This furnace will be able to handle melting aluminum and bronze with no trouble. In theory it should be able to melt iron too but that's probably pushing it.
I've been using my furnace for a couple years now and it has worked out quite well. My design is based on information I picked up from the backyard metalcasting forum. It's a great resource and community. There are a bunch of furnace builds there exploring a variety of ideas. I recommend checking it out. I'd also read this comprehensive guide for a general overview of metal casting and furnace design.
Step 1: General Plan
The furnace is really simple. It's just a metal container with a lid, a drain and a hole for the burner. It is filled with high temperature insulation to retain the heat and a sturdy inner lining (hot face) to withstand the flame from the burner.
I based my design around a small metal barrel that I got from a salvage place. The sheet metal was thick enough to weld to (pretty much) and not galvanized. The zinc coating on galvanazed metal containers will burn and make a noxious smoke so don't use it. Otherwise any metal container should make a serviceable shell for the furnace. I used bits of pipe and angle iron from my scrap pile for the rest of the metal parts.
I made my own insulation because it was substantially cheaper than buying commercial insulation and I just wanted to try it. I did buy a bag of high temperature refractory cement for the hot face.
The shell is filled with insulation leaving an airspace of roughly 2" on all sides between the crucible and the inside of the furnace.
Step 2: Dimensions
Step 3: Mix the Insulation
The insulation needs to:
Be strong enough to withstand some banging around
Not melt at high temperatures, it should withstand 1800 degrees F at least
Insulating enough to retain heat
After hunting around the internet I came across a low-tech method to make fire bricks. It was a mixture of fire clay and sawdust. When it is fired for the first time the clay hardens and the sawdust burns out leaving little air pockets. You are left with a lightweight insulating product that just laughs at extreme temperatures.
The fire clay is a very high temperature ceramic that comes in a powder form. You can get it at pottery supply stores. I got a 50 lb bag of Hawthorn fire clay for $20. I used about 1/2 of the bag for my project. Make sure to ask about the temperature rating of the clay. You want something that won't break down at high temperatures. Do not use Portland cement (the stuff concrete is made with) for your binder; it can't take the heat and will just break down over a few uses.
I used shavings from a planer for my filler because they were fluffier than regular sawdust. You can get it from free from any commercial woodshop.
All of the measurements are by volume.
I used a big coffee can to measure with and a large plastic storage container to mix in.
First make a dry mix of 3 parts sawdust and one part fire clay
Mix it well until the clay is well distributed with the sawdust.
Add one part water to the dry mix
Mix well again
Let the mixture sit at least 12 hrs.
This allows the water to fully hydrate the clay. You will be able to tell the difference once it is ready. The mixture will be less sticky on your hands and form a nice ball easily.
While you are waiting for the insulation to hydrate you can start on making the forms for casting.
Step 4: The Lid
Take the piece you just cut off and place it upside down on a board covered with a sheet of plastic. The side facing up will be the side that faces the inside of the furnace. Make a cylindrical form for the vent. This needs to be 1" dia larger than the final hole. In my case I wanted a 4.5" vent so my form needed to be 5.5" I like to wrap my forms in plastic so that they are easy to remove after the castable is dry. I used a dog food bag (a little tougher and stiffer than plastic sheeting) for this project and it worked great.
Cut some remesh wire or something similar, preferably not galvanized, to support the insulation. Weld it to the lid. Weld a couple tabs with holes drilled in them too. We'll use these to hang the lid later.
Fill the lid with insulation leaving a 1/2" space at the top for the hot face cement. I forgot and had to scrape it out later as well as widen the vent. No worries, it all worked out.
Set the lid somewhere warm to dry. This will take a while. Plan on a week to ten days or more. There is no chemical reaction going on here, the fire clay is just drying out. Cover it lightly with plastic if it starts to get a dry skin. Wait until it is firm to the touch, it can be a little pliable but not squishy before you add the next layer to it.
Step 5: The Body
The inner cavity is straight forward just make a form that is 1" dia wider than the final cavity size. I cut a couple pieces of 1" thick styrofoam into 9" circles and wrapped them with sheet metal to make a cylinder. Use duct tape to hold it all together. Make a sleeve of plastic or dog food bag that fits snuggly around the form.
The tuyere is formed by a cylinder that comes into the furnace tangentially to the inner cavity. This allows the flame to swirl around the bottom of the crucible rather than just blast it at one spot. Place the inner cavity form into the furnace and inset the tuyere tube to mark it for cutting. It enters the shell a little higher than the finished height of the bottom of the furnace. Ideally the tuyere form also needs to be a little wider than the finished burner so that it can get packed with the hot face cement. Don't forget to wrap it with plastic. Weld a piece of pipe to the outside of the shell to support the burner.
I had a 5" finished floor height so the bottom of the tuyere hole was set at 5.5" The tube I used for the form was just a little bigger than the burner tube because that's what I had on hand.
Originally I was going to use a vertical drain but later opted for a horizontal one. When you are melting down metal parts from the junkyard to make ingots it's nice to just remove the crucible and let the molten metal run out of the furnace into an ingot mold. A horizontal drain makes it a lot easier to do that.
I used a piece of 2.5" dia pipe and cut it at a 45 degree angle three inches from the end. I cut a hole in the shell and welded the drain on. The bottom of the hole for the drain should be a little below the finished floor level so that it will, well....drain. This tube is oriented perpendicular to the inner cavity. Looking down at the top of the furnace if the tuyere is entering at 12:00 the drain will be at 9:00.
Once all the form parts have been test fit remove them and fill the bottom of the shell with insulation until it is 1/2" lower than the finished floor height. insert the form parts and carefully pack insulation around them. Fill the furnace with insulation until it is 1/2" from the top. Use a stick to pack the insulation firmly as you fill the walls.
