The basic idea is a short chamber with a depression in the floor (the hearth) which fire is directed across. The flames strike the rear of the chamber (or a baffle) and bounce back into the chamber for another go. The exhaust vent (flue) is either on the sides, straight across from the intake, or even adjecent to it; there are many designs.
The heart of the matter is concentrating heat from the flames, and the longer the path they take, the more heat is radiated. At these temperatures, conduction has little effect; radiation takes over -- if it's not enclosed, it takes a lot more power to get it hotter than it does when surrounded with refractory. Now, that's the basic idea of any furnace but without a crucible to take up space or heat, it's more efficient. Only thing more efficient I would say is a cupola, but those aren't as easy to operate.
Reverberatory furnaces have melted everything from bronze, aluminum and glass to lead, iron, copper ore and have even made steel automatically. (See Henry Bessemer's autobiography, chapter 11. See also ch. 8 for the glass-melting furnace.) Here I'm just going for bronze type stuff. In the above photo (sorry for the quality, AGAIN) you can sort of see the lid gouged out in a 'T' shape, while the hearth is a hemispherical depression.
So let's try pouring this stuff. Here's a mold very similar to one of my first castings, I don't really need another (need? we don' need no steenkin' need) but it's a good small casting to test with. I measured and estimated I'll need about 500 grams (I know, metric? Well I don't have any imperial weights to balance against.. so just remember that 454g = 1lb), plus 100 or 200g for sprue etc.
Click to enlarge. Got the stuff out. You can (well, sorta) see these were originally spare chunks of refractory cast in milk jug ends. :) I hollowed them out (see above) to suit. Only reason I'm able to do this is the stuff sucks, being homemade. It's especially soft after being heated this hot, which does not indicate a long life for this thing. I need to grab some of that clay-like Plibrico pli-brick stuff and make a bigger one of these.
A shot alongside the burner. It's just starting to heat up in there.
A while later and it's melting. The metal inside is around 1800°F, while the refractory is up to maybe 2100°F. At a distance of a foot or two, your face gets warm pretty fast looking into it.
Closeup alongside the burner.
Metal's melted (except for a stubborn bit of copper that found its way into the charge), here we go! You might ask how I pour this, well I remove the lid with gloved hands (I love leather welding gloves, you can grab red-hot stuff with almost-bare hands ... for a moment!), grab the hearth carefully, and pour the metal out the gouge made for the burner. It all went into the mold, and even filled up the sprue! But there should be at least as much beneath as in the sprue.. so the sprue should be like, half full... uh oh...
Another not-too-good photo, but if you look closely around the sprue basin at the top of the mold, you can see the sand looks kinda charred around it. But not anywhere in the casting itself. :(
At least something came out.. You can kinda make out the brassy tone of the metal and the rough sandy surface. The right end is the sprue basin and a little of the casting. Left is the pouring basin and that chunk of copper I lamented about above (I just dumped the molten charge into the mold). Now, it looks solid, but after cutting it open it looks like someone opened a club soda in it! Can someone tell me why I'm getting so much gas?
Now actually, not everything has been a bust. Before this pour, I had tried the same thing and it almost actually worked:
But on breaking into the outer ring it looked more like drinking straw than a casting. Go figure.