I’m moving this content to merge into my regular blog; this will now be at
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.
As I also mentioned before, there was a design problem in that I re-routed the tuyere to come in the bottom of the furnace, which meant that if there was a crucible failure in the furnace, then the molten metal would head down the air pipe to the fan. Which happened. Fortunately the fan was blowing cool air and the melt froze in the air pipe. The remaining small amount of molten metal in the furnace could be easily scooped out with a long handled steel spoon, so no significant harm was done. I was going to be rebuilding the air pipe anyways.
I’ve done so. I’m still not very happy with the design, but it is better. Basically I have a T:
| =========== air input
|| <— aluminum foil plug that will melt
An immediate problem was that ash and coals are going to fall into the pipe and fill up the drain. I solved this problem by putting a pipe cap with twenty or so holes drilled in the sides on top of the air pipe. That certainly lets enough air in, and, bonus, directs it towards the charcoal surrounding the pipe. Whether it will let the metal out in the event of a crucible failure, I hope to not find out. I think it will.
I’ve been trying to come up with ideas for better designs for future furnaces; should there be multiple tuyeres to distribute the airflow around better? Should the floor of the furnace slope towards the drain? If I ever build another furnace I’ll experiment with these ideas.
We digress from our usual topic of me trying to figure out how to melt metal for a brief rant. As someone who owns an old house and likes “do it yourself” projects, I spend a lot of time in “big box” warehouse stores. I try my absolute best to interact with as few employees as possible when I go to these stores because it never seems to go well. Here are a few conversational highlights from over the years:
—— Holy trash bags, Batman ——
Me: Hi there, you probably don’t have these but Western Safety is closed today. Do you have large four or six mil tear-resistant trash bags?
Big Box Store Employee: I don’t think so; what do you need them for?
Me: I’m tearing up a hundred-year-old sub-floor and the test for asbestos contamination has come back positive. The toxic waste dump won’t take asbestos contaminated waste unless it is properly bagged and labelled.
BBSE: Well, I’d just bag it and throw it out in the regular trash and not tell anyone.
(I went to Western Safety.)
—— Circular is the round one ——
Me, speaking to the guy at the tool counter: Hi, I need an eight inch abrasive cutoff wheel suitable for cutting thin, soft steel with a chop saw or circular saw.
BBSE: You mean these?
Me: Those are reciprocating saw blades. Circular saw blades are circles.
BBSE: Oh, so you mean these?
Me: Those are ten inch wood cutting blades.
BBSE: Hmm. You mean these?
Me: Those are concrete cutting wheels.
BBSE: How about these?
Me: Those are metal cutting wheels but those are four inches wide. I need eight.
BBSE: Maybe you should try Lowes.
(I tried Ace, successfully.)
—— It’s not a nuclear reactor, it’ll come back online easily enough ——
Me, fifteen minutes before the store closes: Can I have this twelve foot board sawed into two six foot boards? I need two six foot shelves, and a twelve foot board won’t fit in my car.
BBSE: Sorry, the saw is already shut down for the night.
Me, speaking to the store manager 90 seconds later: I have a question for you: is it the policy of this store that the saw “shuts down for the night” at some time before closing?
Manager: Uh, no… who told you that?
Me: I think it was the guy who just vacuumed up the sawdust and doesn’t want to do it again.
Manager: I know just who you mean.
(They sawed my board but boy, were they not happy about it.)
—— That’s just smurfy ——
Me, talking to a guy in the electrical aisle: Can you tell me where to find one-inch diameter flexible electrical conduit? It is made of thin, ridged plastic and is sometimes called “smurf tube” because it’s that colour of blue.
BBSE: Sorry, we don’t carry anything like that.
As I turned to leave I realized that of course the smurf tube was directly behind me; the BBSE was looking at it as he was telling me he didn’t have it.
—— How useful! ——
Me, talking to a (different) guy at the tool counter: Where are the rivets?
BBSE: I’ve never heard that word before; what’s a rivet?
Me: A rivet is a metal fastener usually used to attach metal objects together. You insert the rivet through the objects you wish to fasten together and then deform one end of the rivet by peening it with a special tool. If you have access to both sides of the objects you can use solid rivets, otherwise you can use hollow rivets.
BBSE: Wow, that sure sounds useful!
(Another employee knew what rivets were, and, bonus, where in the store they were.)
—— Take a number ——
Now, I understand that the people hired at big box stores have no experience whatsoever using any product that they sell, and, as we’ve just seen, often no knowledge of what they sell in the first place. I know that if I want knowledgeable conversation about a tool with an expert I should go to Hardwick’s, which is like paradise for hardware geeks. The trouble is that they don’t have convenient hours; they’re closed by the time I get home from work, and not open Sundays. I try to go to local small-box stores as much as I can. Which is why this experience I had at my local small-business lumber yard yesterday was so disappointing:
Me: Hi there, I need three dozen eight foot two-by-fours and three sheets of quarter inch drywall.
Cashier standing by the front door: I think we have those.
Me: I’m quite sure that you do, since this is a lumber store. Are the two-bys and sheet rock in this building, or in the warehouse across the street?
