Introduction: Dual Fuel Metal Melting Furnace

Picture of Dual Fuel Metal Melting Furnace

I got such a great response from the Easy Dual Fuel Furnace Burner that it only made sense to follow it up with the rest of the furnace.

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 thiscomprehensive guide for a general overview of metal casting and furnace design.

Step 1: General Plan

The basic concept is that we are making an insulated container that can withstand temperatures of up to 2600 degrees. The dimensions don't need to be precise but you will have better results if you stick with the general program. I wanted a furnace that would let me cast a decent amount of metal, about 5 lbs aluminum.  It needs to be sturdy but as inexpensive as possible to make and run.

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

My barrel for the outer shell is is 17" high x 14" dia. I cut the top 3.5" off the barrel for the lid. The shell is filled with 2.5" of insulation and an additional 1/2" of hot face for 3" total. I filled the floor a little deeper- 4.5" of insulation and 1/2" hot face. This left an inside cavity of 9.5" high X 8"dia. There is a 4.5" hole in the top to let the exhaust out and to access the crucible for feeding it metal.

Step 3: Mix the Insulation

Picture of Mix the Insulation

I wish I could say that I thought of starting with mixing the insulation ahead of time when I did my build. You can benefit from my experience and start it first.

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.

Recipe

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

Picture of The Lid

The first step is to cut the top off the barrel for the lid.   I used a grinder with a cutoff wheel to slice off the top 3.5" of the barrel.  File or sand the ground edges so they aren't sharp after you are done.  

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

Picture of The Body

The body is a little trickier to mold but nothing that we can't handle.  There are three components that we need to consider when making the mold: the inside cavity, the tuyere (hole for the burner tube), and the drain.

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

Picture of Other Bits

While you're at it, cast some plinths and an exhaust cover or two from the insulation mix.  The plinth is a little puck that goes under the crucible and I bet you can guess where the exhaust cover goes. :)  Make sure to make it bigger than the vent...

Step 7: The Hot Face

Picture of The Hot Face

The hot face is the inner liner of the furnace.  It is made from a specially formulated refractory (high temp, 3000 degree) cement.  The upside of refractory cement is that it is super hard, resists breaking down and doesn't usually shrink or crack as it dries.  In other words it's good at it's job.  The downside is that it can be hard to source and is pretty expensive- around $90-$100 a bag.  You shouldn't need more than a bag to line your furnace.  Ceramic supply houses, industrial suppliers or furnace repair guys are good places to look.  

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

Picture of Drying

Since we used a clay and sawdust mixture for the insulation there is still going to be a considerable amount of moisture contained in it even when it appears dry.  You need to drive out as much of this moisture as you can before bringing the furnace up to high temperatures.  I was worried about steam being trapped in between the hot face and the shell so I drilled several holes in the top of body with a masonry drill and then some in in the metal shell.  In retrospect I think I went overboard.  Four holes in the top and half a dozen in the shell would have been fine.  

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

Picture of Attach the Lid

Now we need to attach the lid to the furnace.  I made a simple hinge out of pipe that lets me swing the lid in either direction.  The lid has two tabs welded to it that stick up from the top edge.  These are bolted to the V shaped frame which is welded to a piece of pipe that slips into a larger piece of pipe welded to the back of the shell. I had to weld an additional piece of plate to the shell in order to reinforce it and get the spacing right.  Adjust the lid so it sits 1/4" or so above the body of the furnace.  You don't want them to drag on each other.  Getting the lid hinge right is a little fiddly.  Take your time and get it right.  You don't want problems when it's super hot.

I attached a couple wheels on the bottom and a leg in the front too.

Step 10: Accessories

Picture of Accessories

A furnace that gets really hot is nice but without the right accessories it's kind of useless.  You are going to need to make a crucible, some tongs and a hook for the crucible, a paddle/scoop for cleaning the molten metal, some more tongs for grabbing hot stuff and an ingot mold to get started.  A good time to work on this stuff is the week or two while you're waiting for the insulation to dry.

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

Picture of Firing

The first firing of the furnace is a crucial one.  You are firing all the refractory materials and turning them into ceramics.  It is important to bring them up to temperature sloooowly.  Some people recommend a heat lamp first to start the heating process.  I think that's a bit cautious.  

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. 

Success?

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.

Thanks!!!



