How not to Do Backyard Metal Casting

[Lloyd-ss]

(Normally don't include all the quoted text but am this time - - - apologies if that offends someone!!)

This is the kind of reading that I find so incredibly useful!
Someone that has shown that they're doing the stuff and then they're telling about what to watch for in the doing.

IMO - - - I'm getting to learn from their experience (am I allowed to say - - - from their mistakes) so that its far less likely that I'm going to do things that may harm me.

Thank you sir for sharing - - - especially all the details - - - - I have far too many irons in the fire right now but you are making casting more and more appealing!

A serious pile of bouquets from here!!!!!!!!!!!!!!

x2 on what Joe said.
I normally take safety pretty seriously. Part of the reason is that I cut a finger off on a table saw when I was 15, and didn't even know that I was doing anything wrong. That lack of knowledge of the Do's and Don't's will get you.

I have still had plenty of close calls, but nothing like the initial incident. And the dumb ones that I did even earlier than that.

For me, some things are just too dangerous to do and the learning curve is too long. I will leave those activities to someone who knows what they are doing. But these types of discussions always put a boot in my butt to THINK before I DO.


This is a really scary factory video that I ran across several months ago. Scared the pee out of me.

 

[GreenTwin]

Moisture and water are probably the biggest hazard, and the most feared by casting folks.

I learned the hard way that flaming an ingot mold lightly does not dry the residual moisture off of the surface, and so I poured iron into one, and it popped, just like in one of the videos above of the two guys in the lab.

Splashing iron back onto yourself is not really a problem, since you are covered head-to-foot in leather or something similar, and you have a full face shield and safety goggles under the shield.

The rule in casting work is the waterfall effect.
You generally assume that molten metal will go up, and then will rain down on you, or it could be blasted directly towards you.

The art iron folks wear a neck shield under their hardhat to shield the neck, with the shield worn over the shirt collar.

The pants go over the leather boots (some wear spats, but boots are much easier to get on and off).

The art iron folks are pretty religious about safety, and they choreograph every step and every move of every person.
Every person is assigned a distinct task.
They practice it all before they ever start melting iron, and have backup plans when things go wrong.

You can see this in the video below.
My apologies for the wordy video; this is her blog, and she loves iron art with a passion.

Looking at 17:40, there is one person tapping the cupola, and his gloves are over the sleeves, since the metal is flowing out of the cupola towards him.

There are the pouring folks, two per shank, the skimming person, sand shoveling folks for spills and mold breakouts, folks directing the pouring people to the correct mold, and all sorts of support folks.
Often there is a safety person looking over the entire pour group, looking for problems.

Most of the other folks wear their gloves under the sleeve, since metal that splashes up on their jacket will roll down into their gloves otherwise.

So what happened to me is that my gloves were over the jacket, and metal popped out of the ingot mold that was not held in the furnace exhaust stream.
A few beads of molten iron went down my gloves, and ended up on the back of my hands.
I had a few 3rd degree burns before I could get my gloves off.

The problem with wearing gloves under the sleeve is that it is not easy to get the glove under the sleeve, and if you take your jacket on and off like I do, tucking the gloves under the sleeve every time is not very practical.

The moral of the story is to dry everything in the furnace exhaust stream, to drive of the invisible moisture that is on the surface of all metal.



My hand burns.
No pain, no gain, as they say.
Molten iron does vaporize the skin and the nerve endings though, so actually there is no pain involved.
The large spot eventually filled in with scar tissue, and is no longer even visible.

Another thing to keep in mind is to not panic if something goes wrong.
If you panic, you can make matters much worse.
One needs to make a cautious and strategic withdraw if something goes wrong, so you don't trip over something, or drop the pouring shank and crucible, etc.
The furnace needs to remain under control at all times, and the fuel valve needs to be easily accessible, and easy to operate with gloves on.
I don't really like my fuel and air controls up near the furnace, which is where some folks put them.

