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.

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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 www.containment.com/npr.” 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.

How hot will the exterior get?

Before getting into the material science of cement, let’s take a step back here. What’s the fundamental nature of the furnace? That is, what are the simplest, most fundamental parts and functions of a metal-melting furnace? You need:

  • A source of heat. In this case, fire.
  • An enclosure around the heat with low thermal conductivity. That is, some way of preventing the heat from escaping. If heat is being produced faster than it can escape then by definition the temperature is going up.

That’s it; a furnace is an enclosure with a fire in it. It could be a hole in the ground, and of course, for many centuries, a hole in the ground with a fire in it was perfectly adequate. The area surrounding my summer cottage in Ontario is dotted with the ruins of 19th century lime kilns, which were basically holes in the ground in which limestone was turned into quicklime by the application of intense heat.

So what we need is a substance that first, has a low thermal conductivity; that is, heat energy moves slowly through it, and therefore builds up the temperature on the hot side.

The thermal conductance is the quantity of heat energy that is emitted by a unit area in a unit time (and energy per time is of course power) when there is a unit temperature difference between the two sides separated by a unit thickness of material. A typical refractory cement will transmit on the order of 2 joules of heat energy per second (so, two watts of power) through a square meter of cement if the cement is 1 cm thick and the temperature difference between the sides is one degree Fahrenheit.  

In our case, the furnace will have a total surface area of about 0.4 square metres, be about 5 cm thick, and have a temperature delta of about 1350°F. Multiply that all out and that’s about 5400 watts, which seems like rather a lot! That’s the surface of the furnace emitting the same heat as that of eleven 500W shop lights, which is more than I’d like to touch, even wearing gloves.

But we’ve forgotten something important in this back-of-the-envelope calculation: the thermal conductance that we’ve just worked out is the steady state. That is, if we left the furnace running indefinitely, burning charcoal such that the interior was exactly 1400°F, then of course eventually the exterior would get quite hot as it attempted to radiate that heat out into the air. But we’re not going to stoke the furnace with fresh charcoal indefinitely; it’ll typically run for less than half an hour. Also we have completely neglected the fact that a huge amount of heat is going to leave via the top when it is opened. And that when the crucible full of molten metal is removed, all of that heat disappears from the furnace.

Actually solving analytically how hot the surface of the furnace will get is more calculus than I’d care to do; it has been a long time since my 3A term Partial Differential Equations One, and we only solved the Heat Equation in one dimension. I suppose I could write a program to simulate it; all you’d need to know is the specific heat of all the parts (that is, how much energy a unit volume of a substance possesses at a given temperature), and then use a combination of conservation of energy and Fourier’s rule for heat transfer to work it out.

I seem to have strayed somewhat from my original topic, which was what properties the body of the furnace needs to have. More on that next time.

Avoid Galvanized Metals

As I said earlier, one of my primary concerns in this adventure will be ensuring that the foundry is safe by design. While researching possible ways to form furnaces out of refractory cement, I’ve seen a great many designs on the internet that use galvanized sheet metal and pipes as furnace parts. For example, this furnace looks absolutely gorgeous and I’m sure it produces great results, but the amount of galvanized metal on that thing concerns me.

Galvanization is a rustproofing technique whereby a steel object is dipped in molten zinc, forming a thin layer of zinc-steel alloy on the surface. Though this certainly does resist corrosion, the zinc alloy will vaporize into gaseous zinc oxide at about 400°F. Effects of zinc oxide inhalation include, as Wikipedia helpfully points out, fever, chills, nausea, headache, fatigue, muscle aches, joint pains, shortness of breath, chest pain, burning sensations, shock, collapse, convulsions, yellow eyes, rash, vomiting, diarrhea and low blood pressure.

None of that sounds like a fabulous adventure.

Now, you might reasonably point out that the galvanized portions of the sorts of “steel bucket furnace” build are all on the external surface (the “cold face”) of the furnace, which typically will not get even close to 400°F. To which I would respond that the key word there is “typically”. If the furnace runs too hot for too long, or if something near the furnace catches on fire, or if molten metal is spilled onto the zincky (*) surface, or if you accidentally put a piece of magnesium into the furnace thinking it is aluminum and it starts burning uncontrollably, then odds are pretty good that some zinc will vaporize at exactly the moment when you have some larger disaster to manage. (And in that last case, you have the problem of inhaling magnesium oxide, which is just as bad; more on magnesium in a later episode.)

You might also reasonably point out that the furnace will already be vaporizing all kinds of nasty stuff; many impurities in the scrap will go up in smoke. We are going to need to ensure that the furnace is operated in a well-ventilated space. I would respond to that by saying let’s not make a bad situation worse if we can avoid it.

Therefore I’m planning on avoiding galvanized metals entirely. In fact, it is not clear to me why the exterior of the furnace needs to be faced in any metal at all. Why should the cold side not be plain refractory cement, as the hot side is? 

(*) Good Scrabble word there.  

Furnace Design Considerations

A foundry is an operation that does two things. It first melts metal in a furnace and second casts the molten metal in a mold, forming some useful shape as it solidifies. There are a number of design factors when making a backyard foundry:

What kinds of metal are to be cast? 

