building a melting furnace

[chuckt]

Hi everyone,

I am building a melting furnace out of an old 20lb propane bottle. I am documenting the whole thing on my web site. It's still in the early stages, but I have a working burner. If anyone has any suggestions, it would be great to hear them.

Here is a link: http://chucktilbury.com/pages/projects/ ... urnace.php

[Harold_V]

Hi everyone,

I am building a melting furnace out of an old 20lb propane bottle. I am documenting the whole thing on my web site. It's still in the early stages, but I have a working burner. If anyone has any suggestions, it would be great to hear them.

I hope you're serious about suggestions. I have a few, based on having built a few melting furnaces, including one tilting reverberatory type.

I'm not convinced that will be large enough to be useful. I'd suggest you explore a larger container, depending on your needs. If you intend to use only a very small melting vessel, it may be fine.

I would do pretty much anything to discourage you from using steel as a crucible, or from making your own crucibles from refractory materials. Molten metals are strong solvents of other metals, so the end result, unless you intend to line the steel crucible with a refractory material, will be to contaminate your melted metal, plus you'll dissolve the steel crucible. Such a crucible would not be satisfactory for melting brass or copper, and it's borderline not acceptable for melting aluminum, although there's no shortage of individuals who would argue the point. Many use them, all with varying degrees of success.

Think of your welfare when you make a decision to make a clay crucible. The dollars saved won't begin to pay for the medical care you'll require if you lose a crucible full of molten metal while handling. Commercial crucibles become rather soft when at heat, so they aren't as likely to break as one made from clay alone.

I would encourage you to not use a home made *refractory* to line your furnace. Portland cement has no place in a melting furnace, as it is reduced by the heat, plus you risk steam explosions. I strongly recommend a commercial refractory, as they contain no Portland cement, and are engineered for the purpose. You'll be glad you made that choice when you discover how long a furnace, properly built, can last.

Harold

[chuckt]

Thanks for taking a look and the feedback. I am very interested in what you have to say.

I know a little about the chemistry of a refractory lining. That is to say very little. I do know enough to wonder why those guys were putting like 40% portland in their linings. I was planning to use about 25% refractory clay, green stripe or kaolin, 15% silica sand, and about 60% perlite. I would have to experiment some to get it to hang together well enough to make the lining. Maybe a little bentonite thrown in so it will actually fire on the surface. Green stripe fires at 2150 and kaolin fires at 3150. (remembering off the top of my head) The perlite will keep most of it from firing. I was a little concerned about shrinkage. Do you know of a good (read cheap) source for pourable refractory for the lining? I also thought of making a octagonal furnace out of insulating fire brick. What do you think of that idea? I am sure I can make it well enough. Not too sure about how long it would last, though. Aluminum attacks that stuff.

I also was aware that molten copper will attack steel. But I thought that the effect would be negligible. A steel crucible is so cheap, that I was thinking to just replace them when they wear. But if it will actually pollute the alloy, I guess that is not acceptable. One of the things I want to do is make aluminum bronze. I don't need extra headaches. I am wanting to do high quality work, not just a "hobby" level. After I exhaust my lathe scrap for practice (I have a lot), I plan to actually buy ingots and sell my work. Do you know of a lining that will work with a steel crucible? Shrinkage and expansion at temperature looks like the biggest problem with that.

Does this look like what I need?
http://www.ebay.com/itm/A5-Super-Salama ... 938wt_1144

How many pours should I expect to get? Do you have to preheat these in an oven to desiccate them?

What about tongs? 15lbs + is a lot to hold on the end of a hot scissor tong. Is that something I should try and make?

The size of the crucible that I have settled on is 4"x6" One thing I was not able to find much about is how much space needs to be between the crucible and the lining of the furnace. I settled on a 8" diameter difference. Does that seem reasonable? Less? More?

I don't really see why people use a thick steel shell like a propane tank. All it does is protect the refractory inside. I have considered using sheet metal instead. What do you think of that? I know I can make it well enough, but will it provide sufficient protection?

I have a lot of questions. Thanks for bearing with me on that. If you can point me to information on the web, that helps a lot.

[chuckt]

I found this link just now....
http://www.budgetcastingsupply.com/ITC.php

[Harold_V]

Do you know of a good (read cheap) source for pourable refractory for the lining?