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[ajoeiam]

GENERAL OVERVIEW OF FOUNDRY WORK BOOK

Thanks, I am more than glad to share what I know.
It took me at least six years to sort out all the furnace, crucible and burner options, and find out what a good combination is.
It took me a few more years to learn how to correctly operate an oil-fired furnace.
There is no reason for it to take anyone six years in order to learn the process.
I know of one guy who had a mentor at a foundry, and he set up an iron foundry and was successfully casting iron within a month of starting.

Here is one book I ran across in 2011, written by a mechanical engineering student, who I have lost contact with.
While this book is not perfect, it is a pretty comprehensive book about metal casting.
I had comments at one point about how this book could be improved, but have lost those.
I need to go through this book again, and comment about how to do gray iron pours, and a few other things that can be better done.

This book is good about covering the fundamentals of backyard casting in a wide overview.

http://prometheus-foundry.com/The Hobbyist's Guide to Casting Metal--2nd Edition (web).pdf
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If you find those comments or do another round - - - I for one would greatly appreciate those annotations!!!

 

[DaleW]

When I started trying to cast metal (aluminum was my first successful crude test casting), I wanted to cast engine parts in gray iron.

Generally speaking, in the model engine circles, I was told the following:

1. I has not been done yet on a hobby level.
2. It cannot be done on a hobby level.
3. The intense heat will burn you if you are in the vicinity of the furnace/crucible.
4. If you do succeed in making a gray iron casting, it will have defects and hard spots, and will not be suitable for use.

At first, I somewhat tended to believe what others said.
I had no proof otherwise, and after all, these were some of the masters of model engine building making these statements, so who was I to question that, never having built an engine in my life.
I started to look for examples of other's casting model engine parts in gray iron.

Eventually I found the guy who goes by "ESC", who casts his own model V-8 engine in gray iron.
So there was proof that it could be done, and proof that complex castings could be made in gray iron on a hobby level.

Then there was the "How".
There were examples online of folks melting and pouring some nice iron pieces, but not necessarily engine parts.

Melting gray iron and getting it to a suitable pour temperature is one thing.
Making the patterns, and setting up the mold correctly is an entirely different and complicated undertaking.

My first iron pour used Petrobond (tm) oil-bound sand, and that casting had some sand inclusions where the mold failed.
Since it was my first iron casting, I repaired the holes, and used the castings.

I then experimented with sodium silicate bound sand, and finally discovered resin-bound sand.
Resin-bound sand is designed for iron and steel castings, and so that solved my sand mold problems overnight.

Then it was a matter of learning the correct sprue, runners, risers, gates, etc., and I learned that from Bob Puhakka and his mentor John Campbell.

Then I discovered ceramic mold coat, and solved my casting surface finish problems.

I learned about ferrosilicon, and how to use the critical amount (not too much) to give castings that are easily machinable with no hard spots (chills as some call them). Too much ferrosilicon causes excess shrinkage and hot tears.

I studies videos online of folks using resin bound sand.

I had to learn new ramming and molding techniques with resin-bound sand, since the traditional greensand flasks and molding techniques don't apply when using resin-bound sand.

I learned how to correctly tune an oil burner so that I could reach iron pour temperatures with good fluidity.

And I build a medium/low mass furnace, for quick heating and melting, using materials that would withstand up to iron temperatures and slag for extended periods of time.

So people ask me how I went from making initially flawed gray iron castings that had sand inclusions and hard spots, to making iron castings that rival what a commercial iron can produce.
The answer is that it was a long series of steps, as detailed above.
Like climbing a tall ladder.
You take one rung at a time.

Some don't believe I can do it.
Come over to my house, I can show how it is done and make you a believer.

In the beginning I had little confidence that I would ever figure out how to cast iron parts successfully, and I really never fully expected to figure it all out.
Many told me I would never figure it out; they were wrong, but I can't fault them for saying that, given the effort that was required to do it.

