Processing Trees to Lumber for the Hobbyist and Small Business


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PREPARING LUMBER FOR DRYING

It is important to prepare lumber and begin drying it soon after the lumber is sawn. Green lumber will begin staining immediately if not stacked and dried--especially in warm weather. If polyethylene glycol (PEG) is used, to prevent green wood from swelling or shrinking during or after drying, the lumber should be put into the solution immediately after cutting.

For optimum lumber quality, load the dry kiln according to the following suggestions. Place "stickers" on the kiln floor, perpendicular to the lengthwise direction of the lumber pile. Stickers are dry pieces of wood 3/4- to 1-inch thick, 1 and 1/2-inches wide and as long as the wood pile is deep. Never place lumber directly on the floor.

Stack lumber in single layers of uniform thickness, with the thinnest material on the bottom and the longest boards on the outside edges. Separate each layer with stickers of uniform thickness. Place the stickers every 12 to 18 inches along the length of the pile, directly above the stickers in the layer below. Stickers allow air movement through the pile and prevent warping. Make sure there are stickers above and below every board end--whether at the pile's end or somewhere in the middle. A board with a loose end will warp.

Fill the kiln to its designed capacity, to within 6 inches of the bottom edge of the baffle, and to a width that leaves one foot of air space on the sides (Figure 2). A smaller load will dry more rapidly than is desired.

Cut several 30-inch-long sample boards from the lumber (Figure 3). Because the wettest lumber generally has the highest risk of degrade development, the sample boards should represent the wettest lumber in the dryer; that is, the most recently cut boards, the widest and thickest boards, and quartersawn boards. Avoid cutting sample board from areas near knots and areas closer than 12 inches to the ends of the lumber. Cut a 1-inch section from each end of the sample boards. Weigh the sections and the sample boards to the nearest gram (1/25 ounce); record weights directly on the wood pieces. End coat both ends of the sample boards and return them to the wood pile in a spot where they will receive the same air velocity as the rest of the lumber. Number and save the 1-inch sections.

Figure 3. 28-inch sample boards and 1-inch sections
are used to monitor the drying rate of the lumber in the kiln.

After stacking, end coat any uncoated, exposed end of the lumber. Cover the stack with a black- painted sheet of plywood or scrap lumber (separated by stickers from the top layer of lumber) to protect the upper layer from repeated exposure to direct sunlight. Place rocks, scrap iron or other heavy weights on top of the cover to keep the top layers of lumber from warping.


DRYING LUMBER

Lumber can be air-dried or kiln-dried, or both. Air drying will bring the moisture content of the lumber down to around 20%--low enough for exterior use. Air dry 1-inch lumber 60 to 90 days, in a breezy, warm, not too humid place to reach 20% moisture. For interior use, however, lumber must be kiln dried to a moisture content of 7% to avoid splitting or warping in use. The following procedures apply to drying in a solar kiln. Kiln drying of lumber requires some attention. Because wood dried too quickly will develop degrade, the fundamental rule of drying lumber is that the quality of drying is controlled by the rate of drying. Air drying rates are difficult to control, and thus, solar kiln drying is recommended--especially during the critical initial drying of "green-from-the-saw" lumber.

When first learning to dry lumber, or when drying lumber thicker than one inch, monitor moisture content daily to avoid drying too rapidly. Drying speed is monitored by measuring the moisture content of the sample boards and comparing the daily rate of moisture loss with the "safe rate" for that species (Table 1). Exceeding the safe rate drying speed for a given species can cause defects in the lumber. If drying is found to be too fast, it may be necessary to cover part of the roof, or turn off the fans and close the vents during the hottest part of the day. While this may increase the temperature inside the kiln, the trapped air will quickly reach 100% of its capacity to hold water and the lumber will not dry any further.

Table 1. "Safe rates" for drying 1-inch-thick Wisconsin lumber. (1)

Species Maximum Rate of MC Loss Per Day
Beech 4.5%
Birch 6.1%
Hard Maple 6.5%
Soft Maple 13.8%
Red Oak 3.8%
White Oak 2.5%
Walnut 8.2%

(1) The maximum "safe rate" for 2-inch hardwood lumber can be obtained by dividing the 1-inch safe rate by 2.5. Thus, the 2-inch safe rate for beech is 1.85 MC loss per day (4.5 + 2.5 = 1.8).

At night, as the dryer cools down and the humidity builds up, water may condense on the walls. This is an essential part of the drying process as it relieves stresses in the wood that develop during the day as the wood dried. For this reason, do not run the fans at night.

In a solar dryer like the one presented here, the roof area and capacity are designed so that even on hot, sunny days, the dryer cannot dry 1-inch oak lumber too rapidly. That is, the maximum drying rate will not exceed 2.5% moisture content (MC) loss per day for white oak; 3.8% MC loss per day for red oak (Table 1). For thicker lumber, however, with slower maximum drying rates, samples must be used to carefully monitor the drying rate. For 2-inch, heavy, green hardwoods, covering half of the roof area to reduce the amount of solar radiation will slow the drying process.

It is important to note that safe rate refers to the loss in one day, not the average loss over several days. An 8% loss one day and 2% loss the next is not equivalent to a 5% per day MC loss rate. Safe rates must be adhered to until lumber moisture content drops to 20%.

Sample boards provide the means for determining moisture content loss rates. Use the following procedure to measure daily moisture content:

  1. Place the 1-inch sections (sawn earlier from the sample boards) in an oven at 215 degrees F (102 degrees C). After the pieces are dry (usually 18 to 24 hours depending on the oven), reweigh them and obtain the oven-dry (O.D.) weight.

  2. Calculate the moisture content (MC) of the sections with the formula below. Average theMCs of both sections taken from each board to obtain the MC of the sample boards. The wet weights are recorded on the sections from the earlier weighing.

    %MC = wet weight of 1-inch sample - 1x 100
      O.D. weight of 1-inch sample  
  3. Estimate the oven-dry weight of each sample board using the formula below, the average MC from step 2, and the wet weight recorded on the sample board.

    Estimated O.D. weight = wet weight x 100
      (100 + %MC)  
  4. Record the estimated oven-dry weight of the sample boards and return them to the kiln.

  5. Reweigh the sample boards daily and calculate the current moisture content.

    Current %MC = current weight -1 x 100
      estimated O.D. weight  
  6. When moisture content reaches 20%, a more accurate estimate of moisture content can be obtained by cutting a new 1-inch section six inches from the end of one of the sample boards. Weigh the shortened sample board and the new 1-inch section. Follow steps 1 through 5. Be sure to use the new wet weights and the new oven-dry weight of the section in finding the current MC of the sample board (step 5).

If you do not have access to precise scales or a temperature-controlled oven, there are other methods to determine and monitor moisture content loss. Write the authors for that information.

GOOD LUCK! Enjoy the satisfaction of collecting, sawing and drying your own lumber.

Gene Wengert is Extension Forest Products Specialist and Dan Meyer is Associate Outreach Specialist at the University of Wisconsin-Madison, Department of Forestry, 1630 Linden Drive, Madison, WI 53706. Write for plans for a larger, 1500 bd. ft. solar kiln.

Via Oregon State University

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