Sawmilling Concepts – Part V: Wood Drying Defects and Warpage

This is Part V 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 V, we will learn about wood drying defects and types of warpage commonly encountered in lumber operations.

Wood Drying Defects and Warpage

Importance of Proper Drying Schedules

Each species of wood varies in its microstructure: size of pits between fibers, size of fibers, number and size of pores, presence of tyloses, aspirated pits and possible drastic variations in the extractive contents.  The moisture movement during the drying process can vary drastically between species.  Therefore, each species has been tested by experienced kiln operators, and drying schedules have been determined that are the best for each individual species. If these schedules are deviated from too much, severe drying defects can occur.

Wood Drying Defects

If wood was dried very slowly we would not have to worry about many drying defects. However, since most kiln drying is done to get the wood into service as quickly as possible, several of the following defects may occur:

Surface checking – surface checks are failures that occur in the wood rays on the flat sawed surface of the lumber. They normally occur in the initial stage of drying when relative humidity is too low.

End checks/end splitting – end checks are cracks at the end of the board that are caused by the water moving out the end of the board more quickly than from the sides or edges.  It normally occurs in the initial stage of drying when relative humidity is too low.  It is a common drying defect and can be somewhat controlled by end-coating boards before kiln drying.  End splitting is an extension of end checks.

Wood cell collapse – cell collapse results from liquid (water) being removed too quickly during drying, causing the board to actually collapse into itself.  Extremely high temperatures cause this defect in the early stages of drying.

Ring shake – shake is the separation of the wood parallel to the grain along the growth rings. It can be caused by excessively high temperature in the early stage of the drying process or by felling the tree along uneven ground, causing the bole to bend and separate along the growth rings.

Warping

Warp is defined as any deviation from the true plane surface of the wood.  Warping normally is caused by the shrinkage value differences in the different parts of the tree; for example, juvenile wood on one side and mature wood on the other, or domination of latewood growth in the grain pattern of the board.

Other warping may occur because of the natural grain direction of certain species. Warping is a natural occurrence in wood and cannot be completely controlled.  However, the use of weights on the kiln charge will reduce some warping.

Types of warps that commonly occur are:

Cup – is a deviation from edge-to-edge on the end of the board.

Bow – is a deviation from end-to-end on the flat surface of the board.

Crook – is a deviation from edge-to-edge along the long surface of the board.

Twist – is a distortion of the board so that the two end surfaces do not lie on the same plane.

Kink – is an abrupt deviation from flatness or straightness due to localized grain distortion (around knot); deformation caused by misplaced stickers in kiln.

In the Next Edition

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

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 Summer/Fall 2005 (Volume 40) 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.