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WHY DRY WOOD? The most important reason to dry wood is that your customer will request it! Green lumber is used mostly for local uses such as fence boards, barn siding and so forth. Lumber that will be glued or finished has to be dried, however, and that includes just about all the higher-value wood products used indoors such as flooring, furniture, wall paneling, cutting boards and so forth. Dry lumber can be used for more types of products and has greater marketability. Dry lumber is also worth more than green lumber. As I write this the price for a thousand board feet of kiln-dried 4/4 No. 1 Common red oak is almost 60% higher than the price for the same grade of unseasoned red oak. Additionally, dry lumber weighs less compared to green lumber and this might affect your ability to load/unload or deliver lumber to customers. Dry lumber is that dry lumber is stronger than wet lumber. Wood that is properly dried will machine better, it will have better strength properties, and it will take a finish well. Too little or too much water can cause problems, so it’s up to the kiln operator to dry wood to the correct degree of dryness (which is what we call moisture content). Wood that is too dry will be brittle when it’s machined and might tear out under rotating knife edges in a moulder or router. Wood that is too wet, on the other hand, will be attractive to insects and fungal attack. This is where it all gets just a little bit tricky: even if it’s properly dried to the correct moisture content, wood won’t stay at that moisture content for long if it’s stored improperly. AN OVERVIEW OF HARDWOOD LUMBER DRYING OPERATIONS Lumber drying operations usually operate in tandem with one or more sawmills. After sawing, the lumber is stacked with wooden spacers (called stickers) so drying can proceed. Depending on how quickly the lumber has to be dried, and depending on the equipment that’s available, hardwood lumber might be dried on an air drying yard until the moisture content is reduced enough to finish it off in a dry kiln. AIR DRYING ON YARDS AND IN SHEDS. Before hardwood lumber is placed in a dry kiln, it is often placed under cover someplace where the ambient temperature and air movement will begin the drying process. It probably seems counter-intuitive, but drying occurs fastest when the piles and alleyways are aligned with the prevailing wind direction. Air-drying stacks of lumber need to be kept off the ground because drying proceeds more slowly there; stacks of lumber that are not under shed roofs also need to be kept covered to prevent sun damage and rewetting due to rain. Sheds allow you to slow down drying by installing screen-like curtains to minimize sun or rain intrusion, and you might find that to be helpful in your operation. No matter how long you air-dry your lumber, though, it will never get as dry as it can in a kiln. At best, air dry lumber might reach around 12–15% moisture content; this is fine for barn siding, fence planks and such, but it’s not dry enough for lumber to be used indoors for furniture. In my experience, lumber usually goes into a dry kiln either green from the saw (when the business needs to sell the lumber as quickly as possible), or when the moisture content drops to 20– 25% or so after air-drying. 7PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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