What precautions are needed to keep the dark stains off curly maple while bending ithe sides? Can it be done over a plain steel hot pipe or is it better done with the blanket and bending machine route?
For what I know about it the stains should wind up on the inside,can be sanded if not too charred and the blanket bending machine methods probably negate having the stain /scortch.If done manually spray w/water a lot to create steamy damp conditions while bending.and keep the wood moving.
It's better done with a bending machine and blanket. Wet the sides LIGHTLY with distilled water (tap water has enough minerals in it to cause little blue spots), then wrap the side in parchment. Go slowly in bending the sides on the form and use the lowest heat that will get them to bend. Go too fast, too hot and too wet and things go to hell in a hand basket.
Is it the contents of the water that causes the blue spots or the contact with the wet metal? I have been building a maple workbench and every place a pipe clamp has touched the glue or moist maple surface it has stained it blue. It also did it with aluminum bar clamps so I guess its not an iron oxide issue. I have gotten most of it off but I really dont want to go through all that on an instrument. Also I am told that oxalic acid will bleach out those stains but I'm not sure what effect that might have on the rest of the building process.
The stains are mineral stains. They can come from tap water with a high mineral content or from metal objects that contact the wet maple. That's why the wrap with parchment is effective. The rub (no pun intended) comes when you have to touch up the bent maple on a manual bending iron. Again, use distilled water to moisten the wood and keep either a thin sheet of veneer or parchment between the side and the hot iron. As for oxalic acid, I doubt that it would have any effect on subsequent finishing but, to repeat what has become a mantra, try it out on a scrap first.