Question
I would like to try to build a fairly large sculpture by using recently milled (wet) white oak. I plan to saw out a beam from the log, then split it to remove the pith, dado a slot to add some steel to support the piece, and then re-glue the two pieces, which will then be shaped. Is the regularly available polyurethane glue going to work for this?
Forum Responses
(Adhesive Forum)
From contributor M:
Perhaps, but since this is a carving and you won't mind having a mess, I'd slather the fool out of it - use a good bit of it. I'd lean more toward using a glue that cures via catalyst (epoxy), though poly glue does use moisture to cure. I'd still use West System 105 epoxy, and I'd use it heavily.
However, you'll have to take some extra steps to make sure you get a good bond. First, use a good heater and fan to blow on the wood glue joint for as long as you can stand it - at least as long as it takes to make the surface of the wood feel dry. The more surface moisture you can remove, the stronger the bond you'll have. Pile in the glue, clamp it with the best clamps you have, and let it set for a while (epoxy will cure plenty hard in a few hours, but I'd still wait). Poly glue, I'd wait overnight before messing with it.
As a side note, are you not worried about the wet wood checking and splitting after you carve it? Especially oak.
The idea expressed above about pre-drying the surface can help. However, uneven drying can lead to a lot of checking. White oak should dry slowly (though cracks can probably be filled easily in a sculpture).
Keep in mind that your steel support will not move, but the wood will, adding tension to the wood as it dries, and creating larger checks. It will take a year or more to dry a beam like this, and cracks may show up more than a year later.
White oak is most sensitive to drying conditions as the first half of the moisture is removed (the first year or so for thick wood). This is when checks show up. Getting it under 30% (slowly) before you start working with it will be of great benefit. If you resaw, then air dry the wood to about 15% (a year or more), you have a better chance of the glue holding and you are well past the checking stage.
I don't have a lot of success using epoxy on moist wood, and I've tried it many times. Other than a poor bonding surface with moist wood, epoxy is brittle, and as the wood shrinks during drying, the bond breaks. Epoxy could be a good filler for those checks that are going to appear, after they stop moving.
I like using urea formaldehyde glue for wood that has a little more moisture than normal. Unibond 800 and Urac 185 are examples of this. They are quite unpleasant to work with though, and you need fans and a respirator (and a heated shop).
