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lating fan. The idea here is to install the fan so it can draw warmedair from the top of the kiln down through the baffle and then directit up through the bottom of the stacked lumber. I used a 150cfmsquirrel-cage fan with an integral VlO-HP motor, which costs about$50. It's similar to the exhaust fans commonly used in bathroomsor kitchens, and it is sold by building-supply stores. Install a linevoltagethermostat that's adjustable in the range of 35°F to 90°F. Iused a Dayton #2EI58A, which can be purchased from heatingandcooling-equipment suppliers for about $24. The thermoswitchis wired in the same way as a commonly used single-pole switch.You'll need to adjust the switch seasonally so the blower turns offwhen temperatures drop below 55°F in the winter and 80°F in thesummer. You may also want to install a power onloff switch so youcan control the system manually. If your site is remote and electricalpower is unavailable, you can do without the fan and baffle, butyou'll need to provide more space between pieces of lumber inthe stack to improve the air circulation. Leaving a 6-in.-wide chasewayup through the center of the stack will help conSiderably. Thekiln will work this way; drying just takes longer and takes placeless uniformly within the stack. Lumber located in the "dead"areas, away from the freely circulating air, will dry more slowly.For example, I've found that after four months of drying, the topone-third of d1e stack has reached 6%MC to 8%MC, but the remainderof the stack is still a "wet" lO%MC to 12%MC.If you already have your wood, load it into the kiln now; you canuse the stack as a platform while building the roof. Notch therafters a shown in figure 2, p. 75. The notch, or bird's-mouth,which joins the top plate of the front wall, holds the front glasspanels in place. So, it needs to be cut large enough to accommodatethe thickness of the glass. Take care to maintain this space forthe glass when you install the rafters. The rafters support the glassroof panels along the edges and centerline of each panel. If youhave smaller panels than those used here, the center rafter can beeliminated. Tempered glass is strong-it's used for skylights-andd1e rafter layout used here provides ample support.When you install the glass panels, be especially careful to avoidknocking d1e edges: They shatter easily. It's a good idea to use stopblocks or a toeplate at the lower end of the rafters to hold the roofpanels in position. Cover the edges of the glass panels with I-in. by1 Y2-in. wood strips, which can be screwed to the rafters or studsthrough the %-in. space between adjacent panels. For a more permanentsetup, apply a silicone caulk bedding for the strips andcover them with an aluminum cap. One final comment: A 2-in. gapis intentionally left under the roof panels at the top of the frontwall so moist air can escape from the kiln.Using the kiln-My experience with this kiln isn't much differentthan the results reported for the more complex Forest ProductsLaboratory design. The time required for drying depends on yourparticular location, the time of year and amount of available sunshine.The species of wood also affects the drying time: High-densitywoods like oak dry slowly; low-density woods like pine dry morerapidly. In southern Michigan, a batch of I-in.-thick hardwoods canbe dried to about 8%MC in four to five months if started in March;a second batch can be dried by November. TI1e winter months aremarginal because there are too few sunny days.I'm continuing to experiment with the kiln to reduce the dryingtime. I've found that the air circulation can be improved bypositioning the 4-in. stringers (which support the wood stack),as shown in figure 1, to form a baffle, which helps force the airthrough the stack. I've also tried reducing the air gap at the topof the front wall during the final stage of drying to reduce airflow and increase temperature. The effectiveness of this finetuningremains an open question.If you decide to experiment with your kiln, you'll find a moisturemeter to be a good investment. Moisture meters are availablefrom woodworker supply houses for about $100. FWW' on Woodand How 10 Dry It describes a shop-built model. Moisture contentcan also be determined using a simple weight-loss method describedin FWW' #73, p. 20.DJohn Wilson teaches woodworking part-time at Lansing CommunityCollege in Lansing, Mich. He specializes in making andselling Shaker oval boxes, and he depends on his solar kiln forhis wood supply.Wood-drying basicsYou can dry wood simply by passing air over its surface. Left outdoorsand protected from the elements, a stack of 4/4 hardwoodwith freely circulating air will dry to about 15%MC in about a yearin the northeast United States. Further reduction isn't possible, becausethe moisture in the air, normally about 75% to 80% relativehumidity (RH), is in equilibrium with that of the wood. (Relativehumidity is a measure of the air's capacity to hold moisture and isdefined as the ratio of the amount of moisture contained in the airat a given temperature to the maximum amount of moisture the aircan hold at that same temperature.) Our home environments aremuch drier, however, typically about 40%RH to 50%RH and oftenas low as 20%RH in the winter. This difference causes problems forwoodworkers. Furniture, to remain stable, must be constructedfrom woods with a moisture content no greater than 6% to 8% tobe in equilibrium with the drier indoor air. In the very dry Southwest,the moisture content will be lower; in the more humidSouth, it will be higher. So the problem of drying wood comesdown to circulating dry-enough air to produce these lower moisturelevels within a reasonable amount of time.One way to dry the air is to dehumidify it using a compressor.This is exactly what many of us do to prevent our basementsfrom becoming too damp. Some wood kilns operate on the sameprincipal, but of course on a larger scale: They use large compressorsto remove moisture from the air inside the kiln; heatproduced as a byproduct of the compressor operation keeps thetemperature in the 120° range. This option is usually too expensivefor the home woodworker.Most often, dry air is obtained by heating it. More precisely,moisture is not removed from the air but rather the air's capacityto absorb and retain water is increased as a result of increasing itstemperature. Saturated air (100%RH) at one temperature is somethingless than 100%RH at a higher temperature. This difference,by the way, accounts for our homes feeling drier in the winter: Thecold infiltrating air is heated, making it relatively dry.When air is heated, its ability to absorb moisture is increaseddramatically. For example, air saturated at 30°F can hold threetimes as much moisture when heated to 70°F. Heating the air anadditional 40°, to 120°F, increases its capacity to hold moisture bytenfold. This is the practical working range for solar kilns. A kilncan start the day with the ambient air at 70°F and 100%RH, butafter being warmed by the sun for an hour or so, the temperaturewill be close to 120°F. The air will be very dry at this temperature-about10%RH-and able to absorb a large amount of moisturefrom the wood.For more information on the relationship between wood andair-moisture levels, see FWW on Wood and How to Dry It or BruceHoadley's book Understanding Wood, The Taunton Press, Box355, Newtown, Conn. 06470. -J W.January/February 1989 77