View Full Version : Discoloration of wood during side bending.

08-20-2018, 11:00 AM
Iíve been practicing bending my first ukulele sides on my Fox type side bender that I built. I am using a sandwich of ,stainless spring steel slat - white paper , wood. White paper - stainless slat , and the heating blanket on top. The mahogany is spayed with water , I am running the temperature up to 350 deg. F. (186c). Bending and determined set points along the way then holding for 6 minutes, then reduceing the temperatures to
250 f. (122c) and holding for 15 min. to remove any moisture in the wood. I have been successful in bending the sides but I am getting a dark discoloration transferring to the wood. I am allowing the wood to cool & remain in the sandwich overnight
My previous attempts were with non stainless spring steel that had a blue coating on it and the results were the same though more pronounced. I tried to remove as much of the bluing that I could but still had the transfer of color although to a lesser extent, thus the reason for trying the Stainless Spring Steel. . I resorted to wrapping the wet wood in aluminum foil and used the non stainless steel slat and was able to eliminate the discoloration transfer but left wrinkle marks from the foil imprinted on the wood.
Im baffled , what am I missing? Any suggestions would be greatly appreciated.

08-20-2018, 06:46 PM
This is what I do.

Use a hot pipe to bend sides close to the shape require. They don't have to be exact, just getting them well on their way. I only spritz them with a spray bottle as is require for where I'm working the timber.

After the 2 sides are somewhat to shape I then transfer them to my Fox style bender. No more water. My heat blanket goes on top of the timber. Then my 1 metal slat (I use aluminium flashing).

I can then clamp everything in place without worry of damage to the side.

Set heat to 140C and cook for 15 minutes. Then let cool.

08-20-2018, 07:34 PM
Funny but bending wood is the hardest easiest thing to do Or the easiest hardest thing to do... I don't use your method so I really can't comment. However you did say that you used aluminum foil and it left marks imprinted on the wood. I use aluminum foil and this discoloration is normal after bending and the marks just sand out. Strictly skin deep. Keep in mind that bending does leave marks on the wood no matter what method you use, but they are just superficial and you take them away during sand out. I've had some deep marks that seemed hideous at the time, but disappeared after sanding... Now deep scorching, that might be different.

One thing to realize is that sides that come off the bender can look pretty gnarly sometimes, but it is all part of the process and they will look great in the end. My advice: Don't obsess too much on this part of the process... I bend by hand which is ridiculously easy and I don't measure heat. When the wood bends it bends. However, I think your temps are too high. You want to steam the wood and not cook it. Too high a temp just drives out the moisture and drys the wood. Dry wood doesn't want to bend but crack. 350 degrees is way too high in my opinion. Slower and lower with patience. It is not the heat but the steam that causes the wood to give it up. Good luck!

08-20-2018, 10:06 PM
Wet the mahogany wood, wrap it in aluminium foil envelope put it in hot (300 deg approx) fox bender, apply pressure 5 mins... switch off let it cool down for about 20 mins..leaves a few stains but they sand out easily.

08-20-2018, 10:09 PM
Thank you for the suggestions I’ll give both of them a try. It’s just very puzzling as to why the metal would cause a reaction to the wood in that way ,it’s definitely not scorched. I’ve have not been successful in sanding out the discoloration otherwise I’d would have just moved on. Pipe bending, lower blanket temps, & aluminum flashing is my next attempt at it. Thanks again.

08-21-2018, 02:59 AM
Iron in the slats may react with the wood at a higher temperature. Also stainless steel comes in many varieties, some are more stainless (less iron more nickel or chrome) than others. Try putting a piece of damp oak on a piece of steel for a short period of time, will go black. On the temperature, 350 is way too hot, the most I go is 300. I do not bend much using a blanket but use a pipe. I have done what Allen does, get the general shape on the pipe and finish up on the bender. Most people use very little water when bending, I found the less water used the lower temperature when bending. Seems the higher heat was just used to boil the excessive water. This all come down to how fast you bend also, too hot and little water results in the temperature going up too much before the wood gets bent. My general method is to bend on a pipe with a damp rag over the pipe. Steam comes off the pipe and heats the wood. If you think of it the steam is roughly 212 F and the wood bends. It can take a little longer than a bender but I do not have to worry about cracking with the feedback to my hands, no worry about burning the wood with 212 F.

08-22-2018, 01:00 AM
It’s just very puzzling as to why the metal would cause a reaction to the wood in that way ,it’s definitely not scorched.

Tannin in mahogany, soluble in water, reacts with ferrous metals to form dark coloured complexes.

08-22-2018, 03:14 AM
Gentlemen , thank you for your input. Before posting this question I tried a number of approaches to the issue with varying results but nothing that I was satisfied with. Your comments give me reassurance and a new approach to address the issue. I’ve built solid body electric and thin hollow body guitars in the past but have never had to bend wood. This adds to the learning curve.My experience with the guitars has helped but building this ukulele has demanded the learning of new skills and understanding of materials and of course the construction of countless jigs.

08-24-2018, 03:12 AM
I too think that 350 is too hot. I wrap the sides in parchment paper(the cooking kind) and it helped keeping the sides cleaner, not clean but better. I use a blanket to bring the side to steam and then into the press and turn it off. Good luck