Cashier: I don’t know. I think you’ll have to ask someone else.
Me: You don’t know where the two-by-fours are?
Cashier: This is only my fifth day on the job. Take a number and someone will help you.
I would have thought that “where are the two-by-fours” is the kind of thing you’d sort out on day one at the lumber store, but, whatever.
At this point I note that I am the only customer in the store. Behind the counter there are five employees. Three are talking amongst themselves. One is typing on a computer. One is on the phone. As instructed, I take a number, and walk over to the paint aisle to browse spray paint while I wait for one of the five people behind the counter to call my number.
They do so immediately. The moment my number is called, the three employees who were talking amongst themselves immediately leave the building by the back entrance, and the guy on the phone hangs up and leaves by the front entrance, leaving in the building me, the guy on the computer, and the cashier who does not know where the lumber store keeps their two by fours. I point out to the guy on the computer that my number has just been called, and he says that someone else will help me shortly.
I waited ten minutes watching him silently ignore me, typing away, and then I left and went to the big box store at the other end of town; I knew where the two-bys were there.
Attention small business owners: I am doing my best to give you my money. Stop making it so hard.
Attention big box store owners: You run vast multinational corporations with huge profits. You can afford to hire and/or train employees to familiarize them with the products you sell and their basic functions.
I emailed the last portion of this blog entry to the owner of the small business involved, and:
I really appreciate the opportunity to address such an egregious example of poor service. I won’t bore you with the details but it was a bad intersection of shift changes, yard service people hanging out at the counter and too few sales people. We watched the tape of your arrival and departure and have talked it over with everyone involved. Please let me tell you we’re embarrassed and ashamed of the way we treated you. Please accept my sincere apology.
The owner also offered me a discount on my next order and free delivery, which was I think a very nice gesture. As I have said often, you can tell the quality of customer service at an organization by how they deal with mistakes. Good service means recognizing the mistake, taking ownership of it, identifying the structural problem that allowed it to happen, and making a gesture of goodwill to the customer; this is an example of really excellent customer service, and I appreciate that very much.
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.
That chemical reaction can of course be driven in the other direction; it is possible to turn an oxide back into a pure metal by a variety of means. My favourite technique for doing so is to put a tarnished silver object loosely wrapped in aluminum foil in a plastic tub full of boiling water with baking soda dissolved in it. The silver oxide is reduced to metallic silver, and the aluminum is aggressively oxidized far more than it would be when exposed to gaseous oxygen. Essentially the oxygen is moving from the surface of the silver to the surface of the aluminum. The cheap aluminum can then be discarded. This technique is not only less work than polishing by hand, it saves the silver. (Conventional silver polish is simply a chemical which dissolves and removes the silver oxide, destroying the silver.)
Now, that description of polishing silver with chemistry should be making you scratch your head (provided that you do not remember your grade twelve chemistry): what’s the baking soda for?
It’s to help move electrons around; it make the water more conductive.
But wait, what do electrons have to do with it?
Oxidation is in these specific cases the reaction of a metal with oxygen to form an oxide; 18th century chemists knew that, and hence gave it the name “oxidation”. And those same 18th century chemists also observed that when oxidized metals were turned by chemical means back into pure metals, they appeared to lose mass, and were hence “reduced”. Of course the “lost” mass was nothing more than the oxygen leaving the metal and going into solution or into the atmosphere, which those same clever chemists did manage to work out for themselves as well.
We now know that the hallmark of an oxidation is that the oxidized substance must give up some electrons in order to bond with the oxygen, and the reduced substance must accept those electrons. The key fact for the chemist to understand is that the number of electrons is always conserved. If the oxidized substance is giving up, say, two electrons per oxidized molecule then some reduced substance must be accepting those two electrons. There is no oxidation without reduction. (*)
I said last time that what I wanted to talk about the air blast into the charcoal furnace. A charcoal fire is essentially oxidation of the fuel at a tremendous rate, and the thing oxidizing the fuel is in fact oxygen.
Suppose the air blast is set so high that the rate of oxygen arriving is outstripping the speed at which the oxidation reaction can occur. What characteristics will such a fire have?
First off, it will be incredibly hot; the heat-producing chemical reactions are happening as fast as they possibly can. Second, the atmosphere, despite having been reduced enormously by the oxidation of the fuel (remember, if something is oxidized then something else has to be reduced, and that would be the atmosphere) there will still be oxygen available for more oxidation of something else. Such as the metallic body of the crucible itself, or the metal in the melt. Third, there will be a strong flow of air out the chimney, carrying the heat with it. Fourth, the exhaust gasses will be extremely hot and quite “clean” for a charcoal fire, because more of the combustion will be “complete” combustion.
Now suppose the rate of oxygen arriving is just barely enough to run the oxidation reaction, or even slightly less. What characteristics does such a fire have?
It will be less hot because the chemical reactions are not running at peak speed. The atmosphere will be entirely reduced, leaving no oxygen to oxidize the crucible or melt. In fact, the atmosphere could be so reducing that it starts pulling oxygen out of the crucible and melt, unrusting them. The airflow will be lessened, and the smoke is likely to be dirtier.
