How to Make a Small Gas Furnace

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Posted in WorkshopMetalworking

Introduction: How to Make a Small Gas Furnace

About: I shouldn't have to tell you that using a dagger to undo this little, fiddly screw's a bad idea. AAAAARGH! big project ^^ so practically no chance of instructables from me till july, p'raps? maybe a bit la...

Why is it that all the things i make involving heat are small?
There's the coal forge, that was fairly diminutive, but i digress.
by-tor the snow dog asked me if the coal forge got hot enough to do glasswork. ( https://www.instructables.com/id/EG8B14NNJJEYMW0DC9/#CEXLBVSRGSWEZ7BDCQJ ) Basically, it does, but i wouldn't recommend it, because the hot coals will stick to the glass ans it'd be manky, so i decided to make a clean, hot-burning furnace using what i had lying around.

Step 1: Get the Stuff

The first step is, as always, get the things you'll need.
I used the Refractory glop i sued for the coal forge -i'll keep using this until i find something better, IF i find something better- and a can. The can should hold about half a kilo and, even with the refractory taking up some of the space my torch was still struglling to heat all of it up, so if you want a bigger hotspot, you'll need a bigger can,obviously, and more torches for the heat, but that presents some new problems because you don't want the torches to overheat one another. This can held 540 grams of grapefruit. Because the can has to be emptied first, choose something you'd like to eat. If it's beans, work outside.

Step 2: Make a Hole in the Can

The can needs a hole, through which goes a pipe, into which is inserted the blowtorch. This is a bit like the blast pipe for the coal forge, but instead of blowing air through to fuel a fire, this is where the heat comes from. a chisel and some pliers are all you really need, the hole doesn't have to be precise, just big enough to fit a bit of pipe in. (don't use glavanised steel) Once you've made the hole, cut a length of tube or pipe about 2 inches long. Under no circumstances should the tube obstruct the torch's air holes. If this happens, the fuel mixture will burn too rich and you'll get crabon monoxide poisoning instead of heat. The effects of carbon monoxide are cumulative, each does gets piled on top of the last. The pipe should protrude about half an inch into the can, offset from the top by about 30 degrees. The picture should hepl you determine what it needs to look like.

Step 3: Add the Refractory

This is the easiest step, though it needs you to get your hands dirty. Using a spoon or a knife or your fingers, coat the inside of the can with refractory. Make it fairly smooth, too, this will make sure there are no nasty eddies or swirls inside the can that can do unpredictable things. The refractory should be between 1/4 and 1/2 inches thick. Mine was somewhere between those. Don't obstruct the pipe!

Step 4: Some Quick Notes

For those people who can't get XL refractory clay, there are others, more available and less available. The cheapest one i can think of is Adobe. This is simply a 50-50 mix of sand and clay. (no pebbles or rocks) Mix the sand and clay and apply them immediately, then let them dry completely before firing the adobe. After it's been fired once, it's ready for use.
Anyone living near a ceramic supply house or brickyard should be able to find something that'll do the job.

Step 5: Fire It Up

Once the refractory's ready to be used, use it. As with the coal forge i posted, i recommend that you fire it up without doind any work at least once, just to get a feel for it. Place the torch into the pipe making sure that the burner head of the torch doesn't protrude into the main area of the furnace. For god's sake don't obstruct the airholes. Also, don't even USE your torch if it's in anything other than perfect working order. To light the torch in the furnace, turn on the gas slowly while holding a burning stick, or a bit of rolled up paper with the end on fire in front of the burner. Once it catches, wait a minute or two till it gets up to working tempertaure.

Step 6: Making Stuff

My thanks to Larry Zoeller, of Zoeller forge for giving me the idea for this. The link leads to a similar one he made that geve me the idea to make this.
http://www.zoellerforge.com/coffee.html
This furnace was designed and made with glassworking in mind, it's like one of the furnaces i used when i went to The World of Glass in St Helens to try glassblowing. There's no reason not to make small forged items in it, though, like arrowheads, but that might necessitate some small adaptations. There's no question about it getting hot enough, certainly.

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    81 Comments

    This looks good, a mains gas supply and you're there. Could you give us some specifics on sand and clay? Are you digging this out of the ground, or is this e.g. potter's clay? Sharp / concreting / acid-washed / KD sand, or does it not matter?

