How I Made My New Aluminum Melting Furnace
After much research, a little spending, and an afternoon's labor, I finally finished my new furnace. This one is made of durable, high temperature materials, and I expect it to last for many years of use. I'll explain below what was wrong with my previous one, and what I did to make my new one better. I am extremely happy with how it turned out. If you have more questions about anything, feel free to email me.
First I am going to talk about what went wrong with my first furnace. There are two reasons that my furnace didn't work well. The first reason is that I didn't do any research into refractory materials. The second reason is that I didn't even use materials that work as refractory. I used a mixture of Plaster of Paris and sand. Plaster of Paris isn't meant to be used in high temp applications, so after multiple firings it started to crumble. It is also very brittle. The lid I had made broke into pieces after it got hot a couple times. Another problem with my old refractory is that it didn't insulate very well. Instead of the fire heating up the aluminum in the crucible, it instead heated up the walls of the furnace. All the heat was being used to warm up the walls, not to melt the aluminum. This made for a furnace that burned a ton of coal, and took forever to get up to temperature. Here is an look at what my furnace was like before I replaced it. Notice the cracks on the sides.
Because I wanted my furnace to actually work, I started researching what makes a good furnace lining. I spent a lot of time on Alloy Avenue trying to figure out what everybody else does. The general consensus was that it is best to have a insulative material lining the furnace, covered with a hard, protective hot face. I decided to use ceramic blanket as the insulation, and then cover that with refractory cement. I bought a 31"x 24"x 1" ceramic blanket from Amazon, which I calculated would be enough for two layers lining the furnace, 1 layer for the bottom, and 1 layer for the lid. The blanket cost $50. For the refractory cement, I bough Meecos Red Devil Castable refractory from Menards. It cost $30.
My first step was to remove my previous lining. I made quick work of it with a steel pipe. Then I was just left with an empty bucket with a hole in it.
One thing to note about working with ceramic wool is that it can be dangerous to breathe in the particles that it releases. It can also be irritating to the skin. As a precaution, I decided to wear a dust mask and gloves when working with it.
Since my bucket was 10 inches tall, I started by cutting out a 31" x 10" rectangle that would form the first layer of the lining. Unfortunately, my bucket is smaller at the bottom than the top. After spending a good while trying to figure out how to cut it to fit better, I decided to cut out triangles from each side so the bottom would be the right diameter. It then fit perfectly.
I then started work on the second layer of the lining. I measured the inside diameter, and then cut the ceramic blanket out to the appropriate size. Once again, I cut the triangles from the sides so that it would fit well. I also measured and cut out a piece to place in the bottom.
One additional thing I wanted this furnace to do was circulate the air inside instead of blasting the charcoal directly. To do this, I wanted to aim the air pipe so that it blew air at an angle, instead of straight in. But first I had to cut the hole for the pipe. I found the easiest way to do this was to take the saw blade off a coping saw, push it through the two layers of blanket, and then start sawing a hole out. I then cleaned up the hole and widened it as necessary to fit the pipe.
The lid was next. I first cut out a square piece of blanket that was large enough to cover the opening of the furnace. This used up almost the rest of my blanket. I decided to use a piece of sheet metal to attach the blanket to. My idea was to secure the blanket to the metal with some wire. I drilled holes around the outside for the wire, and 4 holes for the handles. I just used some wire for the handles. I then cut an aluminum can sized hole in the sheet metal and the blanket. After placing the blanket on the metal and lining the holes up, I secured it with wire that I strategically wove through the sheet metal. Here is the resulting lid.
All the assembly was now done. The only thing left to was cover everything with refractory cement. Well, everything that was going to be in direct contact with the fire. The lid and the inside of the furnace. Coating everything with refractory cement actually has a couple of purposes. First, it keeps the ceramic blanket from being damaged. It protects it from being damaged by poking and general wear and tear. The second thing it does is keep ceramic fibers from becoming loose and airborne. Like I mentioned earlier, ceramic fibers are dangerous to breathe in. With the cement in place, I don't have to worry about wearing a dust mask every time I want to melt something.
The instructions for mixing the cement were very specific. You had to mix a certain amount of water depending upon how many pounds of cement you were using. Also, you had to mix for at least 3 minutes, but not more than 6 minutes. I had no clue how much I needed to cover everything, so I just started with 5 pounds of cement. After mixing the prescribed amount of water, I started spreading it on the inside of the furnace. The cement was a lot courser than I thought it would be, which made it a little difficult to spread. 5 pounds wasn't quite enough to cover everything, so I mixed up another batch. The second batch was enough to successfully cover the entire furnace, except the lid. After mixing up my third batch, I covered the lid with a nice layer. The only thing I had to do then was wait for it to cure, which supposedly takes 24-48 hours depending on the humidity.
After less than 24 hours (because I couldn't wait), I fired it up for the first time. It heated up quickly, and the cement was holding firmly without cracking. I melted down some aluminum, and everything was just dandy. However, I wasn't able to use the lid most of time. The raw aluminum I was melting (aluminum gutters) were too tall. After I was finished melting aluminum, I decided that I wanted to see what this new furnace was capable of. I had tried, but failed to melt brass in my previous design. So I found some brass, threw it in a crucible, and then placed it in the furnace. I then put the lid on and cranked up the heat! After a couple of minutes, the crucible was red hot. Then I peeked in, and I saw the brass had melted! The melting temperature of brass is 1,700°. Next time I am going to try to melt copper (2000°). I'll let you know how that goes.