What I think will work, and I’ve been trying to do, is to run the furnace so that the air blast produces a reducing environment until the metal melts. Once I’ve melted as much as I want, I turn the air blast up to rapidly increase the temperature from the melting temperature of 1220F to the pouring temperature of 1400F.
When it achieves this temperature the melt should be glowing with a red heat, and it will not stick to a steel rod used to stir the melt.
Lacking a pyrometer, it’s going to take some practice to figure out exactly what the right moment to pour is. I’ll continue to report my results as I practice.
(*) There can be oxidation without oxygen; perhaps the oxidized substance is giving up its electrons in order to get together with sulphur, rather than oxygen.
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.
Thus far I’ve been using a cheap 4 inch diameter stainless steel tub made of pretty flimsy steel. Though it has almost no thermal mass and therefore heats up red hot very quickly, the thin steel will (1) be dissolved by the molten aluminum on the inside, and (2) will oxidize on the outside, and will eventually fail. I’ve therefore obtained a 3-inch inner diameter black steel pipe nipple and pipe cap from Ballard Hardware. It needs some modifications before it will be a useful crucible; more on that later. I wish I had obtained the crucible first, because then I would have chosen the larger 4-inch inner diameter nipple, which does not fit well in the bore.
My fourth mistake was one of operation, and was very simple to fix: I did not use nearly enough charcoal the first couple of times I tried to melt aluminum. Once I started putting in a good four inches or so in the bottom, and more on the sides, it melted nicely.
My fifth mistake was also one of operation: when I went to go for my first pour, when the metal actually melted, I got too excited and poured too soon. I was not molding anything other than ingots in a muffin tin; the metal froze in the crucible when halfway poured into the muffin tin. The combination of pouring too soon and having not enough fuel was not good.
It is somewhat dangerous to have a mass of solid metal in the crucible, because when it heats up again, the metal will expand and possibly break the crucible. Fortunately, after I allowed it a day to cool down, the metal came out of the bottom pretty easily in a big lump. Of course, I will melt it again. One of the truly nice things about metal casting work, as opposed to, say, fine woodworking, is that you take only a small loss of materials for mistakes.
The problem of getting the metal up to pouring temperature is, once there is enough actual charcoal in there, essentially becomes an oxygen supply problem. More on that next time.
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:
- In a propane furnace, the crucible is raised upon a refractory cement block called a plinth. There is a hole in the side of the furnace, called the tuyere. The tuyere is angled tangentially so that the burning propane swirls entirely around the air gap between the crucible and the interior “hot face” of the furnace. If you imagine the crucible and the bore as two concentric circles, and a third concentric circle whose radius is the average of the other two, the ideal tuyere angle is tangent to that middle circle. Blasting straight at the crucible heats up one side far more than the other.
- In a charcoal furnace, the crucible sits directly on the burning fuel, and is furthermore surrounded by more fuel. The air blast enters through the tuyere at a point below the crucible, and blasts directly towards not the crucible, but towards as much fuel as possible. The goal is to raise the temperature uniformly throughout the fuel, which will then heat the crucible uniformly. The ideal tuyere placement as far as symmetrical heating is concerned is directly below the crucible, coming up through the floor of the furnace. Second best placement has the tuyere coming directly in the side of the furnace, pointing towards the fuel. The tuyere must not blow directly on the crucible (because that is actually cooling it down), and must not blow tangentially (because that produces uneven heat.)
As I looked at photos on the internet of various different furnaces I misunderstood this key difference between charcoal and propane furnaces, and constructed my charcoal furnace with a tangential tuyere. And, unsurprisingly, it did not heat up at all evenly when I tested it.
The second mistake was one of construction; the pipe which feeds the air blast into the tuyere should fit the hole snugly, but be removable. I accidentally froze the pipe into place in the concrete, angled in the wrong direction. Whoops!
Fortunately these mistakes were easy to fix. A hacksaw removed the badly placed pipe. (Though it took a while; I have since purchased an angle grinder, which makes short work of cut-off jobs.) I had already made a hole in the bottom of the furnace to act as an emergency drain; fortunately it was exactly the same size as the 1” inner diameter black steel pipe nipple I was using to deliver the air blast. A screw-on flange ensures that it will not fall out the bottom, and a right-angle bend leads it out to the waiting air hose.
This solution works well; I was (eventually!) able to easily melt aluminum with this setup. However, this produces two additional problems.
The first additional problem is a safety concern; my emergency drain now leads directly to a plastic air hose. If there is a loss of containment the drain will shoot molten metal out the pipe into the hose which will then melt. I will be making some modifications to this system so that the drain “tees” off, so that gravity will take the melt down into a waiting steel container rather than flowing down the air hose.
The second additional problem is the fact that obviously the furnace now cannot sit upon the ground, as there is a pipe sticking out the bottom of it. For now, I have it sitting on a convenient antique steel table saw that I rescued from the side of the road many years ago. Eventually when I build the hand truck to move the furnace around I’ll incorporate some legs to keep the furnace up off the ground.
Next time: more design and operational mistakes
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.
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.
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.
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.