Building a Mini Iron Melting Furnace

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GreenTwin

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I built an iron furnace a few years ago, and it will melt up to perhaps an A30 crucible full of gray iron, which is about 60-70 lbs of iron.
I generally operate it with an A10, pouring maybe 25 lbs of iron at a time.

I have been looking at making some lead-free bearing bronze, and I don't really want to fire up the big furnace to do that.

I have always wanted a mini iron furnace that would be easier to use for small pours of bronze and iron, and after pondering over exactly what size I should build for years, I have finally started building a mini furnace.

The crucible range of sizes that would work with this furnace would be between an A0.5 at the smallest, and a somewhat tight fitting A5.

I use Morgan Salamander-Super clay graphite crucibles, and they come in all sizes.

I think an A4 will be an ideal size for this furnace.


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GreenTwin

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The bearing bronze I am trying to make is:

Lead-Free Bearing Bronze C89835, Bismust Tin Bronze
(Copper=87%, Zinc=3%, Tin=6.7%, Bismuth=2.2%, Antimony=0.35%, Iron=0.2%, Lead=0.09%, Nickel=1%, Phosphorus=0.1%, Silicon=0.005%, Sulfur=0.08%)

The intent is to use this material for things like rod ends on steam engines, and other wearing parts, perhaps such as crossheads, etc.

I have some scrap copper, and luckily I don't need very much of the other metals.
Metal prices are rather high these days, along with everything else.

In order to keep material costs as low as possible, I am trying to build this entire furnace using the materials I have on hand in the shop.

I am having to get creative with the hot face, which needs to withstand iron temperatures which will be continuous 2,900 F rated.

I have some hard fire bricks, which stand up to iron temperatures quite well, and so I am cutting up strips for a hot face veneer.
I have a wet saw, and the cutting went pretty well (using a commercial-grade respirator).


A few of the strips broke into two or three pieces, but I have some plastic refractory that is rated 3,800 F, that is good about gluing hot faces together.
If I had enough plastic refractory (which I don't), I would make the entire 3/8" thick hot face using that material.

I added tile flooring to several rooms in a relative's house a few years ago, and so I have gotten good with a tile saw.

The lighter color bricks are insulating fire bricks rated at 2,600 F.
The insulating bricks will not stand up to iron temperatures, at least not the 2,600 F rated ones, but the hot face will protect them.
The insulating fire bricks are soft and easy to cut with a hand saw.

The hard fire bricks have to be cut with a wet masonry saw.

The plinth (the platform that the crucible sits on) is made from two pieces of 1" thick hard fire brick.


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GreenTwin

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And here is the same furnace with an A3 crucible.

The hot face veneer is 3/8" thick, and it is not shown in this photo.

I think an A4 will be the ideal crucible size for this furnace, which will hold perhaps 8-10 lbs of bronze.

I will wrap two layers of 1" ceramic blanket around this furnace, so that the exterior will operate cool to the touch.

The outer shell will be an old hot water heater steel tank that I will cut up and salvage.

The intent of the insulating fire bricks is to provide a rigid support behind the 3/8" thick hot face veneer.

Some folks use coated ceramic blanket, but I wanted a more robust furnace that cannot be damaged easily.


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GreenTwin

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The insulating fire bricks have flat sides, but I ground a round internal surface on them (while wearing a commercial dust mask).

I considered making a mini-oil burner for this furnace, but there is really nothing to be gained from using a small oil burner, and so I plan on using my standard oil burner (the same oil burner I use for my larger furnace).

The Delavan nozzle that I use in my burner will operate at a wide range of settings, and so I will reduce the fuel and oil flow for this furnace.
I will use plastic refractory to neck down from the 2.5" burner tube to a tuyere opening in the furnace wall of about 1.5" diameter.

I trimmed the end of this burner off flat, and no longer use the cut/reduced end on it.

I run clean automotive diesel only, to avoid any contamination issues with waste oil, and to avoid any heavy metals that may be suspended in waste oil.
I have seen some waste oil users have problems with clogging, antifreeze leakage into the oil from a blown head gasket, and other contaminating materials that are in the oil, that will cause all sorts of problems with a burner.

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LJ Peterson

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I really like what you've done here.
For the burner head, I see you have the ends slit and twisted somewhat to create a flame retention head. Is there a blower you use or just the compressed air?
 