It is a awesome thing (a lifelong dream of mine) to be able to look at an old engine, make the 3D design, patterns, molds, iron castings, and finally a working model engine.
I recall looking at the steam engines in the toy shop as a kid, and asking my dad "Where can we buy a real one?".
Dad said "You can't buy them anymore".
I said "Why not ?".
He said "Just because".

So you many not be able to buy many steam engines anymore, but you can definitely make your own.

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I don't know why people say you can't do cast iron at home, I've done it in a blacksmith forge. The one crucial element is the right crucible, it needs to be able to stand high temperatures around 1480 C tap out. I always tell people to use known ingots as not all cast iron is the same. The 3 main elements you need to watch is Carbon, Silicon and Manganese. If you remelt a lot of cast iron than carbon and silicon will become an issue. The cheapest and easiest way to check you silicon is do a wedge test. You make a wedge mold cast it then what we call chase the cherry, immerse in water but not the cherry colour. Once fully cooled break it in half. At the point of the wedge you are looking for a light grey colour that should only be approx 4mm deep, if it's good then pour away. I have also done S.G. at home, though not since moving (the neighbors get upset at the smoke and flare up) As for moulds, I use green sand or Chem set. Green sand I use plenty of graphite paste on it and my finish will rival the best. In chem sand unless it's a large solid casting I don't use any coatings. If I use cores I paint them, mainly to stop burn on.

So I say give it a go at home but treat it with respect, even in industry every major accident I've attended and investigated has had two main causes and they were complacency and a lack of respect for the job.

 

[GreenTwin]

I can't find my comments, but I recall in general what they were.
I will skim it again and post comments.

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[GreenTwin]

I don't know why people say you can't do cast iron at home, I've done it in a blacksmith forge. The one crucial element is the right crucible, it needs to be able to stand high temperatures around 1480 C tap out. I always tell people to use known ingots as not all cast iron is the same. The 3 main elements you need to watch is Carbon, Silicon and Manganese. If you remelt a lot of cast iron than carbon and silicon will become an issue. The cheapest and easiest way to check you silicon is do a wedge test. You make a wedge mold cast it then what we call chase the cherry, immerse in water but not the cherry colour. Once fully cooled break it in half. At the point of the wedge you are looking for a light grey colour that should only be approx 4mm deep, if it's good then pour away. I have also done S.G. at home, though not since moving (the neighbors get upset at the smoke and flare up) As for moulds, I use green sand or Chem set. Green sand I use plenty of graphite paste on it and my finish will rival the best. In chem sand unless it's a large solid casting I don't use any coatings. If I use cores I paint them, mainly to stop burn on.

So I say give it a go at home but treat it with respect, even in industry every major accident I've attended and investigated has had two main causes and they were complacency and a lack of respect for the job.

These are words of wisdom for sure.
Thanks for sharing the information.

I had a definite fear of the temperatures involved with melting cast iron when I started, and I recall visibly shaking while I had the burner running.

When I started, I did not know exactly how to operate an oil burner, and you can get flame-outs and then semi-violent restarts if your burner is not set correctly.

Fear of the unkown was much of it, but also not knowing exactly what I was doing made it more hazardous than it needed to be.
I had to learn as I went and hope for the best.

Now that I have melted iron numerous times, it has become almost as routine as melting aluminum, and there are never any surprises with the furnace, burner, casting, etc.

It is a fantastic knowledge set to have in my opinion, and one that should not be lost with so much other technology that is getting lost today.

If I could find some nickle-mag, I would make ductile iron.
I know the process to use, and how to control the sulfur.


So I say give it a go at home but treat it with respect, even in industry every major accident I've attended and investigated has had two main causes and they were complacency and a lack of respect for the job. (DaleW)

This is so true.
Its like learning to drive a race car.
You have to respect the awesome power involved, and learn how to safely control things.
Don't jump into a race car and think you know how to operate it on day one.
My oil burner produces 122 KW, which is a lot of power in a very small package.

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[GreenTwin]

Here are my comments on the book "The Hobbyist's Guide to Casting Metal" by Ben Baker.
Some of these comments summarize what Ben is saying.