The last furnace I built was quite some time ago, and I now live in a different state, so I am not well versed on what is available. That particular furnace had specific demands, in that the charge was introduced directly to the lining---no crucible was used. For that, a high alumina refractory was used. However, for my crucible furnace, I used a material named Ladlecast. It wasn't particularly expensive, but none of the refractory materials are inexpensive. The real savings comes from doing the job right, for the first time, then using the furnace for a prolonged period of time without issues. That, to me, was far more important than the original cost, as down time doesn't make money. Ladlecast is not a great insulator, but serves the purpose well. There are other options, however. Might be a good idea to search the web, plus take a look in your local phone book yellow pages for refractory materials.

I also thought of making a octagonal furnace out of insulating fire brick. What do you think of that idea? I am sure I can make it well enough. Not too sure about how long it would last, though. Aluminum attacks that stuff.

Probably be fine. They make the brick in all manner of shapes and forms, so they can be assembled according to need. Placed in a rigid container, it would likely work fine. You'd have to get a particular sized container, however, which may not be so easy. Not a problem if you have the ability to roll one from sheet, however. I did that for my tilting furnace.

I also was aware that molten copper will attack steel. But I thought that the effect would be negligible. A steel crucible is so cheap, that I was thinking to just replace them when they wear. But if it will actually pollute the alloy, I guess that is not acceptable. One of the things I want to do is make aluminum bronze. I don't need extra headaches. I am wanting to do high quality work, not just a "hobby" level

If that be the case, don't even consider a steel vessel unless it is fully lined. The link you provided with your additional post looks promising, but unless you coat the entire vessel, inside and out, you can expect less than great performance, in particular if you melt copper. My primary purpose in running my furnace was to melt pure silver, and, on occasion, pure copper. The temperature needed to pour copper is slightly over 2,000°, which would be very destructive of an unprotected steel vessel. The worst consideration is the contamination of the heat, however. In regards to the link, I am not personally familiar with their products, so I hesitate to suggest they would work for you, but I expect they likely would.

Does this look like what I need?

It can be, yes. Also, consider silicon carbide crucibles, which are less demanding of the care required. Graphite/clay crucibles are subject to cracking if introduced to a furnace without being tempered (dried out slowly). Silicon carbide crucibles don't require that operation. I expect you'd get a little better life from silicon carbide, in particular if you don't flux excessively. Fluxing tends to degrade the crucible rapidly. When melting silver after being recovered, I could expect about eight heats from a #8 silicon carbide crucible, at which time it generally would spring a leak at the flux line.

How many pours should I expect to get? Do you have to preheat these in an oven to desiccate them?

Melting aluminum, it's entirely possible you could get a hundred heats or more from that crucible, assuming it is desiccated properly at the outset. The larger the crucible, the thicker the wall, so larger ones will actually yield a better lifespan---plus each heat yields a greater amount of metal, so they are far more economical to use. Do give that some thought when sizing your furnace.

What about tongs? 15lbs + is a lot to hold on the end of a hot scissor tong. Is that something I should try and make?

Two things should make that determination. How well do you weld? If you are not talented, I'd highly recommend you pay someone to weld them for you. Again, you don't want a failure while holding molten metals. The other consideration is the design, and your ability to build the jaws properly. Crucibles should not be subjected to crushing, as they are quite soft at heat. Bilge crucibles nest in the jaws, below the bilge, so the jaws should make uniform contact with the crucible. I made my lifting and pouring tong for silver by machining the jaws from pipe, then welding them to a shank of my own design. They worked well, and, by design, I could NOT crush the crucible, as all the power for holding came from the grip of a single hand. Furnace tongs are often long handles, so you have considerable leverage, making it easy to crush a crucible. In such a case, I'd highly recommend an adjustable stop, so you can't grip too tightly. Once set, it requires no further attention.

One thing I was not able to find much about is how much space needs to be between the crucible and the lining of the furnace. I settled on a 8" diameter difference. Does that seem reasonable? Less? More?

It seems excessive to me, but a great deal depends on the handling tools you'll use. I didn't allow enough clearance for my #8 furnace, although I could use the tools. I would have preferred an additional half inch per side, however. Without knowing what tools you might use, I think, right now, I'd suggest something in the neighborhood of 2" per side, or 4" on the diameter. That should provide for pretty much anything.

I don't really see why people use a thick steel shell like a propane tank. All it does is protect the refractory inside. I have considered using sheet metal instead. What do you think of that? I know I can make it well enough, but will it provide sufficient protection?

