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I agree. I think a solar powered forge would be ideal. One idea I saw for this was using old TV satellite dishes (the large ones) and coating it with reflective mylar. Here in Colorado where the sun is intense anyway this would be a great tool to help recycle and reform a lot of discarded materials. I could even imagine such a setup to melt down polyethylene and PETG bottles and reform into other shapes and building materials.
i think your mistaken. mythbusters made what you call a solar forge and it was barely able to light wood on fire, due to the way the parabola is shaped the hottest spot in it would be too close to the object to allow more then one to focus on the same spot without one shading the other from the sun.
leaving this alone it still would not generate enough heat for long enough to even soften iron let alone steel for blacksmithing. it /might/ be enough to soften decorative metals such as silver and gold.
I didn't see this episode and I can't tell from your description, but I don't think it sounds like they built it correctly. Putting mylar inside a dish in its original shape, which is what it sounds like they did from your description is not the way they are built.
You don't need a dish to do this, a square box would work I hear.
The parabola is made by use of a suction pump. The mylar is stretched across the frame and a seal is made. A small suction pump is used to draw the mylar back into the frame. You have to work out distances etc. But this is supposed to make a small point for a tiny hot beam. I found a website in Africa that has the design and comments by a builder who gives away the idea and plans.
Sounds like myth busters didn't get the full scoop,.. so to speak/
How did they build their unit.
"you can't make a foul ball fair by moving the foul line" Roger Maris.
I had read about the mylar vacuum parabola and the satellite dish on another blog/energy site. I confess that I didn't checked it out or actually think of the power density issue.
Many ideas to tinker with once we have more time on our hands. Old fashioned charcoal forges should work fine too.
This works pretty good:
http://www.technologystudent.com/energy1/solar4.htm
To carbonize the steel you still want to add carbon to the mix. For plastic you don't need too much, but how useful is plastic if it breaks quickly?
Also to get a solar powered forge you would probably need a Fresnel lens 100mx100m (you would end up being able to focus ~2,000,000W, or 2 MW of heat flux onto 1 sq m at high noon.) excluding heat losses and transmission losses through the lens.
A projection-tv-sized Fresnel lens will melt a penny, according to unofficial testing in the parking lot at Science Surplus.
Melting points (deg C):
I've only seen a few Mythbusters and haven't been impressed. A 8.5 x 11 in. Fresnel lens will cause wood to burst into flame with an audible (if tiny) pop.
If the forge isn't hot enough, use multiple lenses and mirrors. Be sure to check out Random Destructive Acts via Focussed Solar Radiation.