1. Use heat shields in strategic places, such as on the end of the pouring shank near the crucible.
A heat shield is a simple piece of sheet metal.
The IR is so strong near a red/white hot crucible that it will burn you through any glove, if you don't use a heat shield.

2. Use clothing and shoes that is not make of anything that will burn, such as nylon.
Use safety equipment that is easy to put on and take off.
Use a full face shield, and safety eyewear under the face shield.
The face shield and/or the eyewear have to be shaded at iron temperatures, else the IR will burn your eyes.

3. I don't recommend lost foam for any application, due to the unknown toxicity of the fumes.

4. I use products that are lead-free.
I don't keep any lead in my shop.

5. Don't ever put magnesium into a melt.
You will not be able to extinguish the fire.
Know exactly what metals you are using.
There are some bronze alloys that are extremely toxic.

6. If you choose to melt aluminum only, you can get away with using a steel crucible, but they do degrade over time, especially thin ones.
I started melting aluminum with a steel crucible, but converted to using Morgan Salamander Super crucibles for all metals.
I use one crucible per metal. Don't melt different metals in the same crucible.

7. Protect your fuel line if you are using an oil burner.
I have seen folks spill metal on their fuel line, and cause an inferno.
Needless to say, keep the furnace away from your house or car(s).

8. Aluminum needs to be poured about 1,350 F (+ - ), in order to get a good surface finish.
If you don't have a pyrometer, you can watch the meniscus at the top of the aluminum melt, where the metal meets the crucible.
When the meniscus goes flat, you are at about 1,350 F.
If you intend to pour very much aluminum, then you probably need to invest in a quality pyrometer, or build one.

Most don't measure the temperature of their iron, since that requires a very expensive pyrometer.
I generally just judge the look of an iron melt to determine if I am at sufficient pour temperature.
An iron melt at pour temperature will disharge sparkles out the furnace lid in the exhaust stream.

Casting brass or bronze is a bit of an art, and the folks I know who do it generally use a glass cover during the melt, to prevent zinc burnoff.
Don't inhale the zinc fumes if you do brass/bronze.
Brass/bronze is in the range where a standard pyrometer may or may not work.

I tried some ready-made greensand, and the surface finish was terrible.
I never used greensand again.

I tried oil-bound sand (Petrobond tm), and while it gives a superb surface finish, I found it difficult to keep from getting too dry.
Oil-bound sand has to be mulled before use, and sometimes a slight amount of alcohol added.
I added some oil to one batch of oil-bound sand, and ruined it.

Resin-bound sand is made for iron and steel castings, and it is the ultimate molding sand in my opinion.
The set time can be varied depending on the mold size (small molds are easy to fill and so can have a short set time, such as 5 minutes).
Resin is not that easy to source, and not cheap either.
The sand used with resin needs to be a commercial foundry sand that is very dry, such as "OK85".

Resin-bound molds need to be made as small as possible, to conserve sand and resin.
Resin-bound molds are not easily recycled/reused, and the art-iron folks never reuse their resin-bound sand.

Resin is toxic, and so you need nitrile gloves and a commercial chemical respirator to work with it.

An alternative to resin-bound sand is sodium silicate bound sand, and it uses the same OK85 sand.
Sodium silicate is a far less toxic version of sand binder.
You can harden sodium silicate molds with CO2 or a catalyst.
The catalyst eliminates the need for CO2, and you can vary the set time with a catalyst.
Many folks ruin their sodium silicate molds and cores by overgassing them.
5 seconds of CO2 is the maximum.

I don't use lost wax or ceramic shell since it requires special slurry coatings, burn out ovens, and a lot of skills and techniques.
Ceramic shell is a very slow process, and in my opinion is only required if you want to make large batches of small very intricate pieces, such as jewelry.

John Campbell's rule number 1 as I recall is use quality scrap of a know composition.
Aluminum for casting should be 356; not aluminum cans and not extruded aluminum.

