Melting aluminum

Today another in my ongoing, seldom-updated series of posts about building my own backyard foundry. Today I’ll describe how the final step works: actually melting and pouring the metal. First, see my previous post on how to make a green sand mold.

Start by assembling all the equipment you’ll need in one place, on a day with no chance of rain. (Click on any photo for a larger version.)

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Casting: making a green sand mold

Today another episode in my seldom-updated series about building a home aluminum foundry.

The technique I use for casting aluminum is called “green sand” casting not because the sand is green (though the sand I use is in fact slightly olive coloured) but because the sand is moistened with water and clay rather than oil. I made the sand myself; it’s a mixture of about ten parts olivine sand to one part finely powdered bentonite clay, and then “tempered” with water until it feels right. (Use a spray bottle set to a fine mist and stir the sand as you temper it.) It should feel like perfect sand castle building material: wet enough to hold its shape but not so wet that you can squeeze water out of it. If you can make a “snowball” of sand with a fist and break it cleanly in half, that’s probably good. Continue reading

Mistakes were made, part three

This post is from my series on building a backyard foundry.

You remember back when I said in part two of this series that I was temporarily using a flimsy stainless steel tub as a crucible until I managed to obtain a 3 1/2 inch (nominal) pipe nipple? Turns out that when you think “I can probably get one more melt out of this thing before it is destroyed”, that is the time to throw it away. The crucible failed. Fortunately, the crucible was still in the furnace. Continue reading

Reduction and oxidization

This post is from my series on building a backyard foundry.

We all have a basic understanding of “oxidization” I think: when a metal like iron is exposed to oxygen, either in the air, or dissolved in water, the metallic iron turns into iron oxide, which has quite different properties. In particular, iron oxide is brittle, flaky, and expands away from the underlying metal, which means that oxidization destroys the metal. Copper produces a green oxide. Aluminum oxide does not flake off; it actually forms a protective “passive” layer on top of the aluminum protecting it from further oxidation. Same with the chromium that is in stainless steel; the surface is actually chromium oxide, which does not flake off.  Continue reading

Mistakes were made, part two

This post is from my series on building a backyard foundry.

My third mistake was building the furnace before I had obtained the crucible. Post construction I went to a number of thrift stores looking for cast iron pots, tall stainless steel tubs,  and so on, to try to find something that would fit the 6 3/4 inch bore of the furnace. It would have been better to obtain a good crucible first, and then ensure that the furnace fits it. Remember, for a charcoal furnace you have to be able to pack charcoal around every side of the crucible; the maximum outer diameter of the crucible should be about 2 inches less than the bore of the furnace.

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Mistakes were made, part one

I said a couple of episodes back that I made some mistakes in the design and implementation of my furnace; fortunately they were mistakes from which I learned something, and that were fixable.

The first mistake I made was a consequence of my not clearly understanding the difference between propane-fueled and charcoal-fuels furnaces. To be clear, the relevant differences for the purposes of this mistake are:
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Royale With Cheese, plus, dividing temperatures

In reading over the previous posts I realized that I am switching between the metric and Imperial systems of measure at will. This is what I get for being a Canadian who has lived in the United States for sixteen years. When doing any kind of “scientific” calculation it is of course far easier to do in the metric system, where converting between litres and cubic centimeters is simply a matter of moving a decimal place. I have no intuition for how many fluid ounces are in a cubic foot; I always have to look it up. But when it comes to carpentry and oven temperatures, I’ve learned how in the Imperial system of inches and degrees Fahrenheit. I’ll probably continue to switch back and forth indiscriminately, so, sorry about that.

On the subject of Fahrenheit, a quick reminder. As I’ve described this project to people, they often ask if I am going to try to melt iron. No, I say, the temperatures are far too high; aluminum pours at 1400°F and iron melts at 2800°F. So far, three people have said “oh, so that’s twice as hot”, without stopping to think about their high school physics. Remember, you cannot divide one temperature by another.

