Sawmilling Concepts – Part VI: Kiln Drying Lumber

This is Part VI and the last of our series about sawmilling concepts.  The purpose of the series is twofold.  It will provide some useful tips to those who own a portable sawmill. It will also assist landowners, who may want to purchase a mill or hire a sawyer, gain a better understanding of proper sawing techniques.

Topics covered in the series include: sawing lumber, using the quarter scale, saw blade maintenance, introduction to drying lumber, determining moisture content of wood, wood drying defects, methods of drying lumber and dry kilns.

In Part VI, we will examine the procedures for kiln drying, including the topics of stickering, stacking of wood in the kiln, end coating, kiln schedules and equalization and conditioning procedures that are used to eliminate stresses.

Kiln charge

The kiln charge is the load of lumber that is placed in the kiln to be dried.  It is best if the charge is all of the same species and all the boards are at nearly the same moisture content.  However, charges of several different species can be dried at the same time if the kiln schedule used is for the most sensitive species.

Drying lumber that has been air dried first

Lumber that has been air dried first requires 24 to 72 hours of conditioning before starting the kiln-drying schedule. This is required because the exterior of the board during the air drying process has set the exterior wood fibers in tension.  These fibers probably have a lower moisture content than the wood fiber in the core of the board.

Conditioning is done by placing the kiln charge in the kiln and setting the dry-bulb temperature at or near 100° F (this temperature is species dependent).  Leaving the charge in this manner for 24 to 72 hours allows the moisture in the core of the boards to move to the outside shell of the boards and to the air around it. This allows the fibers in the shell of the board to equalize in both stress levels and moisture content with the fibers in the core of the boards before the drying process begins.

Proper stickering and stacking

Stickers – are small boards similar to a lath that are used to separate wood stacked in kiln charges to allow movement of air equally across each layer. This allows for the uniform drying of the charge.

Kiln charges that are properly stickered and stacked allow for better movement of the airflow through the stack and less warping of the lumber due to the vertical placement of the stickers.

First-time stickering from the sawmill – the proper stickering and stacking of lumber immediately after the lumber is removed from the sawmill eliminates the redundancy of handling of lumber.  A forklift or loading device can then be used to load and unload the kilns without restacking.

Kinds of stickers – the best stickers are those that are made from the same species that is being dried.  However, this is not always economical.  Stickers should not be made from softwoods because many softwoods have a lot of resin and can easily stain the wood being dried.  The best stickers are made from hardwoods that do not have high extractive contents in the heartwood, such as cottonwood, soft maple, etc.

Sizes of stickers – the sizes of stickers vary by the design of the kiln and kiln specifications. They range from 1/4" to 2" in width.  They range from 3/4" to 4" in thickness.

Stickering – is the process of laying stickers in between the layers of boards as they are being stacked into a kiln charge.  Placing the stickers in the right place is very important for the controlling of air movement and warping in the charge.  The stickers should be placed at the end of each board, less than 4' apart for softwoods and less than 2' apart for hardwoods.  Each sticker should be placed directly in a vertical row, as the layers of wood are stacked.

Stacking – the stacking of lumber for the kiln should be as uniform as possible.  The lengths should all be the same, and the sides of the charge should be as vertical as possible.  Equal spacing should be maintained between each layer.

Figure 2: Note the dark narrow band across the width of this board at its mid point.  It is sticker stain caused by using the wrong sticker material.

End Coating

End coating is the process of painting the ends of the boards of the kiln charge to prevent moisture from moving out of the lumber more quickly through the ends than the other surfaces of the boards.  This helps reduce end checking and end splitting.

Kiln Schedules

Kiln schedules are the schedules produced by kiln operators over a long period of time and experience that allow the wood of each species to be dried as quickly as possible with the least number of defects.  A kiln schedule tells the operator at what temperature and what relative humidity to set the kiln when a certain species reaches a certain moisture content.

Equalization

Equalization is the process at the end of the drying schedule that leaves the temperature and relative humidity at a certain condition, allowing the moisture content throughout the kiln charge to equalize to the same moisture content.  During the drying cycle, some areas of the kiln may have dried more quickly than others.  The equalization process allows the charge to equalize before the conditioning process.

Conditioning

At the end of the drying process, stress samples are taken to test for casehardening of the boards.  Almost always, casehardening occurs because of the nature of force drying lumber.  After the equalizing process, the lumber is then conditioned by setting the temperature and relative humidity conditions to increase the moisture content in the shell of each board.  This relieves the stresses caused by the force drying of lumber.  If too much moisture is added to the shell of the boards, reverse casehardening occurs.  This can be removed by raising the temperature of the kiln or reducing the relative humidity in the kiln.

Dressing and Edging

After the kiln charge is dried and conditioned, the boards can be dressed and edged if desired.  Most hardwoods are dressed (planed) to 13/16".  However, if someone wants to glue this stock and it needs to be planed again, the stock should be left at 7/8" or greater.  Edging allows the end user one straight edge to begin sawing the lumber on table saws in most small operations.  Normally, a three- to four-cent charge is added to the board foot price for this additional operation.

Acknowledgement

The information (including illustrations) for this article is from the Instructional Guide to Wood Technology and Sawmilling, 2000.  A publication of Wood-Mizer Canada Co., 217 Salem Road, Manilla, Ontario, K0M 2J0, 1-877-866-0667, <www.woodmizer.ca>.

This article appeared in the Fall/Winter 2005 (Volume 41) edition of the S&W Report, the newsletter of the Ontario Woodlot Association.

© Ontario Woodlot Association

Visit the ‘S&W Library’ section of our website to read Parts I to VI of Sawmilling Concepts.