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|Bloomery furnace iron smelting -   2010/07/12||Viewed 223 times this month, last update: 2011/02/16|
|I've always loved the idea of working with metal. I've been cutting, shaping, welding and drilling steel for years, making useful pieces, but I've also been just playing with metal my whole life, even as a child. I'd love to acquire the skills of a master blacksmith, and will work toward that goal over time. But first, one needs metal to work! Yes, you can go buy any kind you could possibly want, but why not start at the beginning?|
In my travels through the Southern California deserts, I have found many mine sites. Some seem to have especially good iron ore tailings. I collected some of this ore, and took it home to practice ancient smelting techniques.
My most recent inspiration are these sites:
The mastery and uses of fire in antiquity
The Smelter's Art.
Starting with the Smelter's Art designs, I constructed my own bloomery furnace, using fire brick, piping and a shop-vac. Using fire brick rather than a single ceramic unit means I can break it down, and transport the furnace, or store the brick for use at a later time. The shop-vac at full power puts out far too much air, but using a variac, I get fine control over air flow. I used commercially produced hard-wood lump charcoal for fuel, sorting and breaking up pieces by hand. For instrumentation, I am using a commercial kiln pyrometer purchased from a local pottery supply store. Here is my bloomery furnace in full operation, at 900 degrees Celsius:
The "bloom" of iron, produced in my fourth smelting:
The bloom cut in half with my chop-saw, showing the iron, and voids:
Here is my process, as of now (in the first 1/10th of the learning curve):
Start by finding some ore. I've found several sites in the Mojave desert where Iron mine tailing piles can legally be picked through. I use a magnet on a string to find the most magnetically attractive stones. I break these up using an air hammer, and rolling the pieces in a old metal cement mixer with river rocks. What I get out is a powder, that acts just like iron filings when picked up with a magnet.
Next find a good source for lump charcoal. Not briquettes! Break these pieces up into 1-inch or smaller pieces, and throw out the unburnt wood pieces.
Pre-heat the furnace using a few pounds of charcoal, naturally aspirated.
Start the air blast, at a very low level, about 40 volts out of the variac. Turn the speed of the air blast up slowly until the stack temperature is 900 degrees Celsius. This took about an hour, and 75 volts. When it's hot enough, the fire will be an extremely bright red-yellow color, and too hot to get your head within 4-5 feet.
Start adding layers of new charcoal and iron ore. I'm using a 1:1 ratio of ore to charcoal, by weight. Slow the air blast to near zero, fill to the top of the furnace, and re-set the air blast. Let it burn down enough for the next batch, then add the next load.
When the supply of ore is exhausted, open "tap" the slag from a vent hole in the bottom, allowing the slag to run out. This can be cooled, and re-added to the furnace if desired.
Let the fire burn down, then extract the product.
Here are some more fun/cool pictures of the process:
Bonnie (2010-07-12): Do you know there is a Catalan furnace at Mission San Juan Capistrano? It is the only one in any of the Spanish mission and was built in the late 1700's, modeled after others in Spain. With it, the early settlers here produced wrought iron to repair ships coming into Dana Point harbor and other items to trade.
Erik (2010-07-12): Hi Bonnie; no, I did not know that the San Juan Mission had smelting furnaces, but my next-door neighbor is a tour guide there, so I'm surprised I didn't know about them! Thanks for telling me, I'll have to check those out.
Stephen Dennis (2010-07-20): When I was at Pirate Cove beach in new port recently I found two magnets in the sand. These were not rare earth type magnets, but were on the upper end of strength. The two of them probably had an ounce of iron particles stuck to them. I wondered about the quality of iron beach newport beach sand could produce.
Erik (2010-07-20): Stephen, iron-sand is usually an excellent material to smelt, mostly because it's so easy to get, and to work with. No rock-crushing with that stuff!
In fact, the ancient Japanese swordsmiths used iron-sand from a sacred riverbed to make their iron:
Miquel Segura (2010-08-20): Mi nombre es Miquel Segura. Tambien tengo alguna experiencia en hornos de hierro, Seria interesante si pudieramos compartir experiencias.
Pudes mirar em mi canal de Youtube y tambien en mis Faboritos...
George (2010-09-22): Build yourself a charcoal retort from a 55 gal drum (lots of designs on the web) and save yourself a little coin in the mfg of your blooms.
Erik (2010-09-22): I played with making my own charcoal, but in the end, for me, it is more convenient to just buy. Now if I had my own patch of woodland, it would be a different story!
Jules (2011-02-21): dear erik,
i am a student in an experimental archeology class and stumbled upon your site, I am wondering if you know what the iron content ( percentage wise ) of your bloom was after you took it out of the oven? was the ore magnetite ore ( im kind of guessing from the geographical location of the mojave or maybe hematite- i know you can find that in yellowstone but thats a bit too north from where you are ). thank you ! your furnace is awesome! also out of curiosity, are you going to make something with the iron?!
Erik (2011-02-21): Hi Jules,
I haven't had either the ore or bloom analyzed for iron/carbon content.
From the research I have done, the ore does appear to be magnetite. It is attracted to magnets, and has the grain/color of magnetite ore. I haven't had it tested however.
I'm sure I am going to make something, but what that something is, I do not yet know. My goal at this point is a workable iron bar.
Tom (2011-03-31): Hey Erik, ive been looking into doing some smelting myself. so i was wondering if you could awnser some questions for me?
Is it better when layering you charcoal and ore to do this is smaller quantities or larger? example 1 pound of each or 5 pounds of each?
Also do you restrict the air you allow in the top of your furnace? and if not what are the dimentions of your furnace?
Also the temp you gave to smelt at was 900 Degrees C. in the stack and that was at 75 volts. so when you fill the furnace you slow the air flow to near zero. Once it is filled do you crank the air flow back up to 75 volts? im just not sure if thats what you ment.
Ok one last question, The slag that doesnt join the bloom is it rich in iron? if it is rich in iron do you have to grind it back down to re-add it to the furnace or could you just melt it down and skim the impurities off the surface?
Thanks for your time and your excellent blog!
Erik (2011-04-02): Tom, Check out my most recent smelt page, called February 13th, 2011 - Iron Smelt. It should answer some of your questions a bit more.
To answer some of your questions directly:
1. I don't think the total weight of the charge matters much. The ratio is what is important. Smaller charges will have to be added more frequently, which is more work.
2. See the most recent smelt notes page for dimensions.
3. When the furnace is filled, air flow does not go to zero. You need to increase the pressure of the input air so that it has good flow through the entire stack, and meets the temperature requirements you have.
4. You do not have to re-add slag to the furnace, but it is a good ore to use, and most smelters use it again, rather than throwing it out.
Do check out the Bladesmith's Forum link at the top of this page. You'll find many more people doing this.
Dustin (2011-11-09): hey Erik! can you use coal instead of charcoal for your fuel?
Erik (2011-11-09): Hi Dustin, yes, I think you can use coal, but it won't work as well. Coal has a lot of non-carbon components, like sulfur, which can interfere with the smelting process, or change the chemistry of your product. This might be a good question for Bladesmith Forum.
Jonathan Michelin (2013-08-31): How many bricks did you use total? and what sort of design did you have in mind? I woudl like to copy your design if you dont mind