Why not? First off, we don’t measure temperatures on an absolute scale. There are negative temperatures. If 1400°F is half as hot as 2800°F then clearly it is negative 40 times as hot as -35°F, and 14000 times as hot as 0.1°F! Neither of those make any sense. 

This reason alone is sufficient to reject the idea that 2800°F is twice as hot as 1400°F. Now, we could convert to absolute scale. 1400°F is 1033 Kelvin,  2800°F is 1811 Kelvin, so the melting iron is about 80% hotter than the pouring aluminum, right?

But that’s not quite the right way to look at it either. We’re not starting with the metals at absolute zero to begin with. Room temperature is about 70°F, so the aluminum must have 1330 degrees of heat energy added to it, and the iron must have about 2730 degrees of heat energy, and that is just about twice as much, right?

But no, that’s not quite right either. The specific heat capacity — the amount of heat energy you have to add to a metal in order to raise its temperature by a given amount — is different for every metal, and iron’s specific heat capacity is about half that of aluminum; the same amount of energy increases the temperature of iron for two degrees for every one degree that it would increase the temperature of aluminum. So even though the temperature change of the iron is twice as much, it takes half as much energy, so it’s a wash, right?

Well, no, that’s not right either; somehow the furnace has to get up to the needed temperature and the furnace has to withstand that amount of heat. How efficiently the furnace transmits that heat into the melt is maybe an interesting theoretical question, but the fact is that the vast majority of the heat energy in the furnace is heating up stuff other than the melt.

And finally, we’re still not taking into account the latent heat of fusion! Normally when you put heat into an object, the amount of heat energy that goes in turns into an increase in temperature, in a linear fashion. That is, if putting in one unit of energy raises the temperature by one degrees, then putting in two units will raise it by two degrees. This ceases to be the case when the substance is melting (or freezing) or boiling (or condensing). When the object reaches the melting point it needs extra energy, called the latent heat of fusion (*), to overcome the stick-together-ness of the solid form; this energy breaks down the crystal structure of the solid, rather than increasing the temperature. And, like the specific heat capacity, the latent heat of fusion of a substance is a characteristic of the molecular structure of that substance. Aluminum has a much higher latent heat of fusion than iron: 398 kJ per kg, compared with 272. So, even though iron has to get a lot hotter to melt, it takes a lot less energy to get it from solid to liquid.

The long and the short of it is: don’t think of temperatures as things that you can multiply and divide, and even addition and subtraction is a bit dodgy when going over the melting point boundary.


(*) This has nothing whatsoever to do with nuclear fusion; the “fusion” in question is the fusion that a liquid undergoes when it freezes. The latent heat that you must remove from liquid water to “fuse” it into ice is the exact same amount of energy as the amount you must add to melt solid water, so the latent heat of fusion and the latent heat of melting are the same amount.

Mizzou castable refractory instructions

I’ve built my furnace; mistakes were made along the way which I’ll document in a later episode. I decided on a 10 3/4 inch outer diameter with a 2 inch wall. The furnace is 15 1/4 inches high, and 2 1/4 of that is the lid. Thus the bore is a cylinder 6 3/4 inches in diameter and 11 inches tall. 

This took just slightly less than the complete contents of two 55-pound bags of “mizzou” castable refractory cement, which I obtained at Seattle Pottery Supply. Interestingly enough there were no instructions on the bag. Fortunately the (very informative) high temperature tools web site had detailed instructions, which I reproduce for you here:

  • Material should be stored in a dry place. 
  • Porous back-up materials or wood forms should be waterproofed. Absorption of water can result in reduced flow for the product. 
  • Forms must be stout and water tight. 
  • This product is designed to be mixed with water and then poured/handcast into place. 
  • For best results, water should be maintained at 50-70F. 
  • Approximate Water For Installation: 55 lbs. to 5 pints of water. 
  • Mix for at least three minutes. 
  • For best results, wet mix temperature should be maintained at 60-75F. 
  • Minor adjustments to the amount of water are permissible to achieve desired flow. 
  • Do not exceed 11.0% water under any circumstances. 
  • Place material promptly. 
  • Do not trowel to slick finish. 
  • At temperatures above 60F, air cure, keeping surfaces damp and/or covered, for 16-24 hours typically or until a hard set has developed. Lower temperatures will increase the time before a hard set develops. The best results are achieved at curing temperatures of 90-110F. 
  • Keep material from freezing during air cure and preferably until a dryout can be initiated. Freezing of this product prior to water removal can cause structural damage. 
  • Never enclose a castable in a vapor-tight encasement as a dangerous steam explosion may result.

Typical dryout schedule for a single layer, 9” thick or less:

  • Ambient to 250F at 75F per hour. Hold at 250F 1/2 hour per inch thickness.
  • 250F to 500F at 75F per hour. Hold at 500F 1/2 hour per inch thickness.
  • 500F to 1000F at 75F per hour. Hold at 1000F 1/2 hour per inch thickness
  • 1000F to use temperature 75F per hour

I made a mold out of sheet metal for the inner and outer round surfaces, and plywood disks for the bottoms. The inner mold is held concentric with the outer mold by putting five or six two-inch pieces of wood around the circumference of the inner mold. As I mentioned in a previous episode, I soaked the wood in cooking spray, which was a convenient way to keep it from absorbing water.

The forces on the inner mold are going to be large when there’s eighty pounds of wet cement pushing on it, more than enough to collapse the flimsy sheet metal, so I filled the inner mold entirely with sand.  

I mixed up the cement by putting ten pints — just under five liters — of water in a watering can; this made sure that I did not accidentally put in too much water. I slowly added the water to the cement powder, stirring with a hoe. For easy cleanup, I mixed it in a bin lined with some scrap plastic sheeting.

I then scooped the cement into the mold and rammed it down with one of the wooden sections used to keep the molds concentric, going from one section to the next. I rammed it down pretty hard, and even still, there were a fair number of air bubbles in the finished product. This is not fatal, or even all that undesirable; air pockets are good insulators and lower the thermal mass of the furnace. The risk is that if water gets stuck in a pocket then it could expand and crack the furnace or cause spalling. Ram it a lot.

Once it was done I wrapped it up in plastic for a day while the hydrating reactions hardened the cement. Since the hardening reaction requires water it’s important that the edges not dry out too early.

Then I removed the molds, wrapped the whole thing up in a damp towel and more plastic, put a 60 watt light bulb inside, and left it for a week.

After that, I made some increasingly hot fires in the furnace. There was almost no visible steam at any point and no cracking, so I think I’ve got myself a furnace here.

Next time: however, some mistakes were made.

What If The Crucible Fails?

So, to briefly review, the furnace that I’m going to build is essentially a bucket made of refractory concrete. The bucket will contain charcoal and a crucible: a smaller removable vessle that contains the actual molten metal.

What could possibly go wrong? Metal crucibles can fail in their welded joints or, if made too thin, simply burn through. Ceramic crucibles can crack. Both can be dropped during removal. So as a safe operations consideration, we should figure out how to deal with the containment failure situation.

(Apropos of nothing in particular, I once had a dream where the NPR guy, you know the one, said “NPR news reporting is financially supported by containment. Containment: the property that allows some things to be kept inside other things. For more information, log on to” Apparently I listen to NPR too much.)

If the crucible fails then the bottom of the furnace is going to be full of molten aluminum with hunks of burning charcoal floating in it. Obviously it’s going to be hard to get it out while still molten, and even harder once it solidifies. The solution is to not get into that situation in the first place; the furnace needs an emergency drain. We can put a hole, say 2 or 3 cm in diameter, in the bottom.

This safety system of course will only work if the drain is not plugged on either end. On the interior end, it seems unwise to assume that the cracked crucible is going to float on the spilled aluminum; perhaps it is only cracked halfway up and still too heavy to float. The crucible will have to rest on some sort of grate or plinth that permits access to the drain plug.

