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
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?
Thanks a lot.
1800F (982C) is not really sufficient for a furnace lining material. I
would suggest you try to locate a refractory material supplier near you
and get some proper refractory castable. In my case I have gotten
castable suitable rated to 1650C (3002F) for about £12 ($21) / 25kg and
insulating castable rated to 1400C (2552F) for about £16 ($28) /25kg.
Both these were supplied by Wright Refractories in Birmingham UK. If you
are US based you may be able to locate it cheaper.
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.
The denser wool has better insulation properties although even the
poorest is probably better than the castable. Read the MSDS on the wool
before use, you can get a more expensive wool, Superwool 607 Max IIRC,
which is body soluble so safer. The Kaowool is classed as a refractory
ceramic fibre and you should limit the dust exposure. Some preparations
are available to apply as a rigidiser/sealant. In a similar application
I used fibre with the hot surface sealed with sodium silicate then
coated with a zircon paint. The zircon primarilly help stop bits of
glass that spall off a glass blowing iron from eating through the fibre.
Ask at the refractory supplier they may be quite knowledgeable.
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
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
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
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
Deep Friar: a very philosophical monk.
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.
Makes sense, it's a pretty large pipe.
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).
Deep Friar: a very philosophical monk.