In my opinion, yes. I'm going to show the tilting furnace I made, simply as an example. I had the can made, as I wanted a specific size and could not find anything near. For all my other furnaces, I've used nothing more than surplus military containers, which were all lighter in weight than the can I had made for the tilting furnace. Note that the trunnions for the tilting furnace attach to the skin, so even though the furnace weighed about 700 pounds, the skin was more than adequate for the purpose.

I have a lot of questions. Thanks for bearing with me on that.

Please note that my experiences revolved around melting and pouring precious metals, with a little copper tossed in (it is used to recover silver from solution). You may or may not find my experiences useful for casting, although I expect you probably will.

If you can point me to information on the web, that helps a lot.

Follow this forum closely, as others will have something to contribute. You have to be a little bit careful about where you learn, as most of the information you'll uncover comes from people much like you, who report what they did, which often isn't recommended. That's one of the reasons you see so many using steel crucibles----which are never used in the commercial foundries.

On the subject of crucibles, while you're checking for local refractory sources, take a look to see if you can find any foundry supply places near you. Pay them a visit if you do so. They should be able to advise you on sources for refractories and crucibles, assuming they don't supply them. They probably do.

Harold

[steamin10]

Harold has a vast bit of experience in precious refining, and has most everything right for the home, and semipro founder.

I will add: Stay away from the common gold an Jeweler sites for materials. They are too expensive, and too small for the home foundry guy. The first thing any mechanic finds is he needs bigger wrenchs to do bigger jobs. So it is with foundry work, as the size of the melt available will be your limiting factor, and have a great effect on your time spent and cost efficiency. I have built 3 furnaces from scratch before I knew what I was doing, and just happened to get it mostly right. 55 gallon drum cut to half heigth, and a 30 gallon drum, make for good sizes for crucible furnaces. My furnaces were based on fire brick, cemented in with fireclay, Oildry, pearlite and the like, to cement , insulate and fill the voids in the radial brick lining. Firing should be in a swirl pattern, and not on the vessel. The flame should make on the order of 3 rounds before exiting the fire hole, entering alongside a block the vessel sits on, and passing round to heat all sides of the vessel evenly. I would avoid a reveb furnace for metals above aluminum temps, call it 1800 degrees, unless you understand molding a lining, its retension in the shell, and how to maintain it between sessions. Such a furnace can be a major advatage for aluminum only, early on, because of the elimination of crucibles, or using a crucible simply as a preheated ladle to pour on the molding floor. Otherwise higher temps metals like brass and bronze will be very easy to handle from a crucible with appropriate tongs and shanks for pouring. I recommend you find some books , possibly at the library about molding and pouring, by CW Ammen. he covers most of it very well, from sand formulations, to building a furnace, and a business. Most common grades of casting metals and their properties can be found in the machinists handbook.

Metals in liquid form, are easy to comingle, (read contaminate) until you are lost in the woods for content. Barring elaborate testing equipment, bar metal is the select (but expensive) choice. Keep vigin metals clean, it is unsound to add this and that to a known ingot metal. If you use purchased scrap, knowing the original use is very helpful in knowing what grade of metal it is, particularly in cast, wrought, and extruded aluminum. Auminum has an affinity for other metals, even in its low melting temp, and will absorb and disolve bolts, nuts, and small rivets with a vengance. This small percent of Iron cannot be removed by any conventional means, and has the nasty habit of ruining fluidity, crucial to pouring aluminum in molds, and making it a pasty mess.

Read back in the foundry heading, and gain a lot of rehashed, and arguable knowledge. Particularly when it comes to rudimantary safty to prevent injury should anything happen with all the clever devices cobbled together for the home guy. Make no mistake, it is a dangerous hobby in the least. However, I consider cooking in the home, with hot oils, surfaces, and sharp knives to be of equal danger, so it is knowledge that is most important.

Ok, I wont belabor many more points here, just encourage you to read what already is on the threads. One of the guys here, Pipesecs, Charlie Pipes, is going through the learning curve now, and is the most current on steps and misteps as he makes some parts. Truly informative.

Owning a Grand piano, does not make you a Musician. Only study, practice, and performance can give you mastery, to earn the title. I wish you well on your quest for knowledge.

