Heat treating by radiation

[Santanasaur]

There are flexible heating elements and coils available for fairly cheap online. I’ve been fixing to wire one up for a while now but got caught up tinkering with a light kiln. I have a couple styles on the way, I’ll play around with them hopefully this week and see if I can get a design going.

[DC]

I searched and all I can find so far in flexible are silicone(not hot enough) or elements that are flexible once before you heat it up. Once they've been heated they're rigid.  The wire coils from the common heater work well if you can find insulators to support them. Then I need some kind of support that can be adjusted to varying curves to match the bows limb. It has to take the heat. Also since we're kind of limited to 1500 watts(15 amp breakers) a reflector to concentrate the heat is nice but then it also has to flex. Any ideas.
Simk- do the radiant heaters put out more heat than the conventional wire coil or is there a lot of hype involved

[DC]

I crossed my mind that you could put a straight limb under the heat and poised over the caul. When it was all cooked you could just quickly clamp it to the caul and let it cool. I sort of tried this once with less than stellar results but I was heating with a gun so the whole limb wasn't hot.

[simk]

DC: I'm no scientist nor have I any special education...On wikipedia I can read the following about quartz infrared heaters: Electrically-heated infrared heaters radiate up to 86% of their input as radiant energy. Nearly all the electrical energy input is converted into infrared radiant heat in the filament and directed onto the target by reflectors. Some heat energy is removed from the heating element by conduction or convection, which may be no loss at all for some designs where all of the electrical energy is desired in the heated space, or may be considered a loss, in situations where only the radiative heat transfer is desired or productive. For practical applications, the efficiency of the infrared heater depends on matching the emitted wavelength and the absorption spectrum of the material to be heated.

Finally I just tried this one and it worked well for a proper heat treating in a relativly short time. Still I think also your oldschool conventional wire coil should do the job. What results did you get?

Practically I think it's easy to put the bow loosley on the caul then just heat it up with a good distance until it's hot enough to finally force it proper on the caul (stage one). Once fixed on the caul in the final position you closen the radiator and do the actual heat treating.

My actual r/d shown first was steamed onto the caul in the steaming bag - didn't have the radiator at this stage...I always put them loosley on the caul start steaming and after a while start clamping until i'm on the final form.

I did not claim a miracle all in one device processor but it's imho better than my old heat gun. Still curious with what santanasaur and shannon will come up then  I'm happy with my little progress at the time. cheers

[Stickhead]

Well, I’m relieved.  When I saw this post’s title, I was expecting to see a home-made nuclear reactor!

[Santanasaur]

I’m not saying I have any special heat treating solutions, or start a contest, this is just amateur experimentation. So far I’ve played with cheap flexible heating coils, one was for a dryer. I never found a nice support and it felt like an awful fire hazard, but would definitely be a solution if safety was managed. These are cheap, I bought them for $5-10 a piece. Silicone heat pads only get to around 250f which isn’t hot enough, but many of the cloth coated heating tapes/cables get to 1000. I’m waiting in the mail for one that gets to 900 and is a 1/2” by 48” strip, so hopefully that will cover a limb at a time. Supposedly it remains flexible through use. Ive also seen several tapes/strips for heating frozen pipes that claim to get around 400, but need a temperature controller which can be expensive.

[DC]

DC: I'm no scientist nor have I any special education...On wikipedia I can read the following about quartz infrared heaters: Electrically-heated infrared heaters radiate up to 86% of their input as radiant energy

I think I remember reading that conventional heaters are about 75% efficient but that was in the context of heating a house. No idea where the other 25% went

I think the wire coil will do the job and if we come up with something better we can change up. I did two or three bows with mine but for wiggly reflexed staves i went back to the gun. I will try the radiator again but it will be a while. The horn bow is about ready to shape and I seem to be building an "N" gauge model railroad for/with my grandsonm

[simk]

DC: I dont think those %-numbers are really important. it's only about bringing the wood to a certain temperature, which can also be done with lower efficiency - i dont care about the electricity-bill in this case 
santanasur: i really think you could be on a good run - alone i^m not talented in tinkering electrical devices but will be the first ordering yours 
cheers

[scp]

I occasionally used a heat gun in the way Badger does. But it was too noisy and tiresome. I started collecting heating coils and even fans to make a heating panel to be hung over the limb. I thought about using a radiant heater. Too much energy and somewhat dangerous. Finally I decided to buy the silicon heating blanket used to bend wood for string instruments. It does go up to 200C (392F) using just 300W. I think all I need is 170C (338F) that will make the wood plastic. But the belly started to burn at 150C. The belly was almost black and smoking probably after 30 minutes. I did put some wood blocks on top to keep the blanket in contact with the belly. Not a good idea? That was the first try about two weeks ago. I have not stringed the bow yet.

[willie]

If someone could build a flexible radiant heater that could be adjusted to any shape I bet we would all get one.

the refrigeration industry uses long resistive elements marketed as "evaporator condensate heaters".

[simk]

Thank you scp! Hoping for more details incl. pics soon! I did also google these silicon heating blankets some time ago; they aren't too cheap, are they? Cheers

[Beba]

Willie, those heaters are simple cal-rod heaters. Same as the cooking rings on the top of an electric stove. They will not handle much of a bend before breaking the internal element. They are designed for primarily for conductive (in contact) heat. Used solely in a convection application they will burn out quickly.

Using radiant heat is an option I hadn’t considered. Radiant heat waves heat only the objects the waves come in contact with adding an element of control to the process. so I’ll be keeping track of this thread for solutions to the raised questions.

[DC]

I occasionally used a heat gun in the way Badger does. But it was too noisy and tiresome. I started collecting heating coils and even fans to make a heating panel to be hung over the limb. I thought about using a radiant heater. Too much energy and somewhat dangerous. Finally I decided to buy the silicon heating blanket used to bend wood for string instruments. It does go up to 200C (392F) using just 300W. I think all I need is 170C (338F) that will make the wood plastic. But the belly started to burn at 150C. The belly was almost black and smoking probably after 30 minutes. I did put some wood blocks on top to keep the blanket in contact with the belly. Not a good idea? That was the first try about two weeks ago. I have not stringed the bow yet.

This sounds very promising although the numbers don't work for me. When I heat treat the wood doesn't start darkening until my thermometer reaches 400°f+. maybe my thermometer is wrong, don't know. How many times would this heater last using it like this? How expensive are they? Where can I read about them? etc etc

[DC]

This was my first Google hit. It mentions 400°
http://www.scavm.com/blanky.htm

[gifford]

Not trying to hijack the thread but does anyone else recall a fella cutting a hole in the back of his microwave, I believe it was an old Amana Radar Range (sure looked like the one we had IIRC).  Anyway, like others, the heat gun is a slow and noisy way to go but it does work and is likely a lot safer than microwaving the stave.