The kinds of metals you can cast is primarily a function of the heat of the furnace. Melting points of some familiar metals are:

  • Pewter (an alloy of tin and other stuff): about 400°F
  • Tin: 450°F
  • Lead: 621°F
  • Zinc: 787°F
  • Aluminum: 1220°F
  • Brass (an alloy of copper and zinc): about 1700°F
  • Bronze (an alloy of copper and tin): 1742°F
  • Silver: 1763°F
  • Gold: 1984°F
  • Copper: 1984°F
  • Iron: 2800°F

So as you can see, pewter, tin and lead are pretty easy to melt — heck, you could melt them in your kitchen stove — but also tend to make things that are pretty soft. Zinc is mostly used in alloys, and as we will see in the next episode, is insanely dangerous. Gold and silver are precious; I’m not interested in jewelry making here. Copper needs to get very hot, and iron is a whole other ballpark. That’s out of my league.

But melting aluminum, brass and bronze seems to be within reach, and these metals are hard enough to build tools yet soft enough to machine with easily-available steel bits. We can melt aluminum with ordinary charcoal or propane.

How heavy is the furnace?

I intend this furnace to be portable; I’ll store it in my detached garage when not in use, and run it in the back yard. This means that I’ll need to be able to move it, either by lifting it, or with a hand truck.

Let’s suppose that a furnace is a cylinder that is partially empty and partially full of cement. And let’s suppose, to make estimation easier, that the cylinder is as tall as it is wide. About two thirds of the volume of the cylinder will be full of cement, and about one third will be empty. Cement is approximately three times as dense as water. So the weight of a cylindrical furnace is twice the weight of the same cylinder full of water. Water weighs one tonnes per cubic metre, and the volume of a cylinder is pi times the height times the radius squared; the height, we’ve already said, is equal to the diameter.

So roughly we can expect that a furnace weighs:

  • 12kg if it is 20 cm in diameter – a paint can
  • 24kg if it is 25 cm in diameter – a normal bucket
  • 41kg if it is 30 cm in diameter – large bucket
  • 66kg if it is 35 cm in diameter – a small trash can
  • 100kg if it is 40 cm in diameter – wait, 100 kg is way too heavy

(American readers who do not yet know the metric system: a kg is about 2 lbs. 30 cm is about a foot.)

These numbers are rough, but they seem plausible.

How much metal can we melt at a time?

Molten metal resolidifies surprisingly quickly, so you want to make more than you need for a given casting. You don’t ever want to make a partial casting, then go back and melt more metal. So how much we make at a time is important.

We said that the interior of the furnace is about a third of its total volume. Let’s suppose that the size of the crucible – the removable container that holds the molten metal – is about half the remaining volume, and that we’re going to fill it two-thirds full of molten metal. So that’s one-ninth the total volume of the furnace devoted to the actual metal, which will be slightly less dense than the concrete.  

Heck, let’s just make the math easy. The maximum mass of metal we can melt will be on the order of a tenth the mass of the furnace. So maybe one kilogram for the smallest furnace and maybe four kilograms for the “large bucket” furnace.

What’s the fuel?

The two basic fuels for backyard metalcasting are charcoal and propane. Charcoal has, of course, been used as metal casting fuel for six thousand years; propane, somewhat less. Both are inexpensive and widely available. Propane furnaces apparently require a custom-built burner apparatus and pressure regulator. (The valve on a barbecue tank regulates rate of flow, not pressure.)

There are of course safety issues to consider: propane is a highly flammable gas delivered at liquification pressure which will explode in a huge fireball of vaporized accelerant if heated excessively. Bags of charcoal, by contrast, seldom explode.

Charcoal does present two inconveniences: it produces ash, which could get impurities into the molten metal (where they will float) and is generaly irritating to clean up, and second, the combustion chamber must be rather larger to accomodate the bulkiness of the fuel.

However, choice of fuel actually is one of the criteria that impacts the design the least, because both fuels require basically the same furnace design. That is, there must be a pipe at the bottom of the furnace that admits either blown air, in the case of a charcoal burner, or the propane. It should therefore be relatively easy to change fuels if necessary.

Summing up

A 30 cm diameter cylindrical furnace will weigh about 40 kg, be easily moved with a hand truck, take up as much space as a large bucket, and enable melting of several kg of aluminum at a time. The interior chamber will be plenty large enough for charcoal and a good sized crucible.

Next time: some thoughts on furnace materials.

Safety Third

“Safety third”, is what they say at Burning Man — when I naively asked what was first and second, the answer was of course obvious: having fun, and looking good doing it.

Worse, there was once a sign up in the kitchen of a cafeteria I used to frequent; it listed the top ten rules for cafeteria employees. Things like “Treat the customers and your coworkers with respect always”, and “Actively look for small problems and fix them before they become serious problems”, and so on. ”Put safety first” was, of course, number seven.

As I work on the design and implementation of a backyard aluminum foundry I’m going to think a lot about how to make its operation safe by design. Melting aluminum in your backyard is hardly a new idea; the internet has dozens or hundreds of web sites about people who have done so. It is astounding to me how unsafe many of those backyard operations are, in both design and operation. In the last couple of days as I’ve been researching this topic I’ve seen videos of people pouring molten aluminum wearing shorts, a t-shirt and sandals. I’m not suggesting that you need to obtain a silver suit to melt aluminum, but I personally would not want a 1400°F puddle of molten aluminum splashing onto the floor beside my sandaled feet.