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Seasoning to Prevent Defects in Green Wood

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Seasoning to Prevent Defects in Green Wood Seasoning to Prevent Defects in Green Wood Contents Page Introduction 6 Principles of Wood Drying 7 Protecting Logs and Bolts 11 Outdoor Air Seasoning 11 Piling Your Lumber Properly 12 Drying Time for Lumber 15 Indoor Seasoning 15 In a Home or Farm Building 15 In a Homemade Solar Kiln 16 In the Home Freezer and Heated Room 17 In the Home Microwave 18 Troubleshooting Seasoning Defects 18 Sticker Thickness 18 Sticker Straightness 18 Bolster Thickness 19 Sticker Placement 19 Board Placement in Packages 20 Preventing Stain and Mildew 20 Preventing End Checks and Splits 20 Preventing Surface Checks 21 Preventing Warp 21 Storage of Dried Lumber 21 Air-seasoned Lumber 21 Indoor-dried Lumber 21 References 22 3 List of Figures Page 1. Wood shrinkage and distortion of flats, squares and rounds as affected by the direction of annual growth rings 7 2. The three primary surfaces of wood. Dimensional movement is unique in each of these three planes. 7 3. The recommended moisture content at the time of installation for interior wood and wood-based products varies throughout the country. Producers should be aware of the recommended moisture content values for wood and wood-based products when selling goods to these areas. 8 4. Air-drying map for the Eastern United States 8 5. Drying defects that can occur in improperly seasoned lumber 12 6a. A poor commercial site for seasoning native hardwood. This low-land site has poor air circulation and is subject to flooding. Stacks should be covered . 12 6b. A poor stacking job with no stickers between boards. There is little ventilation under or inside the pile. Lumber piled in this fashion may lead to warp, check, stain and decay. 12 7a. A good foundation is needed for correct stacking and good seasoning. Use treated wood, steel or concrete. 13 7b. Start the pile: alternate placing of all the shorter lumber in the layer. Orderly piling and spacing make for good ventilation and even drying Do not allow lumber ends to hang over more than 6 inches.. 13 8. A new form of sticker is shaped like a dog bone. These stickers provide maximum ventilation between adjacent lumber, because they are curved and have less surface area in contact with the boards. These stickers will provide better ventilation than traditional stickers and minimize the possible area for the transfer of stain from a sticker to a board (sticker stain). It is always important to keep stickers dry and discard stained or broken stickers immediately. 13 9. Good seasoning practices include a level, well-drained open area that is not subject to flooding. Foundations should hold the lumber at least 15 inches above the ground and a fastened roof or cover to protect wood from rain. Orient stacks to reduce sun exposure and increase air flow, usually north to south stacks. 14 10. The front, south side view of a solar-heated lumber dryer 16 11. East side of solar lumber dryer showing principal loading and unloading door and the small access to collector area. 16 12. This lumber has experienced severe bow because these stickers and braces have been placed too far apart. 19 13. A properly stacked pile of lumber has stickers of equal thickness that are vertically aligned. The normal distance between stickers is 18-24 inches. 19 14. These southern pine boards are of uneven thickness. The board that is too thin was miscut and should have been culled or at least sorted out before stacking. 19 15. Checks and separation occurred in unprotected ends of this hardwood log during storage in the log yard at the sawmill before milling. End checks can be reduced by end coatings and prompt sawing. 20 4 List of Figures Page 16. End checks in cants caused by rapid drying and sun exposure. End checks that develop in logs will also be present in the lumber that is sawn. 20 17. Dried lumber is stored commercially on stickers in large storage sheds. Smaller operators will receive similar results in a garage at home or a shed on the farm. 21 List of Tables Page 1. Moisture content (percentage) of green wood of some commercially important species 8 2. Moisture content (percentage) of lumber at various temperatures and relative humidities 9 3. Recommended moisture content for various wood items at time of installation 9 4. Estimated time to air-dry green 1- and 2-inch eastern hardwood lumber to about 20 percent average moisture content 10 5. Tendency to check and warp for various hardwoods 14 6. Effective air-drying days needed to season various hardwoods (1-inch thick lumber) to 20 percent moisture content. 