View Full Version : koa turns greenish when bent

11-17-2014, 03:47 PM
I just bent some koa sides for the first time. They came out with a greenish hue. Oddly enough, where I had some small pieces of tape on the sides marking things, under the tape things were still a nice warm brown. I bent these with a heat blanket, the koa was wrapped in the same brown paper I have used for other sides, then aluminum foil. Not too hot, 230 deg. F

Anyone have a similar experience?

It looks like the greenish hue will sand off, but that is a real pain and may leave things thinner than I want.

11-17-2014, 04:36 PM
Yes, I've had koa turn green, but the hue doesn't run deep. Any wooden instrument has to be sanded after assembly, and this normal amount of sanding has always removed the green coloration.

11-17-2014, 06:08 PM
Koa and Australian Blackwood will do the same with contact to metal while bending. If it's on the hot pipe, you can isolate it with a damp rag between the pipe and wood. On a bender then wrap in kitchen or brown paper to isolate them from the metal bending slats.

It shouldn't go deep, and will sand off relatively easily.

Michael N.
11-18-2014, 06:32 AM
Must be the tannin in the wood reacting to the metal. With Koa I wouldn't expect Green but Black, given the original colour of Koa. In fact it can be a very effective way to dye wood. So Walnut veneer in Ferrous Sulphate gives a very decent Black that is permanent and easily strikes through standard veneer thickness. The same Ferrous Sulphate on a Yellow coloured wood will turn it a Green.

Moore Bettah Ukuleles
11-18-2014, 07:30 AM
I've always used 3" blue tape on the outsides of my sides before bending them. Keeps the koa clean and it seems to help to prevent breaking to some degree. You need to remove it while the sides are still warm so that you aren't picking bits of blue tape off when it cools. The little bit of adhesive left on the wood is minimum and removed with the sanding you normally do.

11-18-2014, 08:07 AM
That's interesting. Under a couple of pieces of blue tape I had on the sides as 'markers' the koa was fine (not green).
Next time it's blue tape for me!