very dry conditions or a log spraying deck has been implemented at the mill keeping lumber cooler and wetter. In any case, the boiler fuel is dryer in the summer, heats up faster allowing for better steam pressure and a more reliable sequence

Sap Movement Issues: How To Protect Lumber In The Yard Before Kiln Drying

Mill personnel have several methods/treatments at their disposal to help prevent lumber degrade (especially end splits, discoloration and damage due to insect infestation) before its later stacked and pushed into the kiln.

Every precaution that can be taken out in the yard, will help kiln operators to develop schedules 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 it will improve that all important bottom line for the mill.

End Coating Lumber, Which Mills Use This Process

Hardwood mills will generally apply a wax like product such as the well known Anchorseal or other compound including highway sealer, to the ends of logs/lumber. It is most often sprayed on the ends of boards after they have been sawn into dimensional stock in the sawmill. Other mills have decided to coat the logs when they first arrive at the mill, 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 have chosen to spray a fungicide on the ends of logs before applying the sealer.

Softwood mills do not end coat lumber because of the cost factor considering that for the most part their lumber sells for $300 per 1000 board feet. On the other hand hardwood mills can better absorb the cost of protecting their lumber from end splits, and the development of mold and a inner wood fungal attack.

In addition to end coating, hardwood mills may either choose to air dry lumber for some time or implement a predry process before kiln drying, knowing full well that hardwood takes far longer to dry down to the levels needed for both indoor and outdoor applications. Lumber is placed in a predry building and fans continuously blo across the stacks of wood. This step is taken before subjecting the boards to a more aggressive drying environment (the kiln).

Another Tool: Sprinkling Logs With Water

Sprinkling log decks are used in various parts of the country as a tool to help protect lumber awaiting kiln drying from developing defects. 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 and also it is 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.

An Example Of A Regional Strategy To Protect Lumber From Discoloration

At some north 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 prekiln processing of lumber using 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.

When stacks of lumber are wheeled into the kiln, readers now have a sample of the challenges and the complex decision making processes awaiting a kiln operator. If all the industry tools that a given mill has been able to implement, for example: an in-kiln moisture measurement system (softwood mills) or sample board weight measurement techniques (hardwood mills) and finally an inline moisture measurement system with a data analysis software component, then at the end of the day the effort of all mill personnel will be rewarded because the mill will have produced the best grade lumber possible.