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) Aluminum 660 Brass 1000 Copper 1084 Cupronickel 1250 Gold 1063 Iron 1150 - 1600 Lead 327.5 621 Nickel 1453 Platinum 1770 Plutonium 640 Silver 880 - 960 Steel 1425 - 1540 Tin 232 Uranium 1132 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.-- zen_tom, Oct 25 2012 Reverberatory Furnace http://en.wikipedia...verberatory_furnacePassively Aspirated [MechE, Oct 25 2012] Not quite so passive Tornado power source http://images.googl...1t:429,r:0,s:0,i:68 [normzone, Oct 25 2012] Wikipedia: Tuyere http://en.wikipedia.org/wiki/TuyereAha, this is the name of the thing I'm talking about adding - though the one in the idea is a kind of multiplex of tuyeres which tornadoise the air they pull in with the intention of getting it travelling quickly enough , and as centrally located as possible to best get to the core of the fire. [zen_tom, Oct 26 2012] http://www.steelgur...Damascus_steel.html [2 fries shy of a happy meal, Jan 01 2013] Simpler …
Build a tall chimney with a hearth in the base. There needs to be a big door for setting the fire, and removing ash, and a tiny door, st higher up, for adding fuel while running.
There is an air duct with a large intake port which can be closed with a baffle.
Also attached to the duct is a much smaller hearth containing charcoal and the crucible.
Light big fire in main hearth with air baffle open. Light small fire in crucible hearth.
Wait until main fire burning briskly. Close air baffle. Convection then has to suck all the air for the main fire through the crucible chamber. Flow is controlled by regulating the size of the main fire and the opening of baffles on the crucible hearth and the main air intake.
Not fuel efficient, but very simple.-- 8th of 7, Oct 25 2012 As an avid shop tourist, I have examined (salivated over) dozens of of forges. I have seen one (1) that self-drafts at operating temperatures. Here is the story of that forge, as related to me by the owner:
When the stone-hearth forge was commissioned (by the current owner's 2nd-grt-grandfather), the stonemason died mid- way through and the job was finished by his apprentice. When the forge was fired for the first time, the mortar around the upper flue slumped, producing a twist in the flue that is visible from the outside today. Another mason was brought in to fix the damage, and at the same time he extended the funnel stack because the smith's neighbors were complaining about cinders coming down on their house.
After those modifications were finished, the smith noticed that, after coking and banking, if his apprentices really worked the bellows for a time and then pulled it out of the cock, the forge would run itself at operating temperature until the coke was gone. Seeing this, he installed a small separate coke oven and had a forge that, if tended carefully, would run indefinitely.
Today, the bellows has been replaced by a powerful fan and the forge burns charcoal. The fan must be run for 20- 30 minutes, then the flue will take over and the forge will run as long as burning charcoal is periodically added. Unlit charcoal cools the bank and will stop the draft. The current owner says he once ran it for twenty hours without turning the fan on.
I hope this helps.-- Alterother, Oct 25 2012 This forge is located at Red Anvil Forge and Iron Art in some otherwise forgettable tiny town in northern NH. They do not have a website nor, near as I can tell, any web presence whatsoever.-- Alterother, Oct 25 2012 [bigs], you're wasted here. You should be writing for the National Enquirer or something ...-- 8th of 7, Oct 26 2012 Cool. I'm looking up stuff for a completely unrelated idea, and find this idea first while searching for a good category. I found a link which directly applies to this posting just minutes ago...What are the odds?
Turns out that this is a very old idea and may have been used in the production of original Damascus steel:
From [link]:"Thousands of steel making sites were found in Samanalawewa area in Sri Lanka that made high carbon steel as early as 300BC. (Juleff, 1996). These steel making furnaces were built facing western monsoon winds and wind turbulence and suction was used to create heat in the furnace. Steel making sites in Sri Lanka have been dated to 300BC using carbon dating technology. The technique propagated very slowly through the world, reaching modern-day Turkmenistan and Uzbekistan around 900AD, and then the Middle East circa 1000AD." (+)-- 2 fries shy of a happy meal, Jan 01 2013 random, halfbakery