GreenTwin

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Thanks LJ-

I tried that slit twisted end on the burner tube, but I could not detect any improvement, and so that part was cut off, and now I use a flat end on the burner tube.
Commercial oil-fired heating units use a spin vane in the burner tube, and so many copy that design, and think that it is necessary or helpful for a foundry furnace.
A commerical heating unit discharges into a large open combustion chamber, and so the spin vane does help mix the fuel with the air.
A foundry oil burner discharges on a tangent into a foundry furnace, and the fuel and air swirl around inside of a red hot small chamber.
I have tried all sorts of spin vanes and other things to spin the air, but never noticed any improvement in burner performance, and so I don't use a spin vane on any of my burners anymore.

I use an air compressor for atomization of the oil (diesel).
A spray nozzle burner acts almost identically to a paint sprayer.
To reach iron temperatures, additional combustion air needs to be introduced into the furnace, and for that I use a Toro variable speed leaf blower, operating on its lowest speed.
.
 

LJ Peterson

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Thanks for your reply.
I use a Becket burner converted to waste oil for my boiler that heats the shop and greenhouse.
You're right about the flame retention head mixing the fuel air mixture in a regular furnace/boiler though. I haven't thought about the mixing in a foundry furnace but i see your point about the shape. I've been thinking about using a similar setup for a foundry. I don't plan on melting steel but would like to do aluminum and brass at some point.
 
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Is there any disadvantage to using a "high temp" furnace like this to melt lower temp metals like aluminum? Melting iron is dream far away but aluminum, maybe. It would seem building a furnace able to melt all metals would be the way to go. Buy once, cry once. Bob
 

GreenTwin

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I have seen a lot of heated discussions about what the best burner and furnace type is.

The best burner/furnace is the one that allows you to cast what you want for a reasonable amount of money, with reasonable longevity.

The trend these days for aluminum melts is the ceramic blanket style furnace, but I would advise that the ceramic blanket should be coated with a high-temperature material, and the raw ceramic fibers never be directly exposed or allowed to get airborn.

Some are purchasing mini-electric furnaces, but I had heard that crucible life may be limited with those units, as well as them having a limited capacity.

Is there any disadvantage to using a "high temp" furnace like this to melt lower temp metals like aluminum?
An iron furnace works well at melting aluminum, but is overkill as far as what is required for aluminum melts.

An aluminum melting furnace can be constructed from much lower temperature materials, and a satanite-coated ceramic blanket furnace is probably the most economical building method.

I don't like to maintain the coating on ceramic blankets that are exposed to the furnace interior, and so I prefer using a hard hot face.

One furnace I built years ago, and still use sometimes for aluminum melts is the one below, which is made from hard fire bricks.
Hard fire bricks can be obtained at wood burning stove stores, and stores like Tractor Supply.
I just stood the hard fire bricks up, wired them together, applied a little furnace cement to the joints (which is optional), left a hole at the bottom of one brick, and I use a pottery kiln shelf as a makeshift lid.

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As you can see, I sometimes even use a small stainless steel sauce pot for a crucible with aluminum.
Steel crucibles won't last forever, but they allow you to play around with an inexpensive setup without getting much money involved.

If you decide you really like metal casting, then invest in a Morgan clay-graphite crucible, make some lifting tongs and a pouring shank. etc.

I have not tried to melt aluminum with a weed-burner, but I suspect a weed burner may work with aluminum.
A Reil-style propane burner is not too difficult to make.


Casting gray iron is something you have to be well prepared for, with equipment and general foundry knowledge, especially safety knowledge.

Casting aluminum is like riding a bicycle on an empty residential street.
Casting gray iron is like driving a race car on a busy crowded freeway.
One can certainly safely drive a race car on a busy expressway, but one has to prepare and train for this, and have the right equipment.

I have seen folks melt iron with satanite-coated ceramic blanket furnaces.
The biggest problem seems to be keeping a coated ceramic blanket lid intact.

A hard fire brick furnace will actually melt iron, and I have seen folks do that, but there needs to be a lot more attention to the details of an iron furnace, such as sealing it, and insulating the outside of it, and making a good durable lid for it.

Edit:
If you decide to build an iron-rated furnace, with the intent of using it for either iron or aluminum melts, do your homework on the iron melt design and get that right, else you may be doing that over again, as I see many doing (and have had to do myself).
.
 

GreenTwin

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My very first furnace (used to melt aluminum only) was just a stack of insulating or non-insulating hard fire bricks.
Insulating fire bricks will begin to crack in a freestanding form, but this worked ok just to try melting some aluminum without much effort expended on construction of a furnace.

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WisJim

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I'm trying to remember details of a furnace I built when I was about 15 years old when experimenting with making alloys of copper etc for a science fair project. It involved a large truck brake drum from the salvage yard and using my mother's vacuum cleaner air output for an air source and used a crucible borrowed from the high school. I'm sketchy on fuel used but I did make some samples of bronze and some other materials.
 

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