PS. Note the apparent quality of the 'cobbled together' tilting furnace. It is well executed, and if there any flaws, they are at least well hidden, by craftsman work.

[chuckt]

I have updated my site (http://chucktilbury.com/pages/projects/ ... urnace.php) again with potential plans and a python script to calculate the actual dimensions of the furnace based on crucible size. Seems like an A8 crucible is the best size for me. An A6 leaves 550+ cubic inches of refractory left over with the parameters I chose. An A8 leaves a deficit of 1 cubic inch, not including the burner inlet. It will take 3 55lb bags of kast-o-lite 26.

inside diameter = 9.500
outside diameter = 17.500
inside height = 8.500
total height = 16.500
lid, bottom, and sides thickness 4.000
vent hole = 4.000

Does that seem reasonable?

(edit, somehow I got the numbers all messed up. fixed now.)

[chuckt]

Firing should be in a swirl pattern, and not on the vessel. The flame should make on the order of 3 rounds before exiting the fire hole, entering alongside a block the vessel sits on, and passing round to heat all sides of the vessel evenly.

I was planning to have the flame enter at as steep an angle as the wall thickness will allow and it to exit from a vent centered in the top. Does that make sense? Or did I miss something?

I would avoid a reverb furnace for metals above aluminum temps, call it 1800 degrees, unless you understand molding a lining, its retention in the shell, and how to maintain it between sessions.

I am intending to make a furnace the uses a crucible to hold the alloy. That's what you mean, correct?

I recommend you find some books , possibly at the library about molding and pouring, by CW Ammen. he covers most of it very well, from sand formulations, to building a furnace, and a business.

Thanks. I have a new Amazon favorite.

Aluminum has an affinity for other metals, even in its low melting temp, and will absorb and disolve bolts, nuts, and small rivets with a vengeance. This small percent of Iron cannot be removed by any conventional means, and has the nasty habit of ruining fluidity, crucial to pouring aluminum in molds, and making it a pasty mess.

I did not realize that, but it makes perfect sense.

Read back in the foundry heading, and gain a lot of rehashed, and arguable knowledge. Particularly when it comes to rudimentary safty to prevent injury should anything happen with all the clever devices cobbled together for the home guy. Make no mistake, it is a dangerous hobby in the least. However, I consider cooking in the home, with hot oils, surfaces, and sharp knives to be of equal danger, so it is knowledge that is most important.

Will do. I wonder why I didn't think of that.

[steamin10]

I will try and cover what you ask, in reference to my own opinions and small work I do.

I go with the high school sized commercial furnaces, built by McEnglevan, (bottom preheat, twin burner) and the lower Speedy Melt furnace that will handle up to a #16 crucible with ease. I donot stray from a common size, as all the shanks and devices I have work with that size without distractions. The McE has a rear vent solid top with swing mech, The speedy melt is less efficient, with about the same size chamber, Lid lifter, and lid vent. Both work extremely well, tho I feel the McE is a bit more efficient with preheated air. ALWAYS use one crucible for each metal. One for aluminum, one for brass/bronze alloy. If you get into using zinc based metals(diecast) use alid to help with preventing smoke and metal overheating at the surface. The McE will make iron, but the vessels burn away fast at that temperature range fairly fast getting maybe 4-8 heats before destruction. aluminum will last for many heats, and usually succumb to a stupid move like setting them in a water spot or some mechanical or temperature shock (mishandling). with the temps of bronzes @2k * they last about 20 -30 heats with ease, for the carbide, so they are worth every nickle in my mind. You will find that making a small part, will have a lot of extra material, for gates and sprues, so bigger is better in a way, especially when you have to stir in silicon, or alloys, it will cool small amounts of metal, and give varying results because of poor volume control for additions. Larger amounts of metals will react and be easier targets to hit for consistant performance, and temperature control. These furnaces have a base block of refractory in the center, and are fired tangentally at the liner. Always run lean, and mean, heating the wall, to allow for a reducing atmosphere, devoid of oxygen in the furnace. You should see a blue haze of fire from the stack at temperature, just above the exit point, as the carbonmonoxide converts to di-oxide as it hits the air. Only a coupla inches. otherwise you are over fueled and wasting. The furnace should sound like a rocket ready to fly, or a jet warming up, as the turbulent burn is quite noisy. You wont get the same heat out of a 5 gallon bucket, or other small size. At high temp, the glory hole will be white yellow heat, and whatever you put in will follow suit.

