(05-23-2025, 01:23 PM)gordon 131 Wrote: John, how long do they typically stay stacked to air dry before they go to the kiln?
Gordon
If it's 4/4 that I mill in the spring, it will be AD (12 - 14%) in 3 to 4 months. If it's milled in the fall it won't be AD until maybe May the next year. 8/4 takes a lot longer, usually at least a year. Species makes a difference, too. Ash only has 40% moisture content when green, so getting down to AD could be a month or two less. White oak, checks just looking at it, so I try not to mill it in the really warm months. I'll wait until Fall or very early in the spring so that it can dry slow enough not to get many defects. In all cases, I've learned the hard way not to put racks out where the sun can hit it directly all day. As the photos show, I have my racks in a hedge row near my mill, to shield it from direct sunlight most of the time. There still plenty of space for the wind to blow through the racks, however.
This is where going directly into the kiln has an advantage. With air drying you are at the mercy of the weather. If you get a long, wet and cool spell, fresh cut maple can start to mold. Really hot and dry weather will cause checking, especially in white oak, as mentioned. But if you put either of those directly into the kiln, then you can control the drying and greatly reduce the risk of drying problems. I would prefer to do it that way, but the drying time is at least 2X over the time for lumber that's been air dried first. The drying season here for my kiln is limited, so I can get more BF/season through the kiln by air drying first.
In really nice weather, meaning warm sunny days and relatively low RH, a load of 4/4 AD red oak or ash will dry in 10 - 14 days, maple and white oak will take maybe 3 weeks. Double or triple that for 8/4. It might be 70F outside, but inside the kiln it will get up to 125 during the late afternoon. The RH outside might be 50% during the day; inside it will be less than 20%. At night, when the RH outside goes back up to 80 or 90%, it will stay less than 40% in the kiln. Drying happens very quickly in those conditions, and w/o defects with AD wood. In contrast, if I load the kiln with an AD load in November, it typically will dry about 1 to 2%/month over the winter, meaning it's not down to 7% until maybe March but more typically April.
How do I know it's dry? I used to cut weigh specimens and pull them out of the stack every day or two to check the weight loss. It's the most accurate way. But over time, I learned that a dual depth moisture meter works well enough. I have a Ligno pinless meter that reads at both 1/4" and 3/4" average depth. If I check a few boards at both depths and see something like 4 - 5% at 1/4" depth and maybe 8% at 3/4", then I know 4/4 stock is less than 7% on average. If it reads 10% at 3/4" then I know it has a ways to go. In the kiln, you will never see a constant reading at the two different depths because the surface dries more than the interior. So you pick a target, 7% in my case, and call it dry when the average is close 7% or less.
John