    15 replies

    It does indeed matter, and it is a very good question for those who don't know.

    Now, you need to buy a particular type of sand if making refractory clay yourselves. But, the good news is that it is super cheap, so much so I have seen it for sale in "Poundland" for 5kg bags for the princely sum of £1 :-) It is sold as children's play sand. It is kiln dried sand. Do not buy the anything else, this stuff is super cheap and makes a super effective refractory clay for kilns. I make a 50/50 mix of this and plaster of Paris.

    I bought 25kg of plaster of Paris for £18 last year, shop around for a good price online. With it I have made two types of kilns and I still have about a third left.

    One of my kilns is used for heat treatment and is fairly big, so the plaster of Paris really does go a long way. I have never had a problem with cracking etc I have read some people have.

    Just be sure when you have made your kiln to heat it very very gently for a long time. Low and slow, I cannot stress this enough. If you are making a small one, a good idea is to heat it (cure it) in your cooker oven.

    I just made some refractory myself, but in this formula, it's easier to find the materials.
    It was based on this, but a little simpler. You'll need Perlite (availible in the gardening section, DON"T get vermiculite), silica sand (plain, clean sand, often sold as playbox sand), portland cement (sold in big bags at some hardware stores, outside. You will probably need help because an average bag weighs 90-something pounds), and bentonite (cheap kitty litter, oil absorbing clay, etc., it doesn't need to be powdered, although that would be better.).

    Combine one part perlite, one part silica sand, and one part bentonite.
    mix well and add a small amount of water, just enough to make it moist. let this sit for 30 minutes, mix again, add a little more water, and let it sit for 45 minutes. Now add one part portland cement, and mix more water in. You are looking for a moist/wet crumbly-ish texture. Grab some in your hand, and squeeze. If it holds well, then you're done. If it is too crumbly, add more water and test again. Remember, it is not supposed to be as wet as normal concrete, so don't add that much water.
    This is like all concretes with portland cements, it hardens by a chemical reaction (hydration), so adding more water will not keep it soft forever, so mold extra into bricks or plinth blocks (they hold the crucible up in gas foundrys; look at the bottom of the page).
    In order for it to turn out pretty, and work well, you need to ram the refractory. I used a small dowel. First, make your bottom layer, then center a pvc pipe in the can, standing up. This will form the interior space. Ram the material in layer, ramming 2 or 3 layers, and then compacting it hard, repeating until you're done.
    I used a 2" pvc pipe, and left about 1/2 to 3/4" on each side( I was using a small coffee can). When I was done, I waited 5 minutes, then pulled out the pipe with pliers.
    I forgot to say that you need to ram with the pipe (the one going to the propane) in when you ram the material so it will leave a hole. So do that.

    I've found it to be an easy formula, although you need to start the torch on low and run it for about 15 minutes to drive off any moisture left from before, before you crank it up to operationg temps. This only needs to be done once.

    Today I was able to melt some aluminum, using it vertically.My crucible was a small steel match holder that I- I mean my friend, actually broke. My only power source was a regular propane tank.I I might post pics if I get around to it. I poured it into a small ingot, using small channel iron (actually steel).

    I'm thinking of making an instructable on the refractory, small forge, and a helium tank foundry. The foundry is the only one not completed.

    Also, I found that when measuring out the materials, using a plastic cup is a good way of measuring. I each part is +3/4 of a plastic cup, then you'll have just enough to complete the forge (in my case). Don't cut it short!

    I hope you appreciate this because it took me a long time to write this comment (I type slow).

    Perlite is Glass and melts at the same temperature as glass.

    Vermiculite is a superior product for making firebrick refreactory.

    John Smith...is the portland cement 1 part to the 3 parts combined of the perlite,sand and bentonite?

    http://backyardmetalcasting.com/refractories.html

    The refractory mix is composed of Portland cement (1.5 parts), silica sand (2 parts), perlite (1.5 parts) and fireclay (2 parts). The first three components are mixed together thoroughly.

    Thats the formula.

    However, its not as great as i thought it was at the time, now that i have tried different refractoires(that post was about a year ago).

    Also, portland cement is a flux at high temperatures. Most people say to skip the portland.

    I use Kaowool now. Its cheap, and truly great for a small gas forge like this. You might want to look into it.

