On the BYMC forum, someone asked me how I came up with the reverberatory furnace. I don't know just how many reverbs there are out in the ameteur world, or even direct-melting hearth style furnaces that just haven't been connected with the word "reverberatory". But I at least seem to be one of the first people to build one and document it online. So, I figured I'd type a detailed story for this guy, of how it came to be. Since it's a long story all told, I thought I'd edit it a bit and post it online here. Since I decided to post it here, it became a long story. And since it's a long story, I'm posting it here. ... Funny how that circular logic works when you're typing away...
If I end up being an interesting or popular person I suppose this page will find itself alongside other stories of mine. I suppose I should write up a whole bunch of stories, but then again with my hit counter as lazy as it is, who cares. :-p
The genesis of the reverb design was probably melting bits with a small propane pencil torch, the kind plumbers use these days.
So, I thought, well I want to burn stuff. So I took a chunk of sandy clay, formed it into a firebrick with a depression in the middle and dried it. This was around the time I made my first Furnace Mk. 1, but I didn't connect the two and think "oh, I want to make a teeny crucible furnace yeah!". I just wanted to melt bits, on a firebrick, so I made that.
So I pointed the torch at it. Then I put another piece of firebrick on top, since obviously it's giving off a lot of heat that I want to keep inside! I found I could barely melt a few copper pennies after about a half an hour.
This was probably 5 years ago, some time after I discovered that our carpet is molten-copper resistant. (A segway from electrical experiments; I was using my bench supply and some pencil leads on that one. On contact, the carpet vaporizes, makes a slight amount of flame, a horrid smell, and leaves a black hole in the carpet. Not much else. I'd be pretty confident with this carpet in event of fire.)
Anyways, once I got a real propane tank and regulator, I built the 3/4" Reil burner and started torching things better. This includes using a larger hollowed-out firebrick, and developing other things, like alloying (at this time I had an idea of what things go into bronze: I knew better than to mix, say, aluminum and copper, unless I'm out to make a copper master alloy, which in fact I did in this "furnace"). I also made a smaller brazing/light heating torch, using a piece of 1/2" conduit, a 0.023" MIG tip and no choke (well, until the flare all but burned away, reducing backpressure and leaning the mixture, forcing me to hold my hand around the intake at all times).
One of the things I torched was a maybe 1 pound capacity hollowed-out-firebrick-reverb. I melted a whole slab of bronze, alloyed to 85-5-5-5 from copper wire, 50/50 solder and zinc pennies, with this reverb. I even tried pouring an ingot or two, and I forget if I made a casting with it. (Direct melting bronze appears to suck: tons of gas, especially since I didn't use any sort of cover flux, only borax dropped on with a coathanger stirrer.) But this is all really crappy. I thought, gee, if I want to melt anything serious (like pounds of aluminum) in something like this, I'm going to have to go out and *make* something good. So I went over to the refractory supply and grabbed a sack of lightweight insulating castable, 2600°F rated (Plibrico LWI-26 to be exact) and started designing.
Now one thing you do in design is check out prior work, as much for inspiration as for laziness (to avoid reinventing the wheel). (Not that I think of that as a caution; I prefer to understand every iota of the subject and personally redesign it inside-out to see why it was designed that way in the first place.) Before all this started I already had a vague idea of the term "reverberatory furnace", having seen encyclopedia diagrams of open-hearth furnaces (as well as anything else that goes into making steel). But the nuts and bolts of anything are never discussed or shown in detail as far as a designer is concerned. So I turned to the great resource in the sky, the internet, which I've been using as a resource since I started (my love of casting began when, one day, I searched for "metal casting"). I didn't find much information, but combined with my "research" (puttering) I was able to decide on a design.
I used most of a 50 pound bag of refractory building the first real one. This first reverb furnace took about an hour to melt a full hearth, 6 pounds of aluminum. I've melted and poured a pound or two of bronze in it, but it gets terrible results, again due to gas. I've never tried using a cover flux or a specialty alloy such as silicon bronze (since I have silicon now, I suppose I could try it!). Aluminum works perfectly, because -- as long as you don't stir it -- the oxide layer seals it tightly.
The slow melt times are due to two things: the steep hearth is only a little wider than it is deep. Just imagine it: you have heat only on the top, cold on the five remaining sides. You have an area of like 4 x 6" on top for heating, and that heat needs to be enough to reach down 3" -- almost as deep as it is wide -- through solid, melting aluminum. The other problem is the burner angle: I set it level horizontal, with the intention that the roof section would reflect heat down into the hearth. It didn't, because I didn't get the flues as I had hoped or imagined.
Both of these problems were rectified in the second, larger reverb furnace. In designing this one, my first plan was to angle the burner into the hearth to guarantee strong heating. In fact, when empty, the hearth has remarkably even heat, althrough the roof is colder, because no flames reach it directly. Even so, it's a good improvement over the first, which had a huge gaping cold spot just below the burner -- note that's a right angle and drop down, so it has something of a "flame vacuum" there.
In designing, I wanted to maximize volume. The most efficient shape for volume vs. surface area is a sphere, but that's really crappy to build, so, I'll live with a cube. Having completed Calculus 1, I had the mathematical method to *prove* that a cube has the least surface area. But I have to throw that out the window because I already know that a cube is going to royally *suck* for melting. Not enough top surface area. In the end, I went for a flat sort of layout: the same 3" depth, but with the side walls much farther apart. There's more floor area, so technically losses are worse, but per pound, it melts much faster.
There was also the choice of scaling up the walls. On the first I went for 2", which is pretty reasonable for small furnaces of the 6 pound size. On this, however, it was looking to turn out about twice as wide, so certainly it's going to take more of a bag of refractory, and two bags wouldn't hurt (indeed, I used I think 60 pounds of stuff in it). I could go for a wider design, using more thickness, increasing efficiency still further. Or I could push capacity up towards a gallon...
Between refractory use and corner-smoothing losses (square corners concentrate stress and suck heat), it turns out I can hold pretty well 15 pounds, filled to the gills. This is remarkably close to the weight of my 3' bed casting, which was my reason for building this furnace. (I started drawing plans for building my own Gingery-inspired lathe* before I decided to build a larger furnace.)
*I wanted to build a lathe completely from scratch, inspired only by pictures and designed with lots of intuitive hand waving, but unfortunately someone spoiled my fun :P by interpreting it as a pitiable situation and sent me the Gingery book set (for free :) ). Which reminds me, that rig is still rusting down there in the basement.. :(
Hmm. Since I haven't build another reverb furnace since (I've built a crucible furnace, but no reverbs), I guess that brings us to the current date. Enjoy!