Never leave a skull in a vessel. If your furnace fails for some reason during a melt, pull the vessel and dump what you can on a sand bed (should be under your molds) or pig what you can get out. Lay the vessel on its side so as not to refill the bottom. A partly full vessel is doomed to crack when reheated, due to the expansion stress from the solid slug.

A pyrometer for non-ferrous metals is essential to hit your pouring temp. Aluminum poured too hot, will give funny shrinkage and added oxides during the pour. Lowering the pour temperature only 100 degrees can give better results, as long as it fills the mold.

Do Not Underestimate the impotance of a good movable lid. Adding material, or stirring and working the melt are important steps, each metal having its own routine. Someitmes killing the burner and relighting is necessary to dross, innoculate,or tmperature the melt, so refiring is important too.

Reverbatory style furnaces are commonly used (or were) for recycle melting of everything from lead batteries, to silver slurry cake. They commonly rock back and forth or rotate, like a cement truck, using the heat from the roof, to become the floor,and back again, and then tilted up where the metals and slags can be discharged on the now lowered end. Hard to explain, but they work well for aluminum, in a 30 gallon barrel size, giving quick reclaim. I do not know about working a charge, but it is quick to pig this metal to ingot. I have not worked a tilting furnace either, so I am ignorant on its advatages or dificulties in working the heat, or topping a charge. Neither of these furnaces use a crucible, so the lining must be made chemically compatable with the materials run, and physically able to handle scrap moving around physically contacting the furnace lining.

I avoid cooking the metals involved, as every melting can have degrading effects. One exception is aluminum, that can degass, and float dross and junk off the top, and leave iron bolts, and junk behind from the first melting, to improve the reclaimed ingot material for future use. Iron of 2% in aluminum, will make it a worthless pasty ball, that wont even flow, and have terribly large crystal structure. Silicon must be added to get good flow and fine detail, as it brings down the surface tension, and grain structure. Aluminum is difficult to maintain good pouring qualities, being so reactive, so care must be taken in every step, or you can run in the ditch and be mentally lost. Bronzes are even worse with its ability to oxidize readily. (copper is very oxy hungry) So porosity (gassing) is the one killer most seen. Heats are often given a cover of charcoal or bottle glass to eliminate oxygen, while reaching for temperature.

When making a pour, make sure your mold will be ready, dont hold any metal at temp, as they will change in molten state, requiring more attention. heat fast, pour quickly, and allow the cast to 'settle' and solidify. Dont rush. Leaving a casting in oil sand will drive the smoke into the sand to be captured there. Breaking it open early will release this to settle elsewhere. Aluminum needs to cool below 600* before breaking out, as it has about half its normal strength then, and a mushy casting can be ruined.

Dont think you can degass, silconize, and dross off a heat, and then let it sit, while you fiddle with something else for 20 minutes. You lose. Pour at the moment you finish your working, and be done for the best quality.

Enough! Read , read, read. Then get busy. I bid you peace.

[Fender]

Much good advice from Harold and Dave.

Regarding the life of crucibles, I have three #8 clay graphite crucibles (aluminum, bronze, iron) I bought 40 years ago, and I'm still using them. Lost track of how many melts, but suffice to say > 25 on the first two, and maybe 8-10 on the last one (iron)each.

I'm the neanderthal on the forum, since I use charcoal or coke for fuel and a leaf blower for blast.

I also have the original furnace, with the original lining. I used commercial refractory concrete, and although it is showing some erosion, is still serviceable. I heat up the empty furnace first, then put in the empty crucible and get it hot, then add the metal. Usually the metal is in the crucible for only 15-20 minutes before it is ready to pour. I use borax as a floating cover over the bronze. Others use glass for this purpose.

Degassing is very important, otherwise you will have porous castings. I bought separate degassers for aluminum and bronze from Budget Casting Supply. Also, skim the dross off before pouring.

Good luck!

[chuckt]

I built a oil burner because I am thinking about free fuel. The thing I built is so unsafe that I am not testing it with anything but water. It draws about 6 inches on the fuel side and blows through about a gallon in 5 minutes. I posted a couple of pictures on my web site. If anyone has any ideas, I would be very interested in hearing them.
http://chucktilbury.com/pages/projects/ ... #oilburner
Thanks!

[GlennW]

Big viscosity difference between water and waste oil.