For gray iron, I use electric motor end bells, since they are very consistent, and I found a supply of them at a motor rewind shop.
They are also somewhat easy to break up, at least the smaller ones are somewhat easy to break up.

The art iron folks use old radiators because they contain phosphorus, which gives the iron extreme fluidity.
The phosphorus is not suppose to be used in engine castings, although I know of one individual who used radiator iron to make a V-8, with no apparent problems.

Never burn the insulation off of electrical wires to get the copper.
Burning wire insulation is some of the most toxic stuff on the planet, and I have heard of folks being partially disabled from one whiff of it.
It will knock a bird out of the air dead 30 feet up in the air (so I am told).

Most folks online insist that you have to degass aluminum melts, and they sometimes use all sorts of toxic materials to degass, or they use useless material to degass aluminum, such as washing powder (there is a cult following of washing powder users out there; but it can be proven to be totally useless).
I never degass my aluminum.
I heat aluminum as fast as possible, making sure not to overshoot 1,350 F, and then pour immediately at 1,350 F.
Never ever stir an aluminum melt; that is the worst thing you can do with aluminum.

Brass/bronze/copper melts are sort of a specialty, and I had so much trouble with zinc burnoff that I abandoned brass/bronze/copper, and use either aluminum or gray iron.
I do have some lead-free metals that I intend to use to make bearing bronze, and the zinc is low, so not so much trouble with zinc burnoff (hopefully).
I am not very familiar with brass/bronze melts, although I have tried a few.
Don't use the brass/bronze alloy that is highly toxic (I forget the name of it).

Some folks use brake rotors as a source of gray iron.
I suspect many brake rotors are made from ductile iron, and the jury is still out on how good this scrap is for casting engine parts.
Most old machines and equipment were made from gray iron, and would be suitable for use as scrap.
If you can break iron with a sledge hammer without too much trouble, and get a clean gray broken surface, chances are it is good scrap.
Don't use window sash weights if you live in the USA. The are made from discarded garbage metal.

The crucible used for iron work should be a clay-graphite ferrous-metal-rated crucible that is rated at least 2,900 F, such as a Morgan clay-graphite Salamander-Super. Never flux a crucible (some moron posted online that you should flux your crucible with borax before you use it, but this destroys the crucible.)
You don't need a ceramic crucible for iron work (as stated in the book); that is false.
Waste oil does not necessarily burn hotter than diesel, and waste oil is often contaminated with radiator fluid, heavy metals, etc.
The viscosity of waste oil can vary widely.
I don't use waste oil, but instead use diesel only.
Use of consistent scrap and consistent fuel is one reason I get consistently defect-free castings.

The burner should be operated with a reducing (rich) flame when melting iron, to minimize oxidation of the iron.
Do not run the burner at a neutral setting as the book recommends.
If you operate at a neutral or lean (oxidizing) setting on your burner with iron, you will have a large amount of slag on top of the melt, which is a waste of metal, and difficult to skim.

I don't use a cover on my iron such as charcoal; it is not required.
I don't use a flux with my iron either.
Some use borax as a flux with iron, but it is totally unnecessary, and will very quickly ruin a crucible.
Iron is not prone to gas issues.
Don't try to fix problems that you don't have.

You will not melt iron for $100.00.
If you try to melt iron on the cheap, with poor quality materials, you will not make quality castings, and you will experience a lot of material failures.
If your budget is tight, stick with aluminum castings.

I build my own foundry equipment, such as lifting tongs, pouring shank, etc.
If you build one of the Rube-Goldberg type combo lifting tongs and pouring shank, you may get Rube-Goldberg type results, and you may get hurt too.
The crucible retainer on the pouring shank must work correctly in all situations. This is critical.

The most cost-effective furnaces are the ones with ceramic blanket linings.
If the ceramic blanket is not coated with something like Satanite, then the fibers will break off during a melt, and are an extreme inhalation hazard.
A ceramic blanket furnace will not take the punishment that a cast refractory furnace will take.