Let the furnace dry for a week and then carefully remove the forms if the insulation feels ready. If there are voids or things crumble a little when you are removing the forms just pack a little wet insulation where you need it and sculpt it to fit. It's very accommodating to work with. Continue to let it dry, lightly covered if necessary, until it is firm to the touch.
Step 6: Other Bits
Step 7: The Hot Face
If a hundred bucks is a bit hard to swallow there are a couple workarounds I can think of. There is a high temperature mortar used for building and repairing fireplaces. It usually comes premixed in a can and can be troweled or brushed on to a surface. It can be found in hardware stores. Make sure it's high temp. It should give you a decent surface and not cost so much. The other route is to just say forget it I'm not lining this thing. It will wear down a little faster but will still work fine. If you go this route just forget the whole 1/2" bigger stuff when you are casting and fill everything to the top.
If you decide to go with the high temp cement (Why not? I only want to build this thing once...) Be careful when you mix it. It goes from too dry to too wet very quickly and a soupy mix won't fly here. You want it wet enough that it is not crumbly but firm enough that you can trowel it onto a vertical wall and it will stick and not sag. Apply a half inch of cement inside the the vent and on top of the lid. I made a smaller form and packed cement between it and the insulation and then troweled more on the top.
Same with the body of the furnace. Pack it on with your hand and smooth it. Line the drain and tuyere too.
The cement should be pretty darn hard in 24 hours.
Step 8: Drying
To speed up the drying process I stuck a 100 watt light bulb (remember those?) inside the furnace, laid the lid on top and left it going for a couple days wrapped and covered with fiberglass insulation. As it warmed up it steamed like a crock pot. Once it was done steaming I pulled out the bulb. The insulation in my lid shrank and cracked slightly but the cement held everything together.
Step 9: Attach the lid
I attached a couple wheels on the bottom and a leg in the front too.
Step 10: Accessories
Crucible - I used a piece of pipe from the junkyard. It's 4 1/2" in dia and cut to 6" high. I used a piece of 1/4" plate for the bottom. Cut it out in a circle and weld it to the bottom. Make sure you did a good job welding the bottom on. You don't want molten metal dribbling out. I figure water tight is a good test. There are a variety of ways to lift the crucible out of the furnace and pour it. I went with putting a pair of lugs on the top and a loop in the back because it seemed the most secure to me. They are made from a couple bolts and a nut I had in the shop. Weld them on securely.
Tongs, hook and scoop- I used metal that was in the shop and bent it to shape. I used washers for the bottom of the tongs. You could bend circles if you were inclined. Make sure they fit over the lugs on the top of the crucible. The hook is just a piece of 1/2" bar bent at the end for tilting the crucible. The scoop is rebar and a piece of sheet metal. The scoop part is 3" wide and bent up at the end to pull out crud from the top of the molten metal.
Extra Tongs- I got a pair of blacksmith tongs and they are really handy for moving hot stuff around. You can make do with a big pair of slip joint pliers but it 's gonna be hot.
Ingot mold- Mine is a piece of large angle iron with some plate steel welded on either end. You can go as simple as a muffin tin but it's going to warp and twist over time.
Step 11: Firing
Put the plinths and exhaust cover into the furnace.
Insert the burner and attach the propane and fan to it.
Turn on the fan but choke it down until almost nothing is coming out.
Light a torch and put it in the furnace.
Turn on the propane until it lights.
Remove the torch.
I start with a very gentle flame just barely licking out of the end of the burner. Let the very low flame heat up the furnace for an hour. This should drive a lot of the moisture out. If after an hour there is no steam escaping from the furnace go ahead and turn the burner up some. Not a lot, just a little, wait another hour. Turn it to "medium" and wait another hour. Finally, crank it. As the furnace heats up especially the first couple times it will smoke as the sawdust burns out of the insulation. It will stop after a few uses. Depending on the power of your burner the furnace should start to glow anywhere from a dull red to bright yellow.
Awesome! Good job!
Turn off the furnace, Let it cool down and take out the plinths and exhaust cover. They should be light weight, yellowish and generally look like soft fire brick. If they fired well then you can assume the rest of the furnace did too.
Use a plinth under the crucible and the cover on top of the vent to adjust the furnace. You can choke it down some but never cover the top all together. The furnace needs to breathe.
Over time you will learn how to adjust the furnace and tune the burner by ear.
Last Safety Talk
Melting metal at home is awesome. No doubt about it.
Going to the hospital......not so awesome.
Here's some cliff notes for being safe with molten metal:
Molten metal will burn you faster than you can react to it.
Leather boots are a must. Smart people wear gloves and safety glasses too.
Molten metal can superheat a little bit of water and spatter.
Preheat any metal to be melted, ingot molds and tools before getting them in contact with molten metal.
Don't drip metal onto damp surfaces especially concrete or stone.
Melting metal or burning out molds can produce poisonous gasses.
Make sure you are in a well ventilated area preferably outside.
Wear a respirator with the correct cartridges if you are creating a lot of smoke.
Metal furnaces are a combustion source.
Don't use the furnace in a closed room. You can asphixiate.
Prep your work area and keep combustibles away from where you are working.
Silica dust from sand can be a hazard.
If you work with sand molds wear a respirator or at least a dust mask when working with the sand. It's the tiny particles that you can't see that can accumulate in your lungs and cause silicosis over time.
This list is not exhaustive. Use the precautions that you feel are appropriate.
If you have any trouble following the instructions or need more explanation please let me know in the comments.
If you build a furnace post pictures in the comments so we can check it out.
If you like projects, and I know you do, check out our Mighty Projects on our Mini-Farm (AKA our backyard) at our website Mike and Molly's House.