Comments

al3ph (author)2016-12-26

Another alternative hotface: Sodium Silicate Gel. Sold on Amazon and eBay. For the miserly or impatient, can be made through a chemically hazardous and time-consuming process involving boiling cat litter crystals (not the clumping clay variety) and lye. If you opt for the latter, please do it outside in a well-ventilated area, with long, chemical-resistant gloves.

spike3579 (author)al3ph2016-12-26

Creative approach but please nobody get hurt from boiling cat litter!

al3ph (author)spike35792016-12-26

It's not the cat litter you'd need to worry about. You could boil cat litter all day long and not get sodium silicate gel. It's the lye/caustic soda/sodium hydroxide, mixed with the cat litter that makes it a somewhat dangerous process. https://www.instructables.com/id/How-to-Make-Sodium-Silicate-Water-Glass/

spike3579 (author)al3ph2016-12-27

Yes, boiling lye can be hazzardous. It will burn you at any temperature. Kitty litter not so much. Still embarrasing when asked "How did you get that nasty burn / lose an eye?" and your reply is "Well I was boiling kitty litter in lye to save $11."

al3ph (author)2016-12-26

Goggles and a respirator wouldn't be going overboard. Just buy the stuff...

Wizardries (author)2016-10-29

forge build

I love creating and this site makes it easy to create better forms!

Mojoe626 (author)2015-05-05

I have a made my first home made foundry.. It's a charcoal fueld foundry and has worked great so far! Question I have and hope someone can give me advice on is, what is the best charcoal/coal to use for my foundry? The current charcoal I am using burns away too quickly and because of the blower, I am getting a lot of ash falling into my crucible. I'm sure I can use some sort of a "kid to put over my crucible but would like an opinion or two on what experienced, charcoal foundry users prefer. Thanks yall!

Dowaine (author)2015-01-28

Could i use the sawdust Fireclay mix for the hot face

spike3579 (author)Dowaine2015-01-28

Yes, but it will break down over time faster than if there is a more heat resistant coating on it. How long will it last? I don't know. One would have to experiment.

pwwcad (author)2015-01-24

Looks great! Did you get to melt bronze with this set up yet?

Ganhaar (author)2014-07-26

Great write up, thanks.

nreed4 (author)2013-09-01

Is there any substitute for fire clay in this ible?

spike3579 (author)nreed42013-09-01

You could try a different clay but I would make sure it can handle high temperatures without breaking down.

nreed4 (author)spike35792013-09-01

Did you get the clay online, or do you have a supplier near you. Great 'ible by the way.

sparhawk7 (author)nreed42014-05-19

you can get a 95lb bag of fireclay at home depot, free shipping to store, if you can't find it anywhere else.

spike3579 (author)sparhawk72014-05-19

Great tip!
Thanks

spike3579 (author)nreed42013-09-02

I went to a ceramics supply house. Look for the kind of place that professional potters go for their supplies.

ivver (author)2013-04-15

You also can get clay stronger with adding some pig hair (belive or not) , I use that method when I need to repair broken clay in furnace. It holds pretty well, try and see at your self. :)

spike3579 (author)ivver2013-04-16

Great idea but where does one get pig hair?

sparhawk7 (author)spike35792014-05-19

paintbrushes?

weish (author)2014-05-08

do you think this furnace design could handle glass? that would require around 2-3000 degrees at most. I'm interested in learning primitive glassworking so a furnace that's easily home built would be very valuable

spike3579 (author)weish2014-05-09

I used to be a glass glassblower so I've given this some thought. The short answer is yes. In fact it's on my list to set up a small glassblowing studio at some point. That said It will take some work to get there from here.

Generally for glassblowing you don't need to go higher than 2000 f. The refactory materials will tolerate the heat. In theory my burner will generate enough heat but in practice I don't think it does. I've seen glass furnaces that run quite happily on waste oil though.

The set up for a glass operation requires more equipment than this furnace which is basically a hot can. You need a place to reheat the glass (glory hole) as well as a place to cool it down (annealer). Most small furnaces have a dome over the tank for reheating and then run the exhaust gasses through a secondary chamber for the annealer. You will also need to come up with a crucible to hold the glass in which is very corrosive when molten.

Check out this movie for a very primitive but effective set up and this one for a more modern take.