I use a 1" Mizzou (dense refractory good for use with iron) hot face, backup up by a layer of insulating fire brick, and then two 1" layers of ceramic blanket.
My furnace operates cool to the touch on most of its exterior during an iron melt.
You can use multiple layers of ceramic blanket behind the hot face in lieu of the insulating fire brick, to save money.

Don't mix more water into referactory than is stated in the instructions.
Refractory is mixed with a tiny amount of water, and requires a lot of mixing.

I thick hot face will be very slow to heat up (such as a 3" thick hot face of dense refractory).

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[GreenTwin]

If you use bound sand (resin-bound or sodium-silicate bound) for your molds, they can be a fraction of the thickness/size of a standard greensand mold.

My resin-bound molds are often 3/4" thick in places.

I go to great lengths to make my resin-bound molds as small and light as possible (to conserve money).
Bound sand molds are also very strong once they fully cure, and you don't need a thick mold as you would need when using greensand.

And the technique for ramming greensand molds is to pound the sand (ram) into place with a rammer of some type.

The technique for ramming a bound sand mold is very different.
You press the bound sand with the fingers, and then with the heels of your hands.
You never pound on bound sand, since there is no clay to compact, and the pounding fractures the mold and shifts it away from the pattern.

The sprue/runner/gating/riser arrangements shown in the referenced book and most other books is in my opinion terribly wrong.
Bob Puhakka explains the correct arrangement very well, and it is about not aspirating slag and air, not causing turbulence, and not splashing metal into the mold cavity.
If you want consistently bad castings, follow what is generally shown in all metal casting books and reference material.
People ask me how I can get consistent defect-free castings, and my response is "do it like Bob Puhakka; he has it figured out".
There is a group of folks who love to hate Bob Puhakka.
Say what you will about his flamboyant personality, he knows what he is talking about, and can prove it with his flawless (x-ray verified) casting work.


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[GreenTwin]

Vent your cores through the center, and lead the vent up and out the top of the mold.

Bake your cores, or flame them lightly with a propane torch.

Core gassing is very common if you don't bake or heat your cores, or if you don't have a hole through the core that is vented out the mold.

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[GreenTwin]

Your lifting tongs should contact the crucible on the lower 1/3 of the crucible.

Some tongs have the main contact low, and secondary no-weight supporting guides about 2/3 of the way up the crucible.

You can cause a crucible failure if you lift it anywhere but in the lower 1/3.

Crucibles are fragile.
Treat them with great care.
Don't pack a crucible with scrap, since it will expand and crack the crucible.

Don't use a damaged or thin crucible.

Don't use a cheap crucible, especially with iron.

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[GreenTwin]

There needs to be sufficient clearance around the crucible to be able to get the lifting tongs into the furnace, and sufficient space for combustion.

There are a lot of bad lid-lifter designs out there.
I build the typical lid lifter on my first furnace, and it jammed partially open and resulted in a ruined crucible.

The best lid lifter mechanism I have used is a pivoting wrist joint.
These will not jam, and they work very well.

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[GreenTwin]

I don't agree with the book about tuyere placement/angle.

The tuyere is the entrance into the furnace for the burner tube.

The tuyere centerline should be at the junction of the top of the plinth and the bottom of the crucible.

The crucible sits on an elevated platform called a plinth, and the plinth needs to be made from either hard fire brick, or with iron I recommend a dense refractory such as Mizzou.

The burner tube enters the furnace offset to one side, so that the flames from the burner do not directly impinge upon the crucible, and you get a tangential swirl of flame around the furnace interior.

The burner tube should be angled down slightly, since the velocity of the fuel/combusion air flow tends to make it climb up the back wall of the furnace, causing a cool spot.

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[GreenTwin]

I see a lot of folks operate their furnace with a propane tank just inches away from the furnace.

I think this is a hazard.

Keep the fuel tank well away from the furnace.

Protect your fuel lines with something resistant to heat.

