I will skip the very long story of how I acquired a bunch of sapele wood. I got it cheap. But just know, the wood I got has been a blessing and a curse. It's beautiful but the boards I got have lots of cross grain fissures. So the boards don't yield good lengths. Then of course the fact that sapele can be challenging to work with anyway.
So I am making a small table for a client. The top is 18" X 50" and is built and sanded. Over the last few days it has decided to cup. Rats! Yes, the wood is dry.
My current plan is to put a caul under the middle of the convex side and clamp the corners down to see if I can "train" the panel back to flat. But do I recall something about moistening one side to promote curling back to flat? Does that work? If so, which side to moisten?
I have fastened plenty of warped tops to frames, then coaxed the top flat against the frame. If your table frame is sturdy just fasten top to the frame with Z clips or some other and so long as the frame stays flat enough, forget the problem. The stress will relax out in the next few years, but who will notice?
If it cupped from stress you need to relieve the stress to get it flat. Wetting the concave side will expand those fibers and it will reverse the cup. But it will likely recup upon drying. Now, if you can plasticize the fibers on the concave side so that they relax and mash flat it may stay flat.
This works.
Mash a bit beyond flat, that is reverse the cup a bit. Take a steam iron, set on the hottest it will go and steam what was the concave side. The steam will relax the stress and it will likely stay flat when it dries. You will have to steam until the wood gets hot. May take 1/2 hour.
Rather than adding moisture to the wood, I usually prefer to just use some heat to dry the crowned side. If your going to have any Sunny days, you can just lay it out with the cup side down on something to prevent air from getting to that side, and let the Sun warm the crowned side. As the wood heats, the air right at the surface warms and absorbs the moisture. This may only take a half to one hour, so keep an eye on it.
Some years ago, a Judge friend ask me to make a couple of Mahogany table leaves for their formal dining table. I had called to tell them they were ready, and we set a time for me to deliver one evening. When I got there, they had forgotten and were gone. I forgot to take them out when I got home, so they were in my car until noon the next day, which was really hot. It must have been 120º inside the car when I remembered and ran out to get them. I was aghast to see how badly they had cupped. They were bundled together in a soft cloth, with face sides in, so had only lost moisture on their finished bottom sides.
Not having many options, other than starting over, I swapped their sides, and rolled them up and put them back in the car for another hour. When I pulled them out to check again, they were back straight. Since they were hot all the way through, I brought them in and wrapped them still in the cloth bundle in plastic so there wouldn't be any new air turnover while they slowly cooled down.
In your case one side lost moisture, or in another situation one side could have gained. Cupping from differential moisture loss of gain is easily reversible by re-equilibrating the panel.
If cupping comes from relieving trapped stress differentially, adding moisture to reverse the cup will be temporary. When the moisture added equilibrates the cup will return. To un-cup the stress must be equilibrated. The fibers on the concave side need to be stretched, then relaxed. It can be done by clamping the panel flat for a long time. Or it can be done by plasticizing the wood with hot moisture.
I have not tried the following. Ethanol will plasticize wood. Unsure if room temperature is sufficient as I have not experimented. If Thomas wants to do a science experiment he could try adding ethanol to the concave side while clamping it flat. Or maybe ethanol soak followed by a heat gun.
This is interesting. I was given a moment of pause by the term Hot Moisture. The moisture would normally cause the wood to swell, assuming it was dry to start with. Heat would normally dry the wood initially causing it to shrink, but if you could get it hot enough to plasticise the fibers it would relax. Also, it is easier to drive heat into wet wood than dry wood. Heat moves easily through water but not so with dry wood.
Ethanol as a plasticiser? I did not know. Just be careful with ethanol and a heat gun. I have not tried it, but it would be pretty easy to ignite ethanol with the heat gun I have and IIRC the flame would be nearly invisible.
How about ammonia? I see discussions about bending wood with ammonia. Nasty stuff though.
The guy I bought this wood from thought it was ribbon mahogany. It was rough cut and I thought it was mahogany too. That is, until I started working with it. I soon discovered there was something terribly wrong with this "mahogany"!! It does not like to be tooled at all. I studied it a bit and realized it was sapele. The fibers in this wood interlock. Even with helical cutters I have to prepare surfaces with care. There are small cracks and fissures that appear when milling. The cracks run perpendicular to the grain. It is very beautiful and has wonderful color and chatoyance. But it is a PITA to work with!!
The last few times I've used Sapele, I had my share of regrets. It had a high precent of unusable boards. I think in the future I'll need to order about 40% extra so I'll have plenty to cull.
Hoadley's book calls the fractures across the longitudinal grain as Compression failure. I'm sure you already know not to use any where the full strength is needed.
I bought a bunch of it for $4 bf. Like you say, some of it will be unusable for my work. But I'll still do okay. That said, most of what's left will be relegated to smaller projects. And definitely NO table tops!
Here is a photo of a table, similar to the one I'm making, using the same batch of wood.
Bill, yes, I intend to. But this table is going to a client in Texas so I want to make extra sure there won't be any problems later on. I can always fix local stuff but anything long distance can be a problem. T.
Can you mash it flat by pushing on it? This is often the case with a 4/4 top; it takes little force to force it flat. If so, pull it flat and sleep peacefully.
I do want to get down to Texas but it won't be this time. The client has a kid in school here at UI. They plan to pick up the pieces when they get their kid and his stuff.
Often I've seen both kahya (sp?) and sapelle veneer listed as African mahogany. And a few weeks ago a hardwood source, when I asked if they still stocked south american mahogany, they said no, just sapelle african mahogany.
not to leave the piece in a hot car in route, like stopping for lunch and leaving windows shut and hot sun a blazing. I am drying some wood in the window of our SUV and it is getting into the 90's inside even here in early spring.
this is a piece of sapele that became a wine cabinet for my son. It started life dead flat and straight. I ripped a 2X2 piece that became legs. As you can see in the pic, there is a little stress. Oddly, the 2X2 piece stayed straight and the 6" piece bent. I really didn't like that wood, although it sure took a finish well.
I must be some kinda weirdo, I actually don't mind working with sapele, it seems to work fine maybe. I get occasional tear out but not bad. It defiantly is not a hand tool friendly wood.
Khaya on the other hand is just nasty, fuzzes out totally unstable and on the west coast pretty inconstant supply.
