I've got a lot of ashtrees that were killed by the ash borer beetle and are standing dead. Ash in general is said to be dry enough that you can burn it without seasoning, and this should be especially true for dead wood.
But does this mean that I could cut these trees into boards and use them right away for woodworking?
Trees that have been killed by ash borer damage are generally not suitable for woodworking. The wood is very rotten and soft and falls apart easily. I am throwing our dead ash bits into a hugelproject instead.
You can see with only one eye open, but you'll probably run into things and stub your toe. The big picture matters.
I think the short answer is yes, standing dead wood, even ash, needs to be pretty dry before doing woodworking. At least anything nice like furniture.
Once a board is cut from a live tree, they say it needs one year of air drying per inch of thickness. I'm guessing a dead tree would still need several months to a half a year to get dry enough that it won't warp on you.
The length of time the trees have been dead matters to the usability of the wood. After several years, it may be rotted and unusable, but within a year or so of death it should still be fine. If the bark has not come free from the trunk in spots the wood is probably good. Cut a tree down and check it.
You can do all sorts of woodworking using green wood that has not been seasoned at all. It's only in the last hundred years or so that this has even become a matter of any debate, as before the advent of commercial kilns for drying lumber, everything started with greenwood. There are accommodations that have to be made for the way the wood will change as it dries and equalizes with its environment. Some of these are elements of why traditional chairs made with green wood can hold together without glue or metal fasteners. The differential shrinkage between the mortise, tenon and wood peg making up the joint can all work together to hold it tight. I work in green wood all the time. It's easier on both tools and the body ;)
If the tree is rotten I think better not to use it for woodworks, because this will have a big impact on your final result. Recently I brought some trees from the forest that looked good and didn't look rotten and I started to cut the wood into several pieces to get some boards and they got pretty okay but they had to be sanded quite well. For the projects I had to do, I had doubts that something normal would be received,but I still tried. I thought it would be easier for me to sand when I will try to use an orbital sander, because it have a better result on woodworks and big surfaces and to my surprise it actually worked. In your case, you can try but again this is not 100% effective.
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