I built a simple metal melting furnace that uses propane. My burner is basically a reil burner that uses no blower or anything. The end pipe is 1 1/2 inches in diameter. My furnace was only good for a few aluminum melts, before the furnace cement I used literally melted.
The furnace cement I was using was refractory furnace cement bought at a home depot. It's rating was 1800F. I mix that with perlite in a 1 to 4 ratio. Apparently, my furnace is getting well above 1800F. The furnace cement is rather expensive, and I'm somewhat dissatisfied with it's performance.
My burner is a bit excessive, does anyone know if passive air burners scale down? Right now, my burner has a hole drilled in it about 1mm, where propane rushes out. Can I make the burner smaller(an inch in diameter), and say use a size #80 bit(.3429mm/.0135")? It's using a lot of propane, and I don't need as much heat as it gives out. Turning down the propane pressure causes the flame to fall back into the pipe and burn at the nozzle, which causes it to make a sputtering popping noise instead of an air rushing noise.
Also, I've looked into refractory cement recipies. Portland cement, silica sand, fireclay, and perlite. Doesn't portland cement store water? Will it crack/explode? Are there any other similar recipies that are reliable?
I know for sure the Mike Porter burner can. I built his 1/2" pipe burner and it works great. I just use it freehand for heating, but it would work fine in a small furnace or forge.
Mike Porter's book is titled something like Propane Burners for Forges, Foundries and Furnaces.
Portland cement does not stand up to forge/foundry temperatures. That's how cement is made, by heating rocks in a rotary kiln.
The cheap furnace cement is typically sodium silicate with clay filler. It worked OK for me with the perlite in a "coffee can foundry" (see
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for some photos), but it won't last. The temperature cycling and cycling of humidity make it weak after a few uses. If you're just experimenting and don't plan to use it more than a few times, then that's OK.
If you can find the sodium silicate solution you could make your own cement paste with ground kitty litter clay and perlite. But for the effort involved I would suggest buying some commercial refractory product and thus getting something that will last.
How big is the furnace? A burner that size is fit for 10-15" i.d.!
No wonder! 8-)
Yeah, listen here... 1800 is like yellow hot. True yellow hot is 2000°F, the melting point of pure copper, but 1800 will look yellow to you (2000°F is much brighter, trust me you'll figure out all this temp-by-sight stuff eventually). Well anyway, that's like nothing. . .
. . .And I always call perlite a flux! It melts at "only" 2300°F, which is more than plenty for aluminum, and iffy when melting bronze.
FWIW, portland cement is *also* a flux, but considering your furnace cement is somehow rated at a mere 1800°F, there's no amount of cement that wouldn't do better than that crap you bought!
Something like 1 part portland, 1 part fireclay (or kaolin or a refractory ball clay, if you go to a pottery store/supply), 1 part (masonry) sand (or better yet, 20, 30 or 60 mesh kyanite) and 5 parts perlite should work well. I'm just pulling that out of my butt, but if it holds together, it'll work (such is the nature of these things we do!).
Ouch! My medium burner (check ABYMC.com, propane burner article) uses a
0.030" diameter jet into a 3/4" water pipe with moderate reducer on the end. It has a usefully wide range of mixture, going from rich (longer greenish flame; no yellow, too many holes in the reducer ;) to neutral blue to lean, thin purple (not nearly so thin that it blows itself out either). It's also an extremely stable burner, from 1/2PSI to 35PSI with no trouble!
1" water pipe (as opposed to actual-size 1" tubing!) will be best served with a #57 or so. That's what my large burner uses, and it melts four pounds of bronze from cold to bright yellow heat (approx. 2100-2200°F) in half an hour.
For a kiln I'm building, I'm going to assemble two 1/2" pipe burners and tune them for stability from at least 1PSI, hopefully 1/4 or 1/2PSI, with wide range of mixture.
To answer the inevitable question, here's a picture waiting for more kaowool:
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I could fire a five gallon pail when it's done. It'd be a little "well done" around the edges though.
Yep, unstable. Work with the design, you'll find - like my medium burner - something very stable that won't flash back until nice, quiet and relaxed pressures.
Yes, yes and yes. All cements hold water, heck all refractory products will adsorb water which can cause spalling (exploding) and cracks. That's why you apply heat slowly the first time. And if your furnace gets left out in the rain... take it up slowly!
Same thing happens to sidewalks as a matter of fact, but that has to do with water expanding on freezing instead of boiling.
Back to refractory: the best stuff contains things that: shrink on first firing; things which *expand* on first firing to counter the shrinking stuff; things which expand and shrink very little with temperature; and some sort of binder, either drying, chemical or air set, or hydraulic.
Portland cement is a hydraulic cement: just add water. It sets without air, as opposed to lime mortar or a water glass formulation, which needs air (rather the carbon dioxide in it) to set. The downside of portland is it breaks down on heating. This is overcome in calcium aluminate cements, which are what commercial refractories use. This stuff is more sensitive and expensive (a friend in Ohio says he can get it in $30/90lbs. bags, on special order since it has a two-day shelf life!), but *will* hold until the whole product melts. It also sets faster, typically one to two days for complete set (compare that to a month for portland!).
If you can get some calcium aluminate cement, toss it in some sand, clay, kyanite, grog and anything insulating (perlite is okay, but melts easy; something porous yet high-melting), add water and you're set.
If you don't mind burning something out, you can add saw dust (sift it through window screen, or even something finer) to provide air spaces and insulation.
Tim
-- Deep Friar: a very philosophical monk. Website:
The diameter of my furnace is about 6 inches. 10-15 in would be best for my current burner? Wow.
I just replaced the propane nozzle cap inside, which had 1/16in hole in it. I drilled a 1mm hole into a new cap and put it in. The thing sucks in WAY too much air now, adding a choke plate helped. The burner works well now with lower pressure, but starts to blow back at ~1psi. I think I should still scale it down some and perhaps add a flare at the end. The choke plate allows the flame to range anywhere from bright yellowish/orangeish to bright blue. If the mixture is very rich it's kind of orangeish, and looks like it wants to go out. It's also kind of loud. If I close the choke a little, the entire furnace will be blue. Is this what I want? Closing it too much and the flame gets kind of loopy.
The flare is for testing outside the furnace, if the furnace's tuyere fits tightly to the pipe it serves as a flare and holds the flame at the end of the burner pipe. If it doesn't fit well, you'll need a flare.
Any soot on the rich side?
Bright blue is neutral. Thin purple is lean. Mind any orange coming from hot surfaces like the furnace wall or flare.
You want the flame to be burning hottest inside the furnace, plus a little fire out the top. Too much fire out the top (long, rich flame) and most of the heat is being wasted. Too short a flame (so it ends inside the furnace) and all the heat is being spent inside the furnace, but not enough and especially the top will be cold. This is usually due to leakage, air being drawn in around the flame, which also cools it down.
In general a lean flame is faster because temperature is higher. If you have enough gas flow, it'll fill the furnace and leave maybe 4 to 6" flame coming out the vent (as seen in darkness).
Tim
-- Deep Friar: a very philosophical monk. Website:
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