Fun with Molten Metal...



Building a Metal Melting Furnace


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I'm not a blacksmith or a carpenter. I don't even play them on TV, so if you decide to try any of this stuff and hurt yourself, don't come crying to me about it...)


I started hanging out in the rec.crafts.metalworking newsgroup shortly before buying a small CNC (computer numeric controlled) milling machine (the MaxNC 10-2, from, but more on that later) for use in making robot parts under computer control. In some cases, though, the easiest way to make a part is to pour the metal into the shape you want to begin with, rather than starting with a big brick and cutting off all of it that isn't your part. Melting and pouring aluminum looked like too much fun to pass up, so I decided to give it a try. Since I get into most of these bizarre hobbies mostly because I enjoy learning new skills, I'm building all my equipment instead of buying it, whenever possible. Using guidance from book 1: "The Charcoal Foundry" of a wonderful series of books sub-titled "Build Your Own Metal Working Shop From Scrap" by a mechanical genius named David Gingery, along with lots of helpful information from the great gang in rec.crafts.metalworking, I have set out to build a propane-fired home foundry and accessories. If I get really good at this, I may attempt book 2: "The Metal Lathe," in which Mr. Gingery leads you through casting, machining (by hand), and constructing a working metal lathe.

The story so far:

The Gingery book calls for an empty 5 gallon metal paint bucket, but try as I might, I couldn't find one that I could beg, borrow, or steal. This is, after all, the age of plastic! There were plenty of plastic ones for sale empty, but the only metal ones I could find came filled with some noxious chemical brew like driveway sealer at a cost of $40 or $50. Just as I was about to despair, I wandered into the housewares section of the local Wal-Mart store and found... (tah-dah!) A 5 gallon stainless steel stock pot!

Step 1: Lose the lid, and bolt on some angle-iron legs made by sawing in half some conveniently-shaped bits found at the local scrap yard, thusly:

Step 2: Obtain a quantity of "castable refractory cement". Just as I was ready to give up hope of ever finding any locally, a wonderful person from a local company whose name I won't mention until I get permission (Jeff - you're a stud! Hope you enjoyed the Jim Beam!) gave me two 100lb. bags of the stuff!

Step 3: Prepare forms for the crucible chamber in the center of the furnace, and the tuyere, ("tweer") where the burner is inserted. I used a piece of 8" stovepipe, "adjusted" so it doesn't snap permanently together when closed, to form the chamber. It has wooden supports on the inside to keep the shape, and on the outside to keep it centered. The tuyere is formed with a piece of 2" copper pipe, inserted tangentially into the crucible chamber through holes in the kettle and the stovepipe.

Step 4: Mix a suitable quantity of castable refractory and water in a large container using a shovel. (I had no idea what a "suitable quantity" might be, so I ended up going back to the refractory bag 3 or 4 times.) This is where it gets a little strange. During a call to the refractory company for instructions on adding water and curing, I was informed that the refractory I had been given was, in fact, designed to be applied with a high-pressure gun of some sort, where it would be mixed with water upon exiting the nozzle of the gun at a high velocity, to stick on the target surface like last week's mashed potatoes lobbed by a petulant two-year-old. Oh, and by the way, the "SS" designation on the bag? That means it contains shredded stainless steel (!!!) for a bit of added strength. How much water to use when mixing it by hand? "Gosh - we've never done that. I guess you'll just have to experiment." (Thanks, guys. You don't happen to work for the government, do you?) I determined empirically (My partner's term for "screw with it until it looks right". What do they teach kids these days? Sheesh.) that one cup of water mixed with a little more than two 1lb. margarine containers of refractory seemed to result in a reasonable consistency. Time will tell... Below is my vast array of concrete mixing and application equipment. In the background, the unsuspecting pot basks in the afternoon sun, totally oblivious to the bizarre fate in store for it.

Step 5: Here, the proto-furnace has been filled to the tuyere hole with refractory. (And we all know how uncomfortable that can be...)

The inner form and tuyere form were inserted, and the outside of the furnace was packed with refractory. The spacer boards were gradually pulled out as refractory was added until the form was able to maintain position by itself. It has been curing for several hours now, and the refractory has reached the consistency of slightly damp mud. Tomorrow, we attempt to persuade the forms to come out...

(24 hours later...) Eureka! It worked! After pulling out the center form, and spending 20 minutes wrestling the tuyere form out, here it is in all its glory! It's been curing for nearly 24 furnace_with_refractory.JPG (4299 bytes)hours now, covered in damp paper towels and a plastic bag. Depending on which information sources I believe, I now need to either a) let it continue to dry for 24 to 48 hours, and then fire it, or b) keep it from drying out until I can fire it. Arrrgh. What to do, what to do? (This picture is a bit on the red side, as it was taken after dark by the light of a 75W bulb about 12 feet away. For some bizarre reason, Ricoh chose not to equip this particular model of digital camera with a flash!)

Step 6: The lid. I thought this would be easy, but it turned out to be the most complicated part so far. Make a cylinder of sheet metal, 2" high, with the edges folded over and the ends riveted together. Drill holes around the edge and weave steel wire through them to support the refractory. Figure out some kind of handle arrangement. (Being the "belt and suspenders" type when it comes to mechanical design, I added both side handles made from 1/8" flat steel, and loops on top for attachment of some sort of lifting device.) I ended up with something that looks like the offspring of an unnatural union between a Veg-o-matic and a "dream catcher". The dowel across the top is there to stabilize the lifting loops until the refractory is set. A piece of 3" stovepipe in the center will provide a sight hole/exhaust port. (I don't know about you, but I've got my doubts about putting my eyeball anywhere near a hole that's been spewing exhaust at 1800 degrees...)

At long last, the completed lid, sitting on the completed furnace! You can see the lifting loops (in case I decide to get fancy and rig up some kind of boom and chains to lift the lid with) sticking up out of the top. Now, the whole thing has to air dry for a few days, and then we can start heating it up!