Drying Willow in a Solar Kiln

      Challenging scenario: a tricky wood to dry and a difficult kiln to control. February 14, 2010

I have just sawn five large willow trees (central Texas) and loaded them in my solar kiln. Total of about 350bf and mostly 4/4, but I did put six 8/4 flitches on the bottom. How fast can I dry this? The kiln is an attic collector, the box is 8' x 20' x 7' high, the collector roof is 10' x 20'. Should I cover part of the collector? It has been over 105 every day for almost a week now. Below is a picture of six bookmatched 12" x 8' 4/4.

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Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
I would dry willow like oak and this means, among other things, that for 4/4 you should have 10 BF for every 1 sq ft. So you have too much collector... enough for 2000 BF of 4/4. For 8/4 we need about 25 BF per 1 sq ft. Mixing thicknesses in the same kiln load is not a good compromise. It would be better to consider air drying under roof for one or the other, especially in a warm sunny climate. City trees are especially hard to dry.

From contributor S:
A publication I have from Purdue says that willow can be dried with a "relatively severe kiln schedule," but doesn't really elaborate further.

From contributor L:
I doubt that the tree in question is a city tree, knowing where the questioner lives.

From Professor Gene Wengert, forum technical advisor:
In "Drying Hardwood Lumber," the facts about black willow (which one of the authors had experience drying) are it is prone to collapse, honeycomb and wet pockets. Further, the green MC is listed as 139% MC.

Although the article from Purdue states that willow can be dried with a "severe" schedule, that conclusion was based on uses for willow in the 1950s. Such uses were not quality uses, so collapse and honeycomb were not considered defects. (We see the same for alder and aspen and a few other species.) I suspect that the questioner is interested in producing a quality lumber product, perhaps for furniture or cabinets. Hence, the best drying approach in a solar kiln (and perhaps any kiln) is one that is similar to oak. This would also be the best approach to material with knots and so on. The old schedules were based primarily on upper grades of material, large logs, etc.

To the original questioner: You state that it has been over 105 F. I assume you mean this is the outside air temperature. A temperature inside a solar kiln of 105 is okay if the humidity is not too low. But in a kiln of your size, if it is well insulated, it would not be unusual to see the inside kiln temperatures to be 35 F hotter than outside (or more), giving you 140 F peak in the kiln, which is too hot.

From the original questioner:
That is correct - I emptied some bur oak from the kiln Saturday morning, once it was below 20% I had uncovered the covered half of the collector and closed up the vents to build up the heat. Friday it was 158 in the box, so yes, it can get hot. I have not put the baffle down, so the air is not being directed through the stack yet, as I have one more log to saw and add to the stack this evening. I will cover about 60% of the collector tonight. I plan to make some tables and other furniture from this. Yes, this tree was in the country by a creek.

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