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[GreenTwin]

I use an oil burner, because it works at any temperature including cold temperatures (with diesel), and you don't get tank cooling problems as you do with propane tanks.

Two common types of oil burners are drip-style, and spray nozzle (such as a siphon nozzle or pressure nozzle).

I tried building and operating a drip-style oil burner, and was very disappointed at how difficult it was to control, how poor the control was at any given time, and how adjustment of the burner was required during the melt.
Some use drip-style burners only to melt lots of iron.
I would never use a drip-style oil burner with a furnace.

I use a siphon nozzle burner with a 10 psi pressurized fuel tank, clean diesel only, 30 psi safety valve on the tank, and an inline fuel filter ahead of the burner.
This arrangement gives rock-solid repeatable performance, very fine control over a wide output range, and it does not require adjustment.

With iron, you want to operate the burner with a reducing (rich) flame, with about 3 or 4 inches of yellow flame coming out the furnace lid opening.
This will greatly minimize the amount of slag that is produced during an iron melt.

Again, if you want consistently defect-free castings, use consistent scrap, consistent methods, and a very consistent burner.
Remove every variable you can from the process and you will get good castings.

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[GreenTwin]

If I were doing small aluminum melts only, I would probably use a naturally aspirated propane burner, similar to the "Reil" design.

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[GreenTwin]

Fire bricks come in two types, hard (non-insulating) and soft (insulating).

You can cut soft bricks by hand with a hacksaw (wear a good dust mask).

The hard fire bricks require a diamond saw.

Insulating fire bricks (IFB's) are relatively inexpensive if rated in the 2,500 F range.
As you go above the 2,500 F rating, insulating fire bricks become very expensive.
Insulating fire bricks can easily be damaged during shipping, and so the shipping cost can be very high if they pack it correctly.

Hard fire bricks are available at places like Tractor Supply, or wood stove supply houses.
You can actually make an iron furnace by standing up a ring of hard fire bricks, and insulating behind them.
Hard fire bricks stand up to iron temperatures surprisingly well.
Hard fire bricks are more difficult to use since they require a diamond saw.

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[GreenTwin]

All dense refractory cracks as it goes through iron-heating cycles.

Patch all cracks after you use a furnace with something like Satanite or similar high-temperature furnace patching material.

Never start a furnace that has open cracks in the refractory walls.

Cracked refractory is not a problem as long as the refractory does not crumble.

Mizzou will crack, but will never crumble at iron temperatures.

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[GreenTwin]

The furnace design shown on page 95 is a pretty good design.

I like a hot face that is at least 1" thick.

Never let your burner tube protrude into the furnace like he shows in his drawing.
You will overheat the burner tube and melt it off.

The burner tube should be back in the tuyere about 3/4".

And a sign of a rank amateur is that they use drain holes in the bottom of the furnace.
Forget drains in the bottom of a furnace.
Use a quality crucible and you won't have spills in the furnace.

If you somehow do dump a crucible of metal into your furnace, turn the furnace on its side, run the burner, and let the metal drain out.

The need for drain holes is another one of those myths that will never go away.

Also, you should not put ceramic blanket under the bottom of the furnace, unless your furnace bottom is pretty thick.
I use insulating or hard fire bricks under the bottom of the furnace, because you can have a lot of weight on the plinth and on the bottom of the furnace, and if the bottom of the furnace gives, you will dump your crucible full of metal into your furnace.

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[GreenTwin]

The slab-rolled hot face is not a bad idea (page 101), however, I would make it 1" thick minimum.

They make a special refractory for applications like this, and it is called "Plastic Refractory".

It has the consistency of Playdoo, and you don't add water, but instead just roll or mold it by hand into the shape you want.

Plastic refractory does tend to dry out over time, so use what you buy as soon as possible, if you use it.

I did find some plastic refractory after I built my furnace, and it is excellent for patching hot face cracks.

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[GreenTwin]

Page 105, never let your burner tube protrude into the furnace like he shows in this photo.

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