    If not, see [www.backyardmetalcasting.com/forums] for more info, or comment back.

    nicely done! there might be more furnace instructables from me in future

    Thanks, I for one appreciate this. L

    i recommend a clay with a high silicate content, to take the heat well, no big rocks, no limestone dust in it -if you heat it, it decomposes into calcium oxide, nasty stuff and carbon dioxide- so perhaps something out of the ground, put in a bucket with all the roots and stuff taken out of it. Don't use modelling clay, all the stuff i've seen here is re-enforced with nylon strands which wouldn't take the heat well.

    i meant to say "put it in a bucket of water, break it up and take out everything that isn't clay"

    If you can't find "fire cement" try looking for castable refractory, at just about any big-box home improvement store. Country hardware stores will often carry it also. If you're brave/foolish/smart enough to make your own refractory from dug clay, what you'll want is a high Kaolin content clay... though just about any clay will work to a degree. The type of clay used mainly effects how long, and how hot before you get spalling. Clean playground sand works pretty well, for aggregate in your clay mix. but it's not so hot for insulation. When Making firebricks, the trick is to inclode about 25% fine sawdust into the clay mixture. When you first fire your clay,apply the heat low, and slow. If you control the heat well enough, the sawdust vaporizes without breaking the finished clay body. The tiny pockets of air provide the best insulation to be had. The finished product should resemble the soft/porous firebrick Wargoth mentioned. Do NOT use cinderblocks... please, oh please NO! Cement retains far too much water and can/WILL explode most violently when subjected to the extreme heats generated in a forge like this. If you upgrade from a soup can, to a coffee can or larger, take the time to also hinge a door onto it, or get some firebrick to partially block the front, as it will drastically improve your ability to achieve, and retain heat.

    "25% sawdust"... is this by weight? seems like a lot.

    It's been a while, and I've lost my exact recipe... but I seem to remember it being about 25% by VOLUME. And mixed into the moist ingredients(mixing into the dry ingredients would require a LOT more water, and take days longer to dry enough to even begin curing) measured mostly by eye and luck. My personal experience doing this was, in a university ceramics department. I suppose, I might call it cheating, as we had 100+ gallon containers of various clay powders, and 1000's of pounds of bags stacked. The large volume is a lot... and leaves a very soft, fragile brick... but it's also what gives it it's insulation ability. It's a bit of a trade off. Looking upward, John Smith's suggestion of using perlite might be a better solution. Heck, it may even act as a better insulation(dunno, haven't tried refractory cement yet.) and having material in place of voids, may create a stronger brick. I'd still start at 25% lmao! sorry, just wiki'd perlite. "When it reaches temperatures of 850-900 °C, perlite softens (since it is a glass). Water trapped in the structure of the material vaporises and escapes and this causes the expansion of the material to 7-16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles." So basically, the perlite is already a bunch of little air bubbles! Instead of adding the sawdust, and letting the material burn out leaving voids, the perlite is pre-encapsulated voids. and the worst that will happen if you over heat(900+C) it is, the obsidian will fuse to the inside of the void like a glaze. Live and learn! I'd still begin with a 25% mix, and test larger percentages as I went along(in small batches). But if my intuition is correct, i could see going as high as 40%, in a cast-in-place material. Higher heat insulation... and very soft, but with the metal acting as a structural element, it should still work. give it a go, and reply with the results? :-)

    What excellent timing, i've just finished making a knife, and rasping down the hardwood handle has given me a nice pile of sawdust!

    so....the recipie for firebricks is a 50/50 mix of clay and sand, with 25% sawdust added after the clay and sand are mixed? anything else?

    pretty bomb design for being improvised as it is. my suggestion though would be to take some wiss snips and made a hole in the back 2in long and an inch wide so you can insert longer bar stock or knife blades through it. then in the front, take a soft fire brick and cut a groove in one edge and set it on edge so the groove is on teh bottom. only the bottom 3rd is exposed and youll get higher temps inside due to the heat being concentrated.

    this would make an excellent tempering forge for small/med knifes, punches, chisels and the like.

    1 reply

    actually, master bladesmith wayne godard made a forge like this and has instructions on how to make it in his book "the $50 knife shop"

    What about Acetylene? Would it work ? I like the forge idea, good work.