15 7. Resulting indoor relative humidity and wood moisture content when outdoor air at 80 percent relative humidity and various temperatures is brought into an unhumidified room and heated to 68 degrees Fahrenheit 16 8. Moisture content of lumber dried in the freezer and heated room 17 5 Introduction This publication provides information on some of The moisture content of green hardwoods may be problems and basic principles involved in seasoning more than100 percent. Extra care is needed from the time lumber, wood disks and other small pieces of wood. It is a tree is felled until the lumber has been correctly sea- intended to help individuals who would like to season soned for the intended use. Most dense, native woods small quantities of wood inexpensively and without (like oak, hickory, sycamore, beech, walnut, ash and elm) sacrificing quality. Small mill owners may also find this require even more careful drying than pine or some of the information useful in the manufacture of dried wood other light woods (like basswood, yellow-poplar, soft products. maple or cherry). Extra care and proper In addition to information on the seasoning processes, seasoning,however, will pay off with these advantages: this publication covers pre-drying treatments that reduce 1. Drying reduces the likelihood of insect infestations, defects like mold, stain, decay, check, splitting and warp. mold, stain and decay during storage and use. Fungi and The pre-drying procedures include treating wood with termites cannot grow in wood with a moisture content of PEG (Polyethylene glycol-1000), salt paste, pre-surfac- less than 20 percent. ing, end-coating and dipping in fungicides. Some solar dry 2. The shrinkage that accompanies seasoning is kiln issues are included. A troubleshooting section deals completed before the wood is put to use. with some potential pitfalls that can easily be avoided when drying wood. 3. Drying reduces weight and increases most strength properties of wood. When wood is first sawn from a green log, up to two- thirds of the weight of the wood is water. Much of this 4. Only relatively dry wood can be successfully glue- water must be removed from the lumber before it can be jointed, finished, machined and treated with chemicals. made into useful products. The moisture content (MC) of wood is defined as the weight of the water in the wood as a percentage of the weight of the oven-dried wood: MC (%) = initial weight - ovendry weight x 100 ovendry weight 6 Principles of Wood Drying Wood dries by movement of free water through fiber cavities, movement of bound water through fiber walls and movement of water vapor through wood fiber spaces. When wood dries to about 30 percent MC, or the condition in which the water has been removed from the fiber cavities but water in the fiber walls remain saturated, it is said to be at fiber saturation point (FSP). Shrinkage of wood begins only when the moisture content drops below the FSP. Because wood is not homogeneous, it shrinks more along the growth rings than across the rings (Figure 1). The least shrinkage or swelling of wood is found in the Figure 2. The three primary surfaces of wood. Dimen- longitudinal or “tree trunk” direction. These shrinkage sional movement is unique in each of these three planes. variations cause drying defects like warping and checking. (Source: USDA Forest Products Laboratory 1987) Shrinkage and swelling cease as the MC of wood ap- proaches an equilibrium with its environments. Species of Wood will give off or take on moisture when sur- wood vary in the rate and amount of shrinkage. rounded by air at a given relative humidity (RH) and An individual piece of wood will display unique temperature. When moisture gains are equal to moisture shrinkage or swelling patterns in the three planes of the losses, the wood is said to be at a constant MC. The wood. The three primary directions of wood movement green MC will vary between the wood in the middle of a are: tangential (perpendicular to the growth rings), radial log (heartwood) and near the bark (sapwood) of most (parallel to the growth rings) and longitudinal (parallel to wood species (Table 1). In the average home, where air the grain, the direction of the standing tree) (Figure 2). conditioning and central heating keep RH at 25-50 Tangential dimensional change is often nearly twice that of percent and the temperature around 70 degrees Fahren- radial movement for most domestic wood species. Longi- heit, the MC of wood ranges between 5 and 9 percent. tudinal dimensional change is almost always negligible for The wood framing (joists, rafters and studs) in a building most domestic wood species unless abnormal wood tissue attain an average MC of about 12 percent. Under warm, is present. humid conditions, as in a greenhouse with an RH of 90 percent, wood will have MC of about 20 percent. Table 2 gives wood MCs for different temperatures and relative humidities.
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