That then of course naturally leads to the question of “where does the molten aluminum go from there?” We’ll need an emergency containment system of some sort under the furnace. A hole with a bucket’s worth of sand at the bottom would do, or a cast-iron pot. The furnace cannot simply rest on the ground. And we certainly do not want the possibility of spilled molten metal on a concrete or cement floor, for the reasons described in the previous episode.

Refractory cement

Last time we discussed how the furnace body material needs to have a low thermal conductivity, to ensure that temperature builds up inside the furnace; this has the nice additional property that the outside of the furnace remains relatively cool, at least in the non-steady state.

The material also needs to have small thermal expansion. Because the thermal conductivity is, by assumption, low, there will be a large thermal gradiant; that is, when it is fired up, there will be areas of the furnace body that are very hot, and areas that are relatively very cold; if the body expands significantly more in the hot sections more than the cold sections then we have a thermal shock scenario, which can fracture the furnace.

A substance which has these properties is said to be refractory; I’m going to make my furnace out of refractory cement.

What is so special about refractory cement? Why not use ordinary Portland cement, or even concrete?

Cement works by undergoing a chemical reaction in the presence of water that essentially causes it to crystalize. Doing so can trap considerable amounts of water in the body of the cement. Concrete is essentially a mixture of cement and hunks of rock. When these substances are used in a high-temperature application, the trapped water will attempt to vaporize and form a high-pressure steam; if the pressure gets high enough, cracks can form explosively. And concrete may also contain rocks that fracture under heat, which can make the situation worse. Do not use ordinary cement or concrete.

Refractory cement is typically ordinary Portland cement plus additional chemical additives which encourage far less water retention in the cement. But since the chemical reactions that make the cement crystalize in the first place are hydrating reactions, there’s got to be enough water in the cement throughout the curing process to ensure that it hardens throughout.

So, some important tips for casting refractory cement:

  • Cement is caustic when wet, presents an inhalation hazard when dry, and undergoes an exothermic (heat-producing) reaction when curing. Keep this in mind and use the appropriate safety gear.
  • Use the entire bag. As the bag was shaken on the truck from the factory, it might no longer be a consistent mixture of the necessary chemicals.
  • Mix in exactly the amount of water recommended by the manufacturer. When it is adequately mixed you should be able to make a snowball out of it and throw it without it either liquefying or crumbling as you do so.
  • If using a wooden mould, spray the mould surfaces that will get cement on them with cooking oil spray. This will discourage the cement from losing water too quickly into the wood as it cures.
  • Cement is extremely strong in compression but has poor tensile strength. Bend some steel coat hangers or rebar and use it as reinforcement inside the concrete, particularly in the lid.
  • Avoid the creation of air pockets; they will contain air which can also expand when heated and encourage cracks. Get a hammer drill and a piece of scrap wood. Use the vibration of the hammer drill against the wood to vibrate the surface of the cement; this will drive out the bubbles.
  • Do not wet the surface and trowel it smooth. Smooth it out with your (gloved) hands.

Once it is cast then the curing process begins. It is very important that the cement have the right water level as it cures; the cement should be very dry when the process is done, but it cannot dry too quickly otherwise the hydrating reactions that make it strong will not have time to take effect. Also, you don’t want to be in a situation where the surfaces have hardened so much that they are trapping lots of water inside.

  • For the first 24 hours, keep the exposed surfaces covered with plastic sheeting or damp rags; the edges will dry first but they need to stay damp so that they harden.
  • After the first 24 hours, put a 75 watt light bulb (lit!) inside the furnace. This will provide enough heat to slowly drive much of the excess water out of the cement. Leave it there for a week or so.
  • The last, and most crucial stage, is the calcining stage, when the last of the water is driven out and the final chemical reactions take place. Build a small fire in the furnace and bring it up to the metal-melting temperature over a period of many hours, the longer, the better.