Using charcoal, propane or some other fuel, along with some externally powered airflow system (bellows/hairdryer etc) it's possible to heat a crucible to the temperatures required to melt glass, lead, copper, iron or steel.
What I want is to be able to attain this kind of heating power, but in as
passive a way as possible.
First design choice to make is to choose a fuel type. Charcoal is my preference as it comes in big bags that can be left at the bottom of the garden, and in the case of an Armageddon scenario, be replenished using readily available materials. Coal and charcoal can burn under optimal conditions at a temperature of around 1,920°C
Second design choice is to eschew any aeration method that requires non-human power - indeed ideally one that doesn't require any kind of external power at all.
Third specification is that the forge needs to be able to melt scrap metals. Based on the optimal temperatures achievable by a coal/charcoal fire, we ought to be able to meet the temperatures found in the table below:
Metal Melting Point (oC)
Iron 1150 - 1600
Lead 327.5 621
Silver 880 - 960
Steel 1425 - 1540
Zinc 419.5 787
Final specification - the design needs to be something that can be recreated using commonly available components; bricks, cement etc - ideally out of doors so that in any non-Armageddon scenarios I get to try it out without causing any cohabitants any cause for concern.
That's the preamble - the point being that assuming the ability to build a pit with suitably insulated walls, the only limitation on achieving these kind of melt-temperatures is getting the air-mix right.
Enter the Tornado Forge. First a circular footing is laid as the foundation of the pit.
Atop the foundation is laid a specially cast ceramic air chamber formed as a series of separate channels each shaped kind of like a horn, with the outer flared ends drawing in air from the periphery and channelling it upwards in a tornado motion into a bowl section higher up at the centre. This is difficult to describe, but imagine a clay representation of a tornado, only upside-down with the fat part at the bottom, and the thin part higher up.
It's this part that's the real essense of the idea - i.e. a passive shape that will purely by nature of its configuration be able to encourage the airflow that we need to achieve the high temperatures required for the forge.
A circular "pit" is then built up around the air-feed section, ensuring suitable insulation is used to line the inner casing. The outer casing could be constructed in brick with a ceramic cement mix for the chamber.
Fill with charcoal and ignite - as the fire builds, it should start sucking air up through the tornado section below which would be pressurised and sped-up as it progresses up the channels into the heart of the fire.
I'm hoping that this would produce a fire that's able to burn hot enough to melt some metals, but for higher temperatures, a collar could be fitted around the air-intakes of the forge and linked up to some kind of bellows device to provide an even greater airflow.