Good luck on your project. PM me if you have more specific questions. I've built a couple glass furnaces and would love to see a low cost glass blowing set up get built.

weish (author)spike35792014-05-09

For a crucible, i had seen post by a guy who was making his own pots from hand mixed clays and whatnot, so I've at least got a recipe for that. My thought on annealing was that once the work is done and the furnace is shut off, place a simple wire rack with multiple levels into the hot furnace to pee heat, then shut in the glass pieces on the rack, maybe with a small wood or charcoal fire in the bottom to keep everything warm, but not hot enough to slump the glass. Then just let the insulated burn chamber slowly cool to anneal the contents. The glory hole is a design element I hadn't thought of yet. My first thought though would be to basically take another matching barrel cut to half height and turn that into a topper lined with refractory and with a hole or door for reheating parts. I'll have to continue my research, ultimately I'd like to end up with a furnace I can use for metal casting, and then reconfigure for glass working. Of course for me, the fuel of choice is charcoal, gas is just too expensive compared to hardwood lump charcoal, and naturally that would have been the fuel of choice for centuries before modern processes and fuels were available.

spike3579 (author)weish2014-05-13

The glass needs to go right into the annealer after it is formed. You can't cool it down and then anneal it later. I would make an oven that sits above the furnace and gets heated from the exhaust of the furnace. You use a damper to control the temp of the annealer and then seal it up when you are done. For the glory hole you can cast a dome top like a wood fired pizza oven. Use that when you are melting glass and just a ring when you are melting metal. The rocket mass heater seems like a good approach for wood firing a furnace. Here's another couple links to some old school glassblowing for inspiration.

http://www.romanglassmakers.co.uk/index.htm

ivver (author)2013-04-16

well, I don't know how is in your country, but here by me we sometimes slaughter a pig (to make sausages, bacon and other) so it left behind. I don't know where else would you find hair, try to deal with some local butcher, he could have one. :)

x3wayassassin (author)2012-06-20

instead of using the clay you could also use refractory cement that is rated up to 3000 degrees F

spike3579 (author)x3wayassassin2012-06-21

The clay is $12 for a big bag and the refractory cement is $95 for a small bag. I did try it with some left over refractory cement and it just made crumbles rather than holding together. There are other lower temp refractories that would work but they too are awfully expensive.

static (author)2012-01-05

Great instructable, saved it for future reference. On the 100 W. light bulbs, LOL. I'm certain that 100 W. incandescent bulbs that meet the energy efficiency standards are available

spike3579 (author)static2012-01-07

I don't know....
I think I might start stockpiling a few. Can't get much heat out of a led :P

hag (author)2011-12-07

I was just wondering what you used for a burner. The only thing that comes to my mind is a propane blow torch, but I doubt that gets hot enough.

spike3579 (author)hag2012-01-07

I used this burner.  Works great :)

Ace193 (author)2012-01-01

This is a great 'ible. Your eye for detail in writing this one made it very useful. Keep up the good work!

lewster (author)2011-12-19

Looks awesome, great 'ible

pj63 (author)2011-12-02

What a Great Instructable....I've been waiting for a while for this to come up...I have Bricks,Fire Cement and a 25kg bag of Fire Clay to try to build a Smelting/Foundry in my Garden..I needed some final information and your Instr was Brilliant..Thanks and well done very informative...

spike3579 (author)pj632011-12-03

Sounds like you're ready to go!
PM me or put any questions that come up in the comments
Post some pics of your project.
We want to see....

robotjim (author)2011-12-01

thanks for the tips, been wanting to try melting
and casting metal for ages...this looks easy enough
I've got no excuse not to !

wasabi237 (author)2011-12-01

Gold melts at around 1800. Whoo Hoo!

kenwork (author)2011-12-01

Loved this instructable on creating a metal melting furnace! Thank you!

BTW, your website address is incomplete at the end of the instructable, it is missing the .com.

Here is the complete link for people that want to see Mike and Molly's house :)
http://mikeandmollyshouse.com/

PS118 (author)2011-11-29

Do you think it would help with the cracking to dry the insulation before pouring the liner?

spike3579 (author)PS1182011-11-30

Probably some. The trouble is that the clay is going to shrink no matter what. The commercial stuff is formulated with parts that expand and contract for a zero net change but it costs more. I don't think a few cracks will really affect the performance of the insulation in this application. You could fill the cracks with mortar if you wanted too.

drewgrey (author)2011-11-28

Awesome, looks like fun!

soul_eater (author)2011-11-28

it looks Great , Im going to make plans to build this one

rimar2000 (author)2011-11-28

GENIAL, thanks for sharing.

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