Greetings, This is my first post here, so hello everyone! Many years ago before i started a business and lost all my free time I built a small propane furnace out of an old air compressor tank, kaowool, and refractory cement. It worked ok, I was only ever able to melt aluminum but really wanted to melt copper. Lately I've noticed I have some free time again and am thinking about building a new furnace. I have an old 20 pound propane tank which is already cut open and has holes drilled for ventilation and for the blower pipe which I made back in the day which was going to be my second furnace. I'm looking for any insight or opinions if the kaowool/refractory cement set up is still relevant and the go to or if there have been any new developments since i last dabbled. Also, looking at this 20 pound propane tank makes me think it'll be too small once its insulated so I'm wondering if it will be a waste of resources, but not really sure what else to use. Thanks in advance for any suggestions and allowing me to be part of the forum.
Welcome to the forum Brandon. What size crucible are you thinking? Dimensions and pictures of the tank so far? I was in a bronze casting class where the bronze was melted in a naturally aspirated (propane no blower) furnace with Kaowool sides and roof and a cast refractory base. I saw the same furnace melt bronze again at another event so it is repeatedly capable. Previously I was under the impression that I would need a blower for bronze. A long way to say that burner tuning probably has as much to do with melting things as insulation.
when in this early of design stage you have a few questions you need to answer in order to successfully move forward.There are also a few things that need to be addressed for proper combustion. 1 What fuel do you plan on using, you can reach bronze temps with any of the fuels available with the proper design on the burner and the furnace. 2. What is the maximum temp needed for a satisfactory melt of your desired metal, you can always fire a cooler furnace but the upper limit is a design factor. IE. you can melt lead in an aluminum furnace. 3. The thicker the refractory cement the longer it will take to get to temp because of the thermal mass. The furnace referred to by tops is familiar to me and it has melted iron in very small quantities With sufficient time. 4. KaoWool is and excellent insulator, but, it is friable and puts fibers into the exhaust if there is any disturbance of the Wool, there are a number of solutions, one is Santanite it is good for 3200F I us ceramic slip to coat the KaoWool on my large aluminum processing furnace, I spray it on at the end of a fire, My furnace has 2" of KaoWool #6 it will reach about 2200 if i let the burners stay on high fire. I haven't ever processed bronze etc because of how large the furnace is, It will hold 250lbs of aluminium. I also coat my ingot molds and metal crucibles with the ceramic. 5. Naturally aspirated burners if designed and sized properly can burn all the gas that is needed. most times that people have trouble with naturally aspirated burners has to do with the venture affect that pulls in the air. most youtube burners are not designed for good combustion. 6. Powered combustion provides the best burner design because the air is adjustable to match the fuel. By using discharge baffles at the end of the burner the flame front can be adjusted to the best point for the specific burner. 7. The combustion space in a furnace is the area left with the crucible installed, gas burners can work OK about 75% of the volume need by an oil burner because of the heat and space needed to vaporize the oil, a siphon burner atomizes the oil but it still needs to be heated to the ignition point and vaporized this takes space, our furnaces are in reality fire boxes because 90+% of the heat is vented out the exhaust. 8. any flame outside of the furnace is actually wasted fuel. if it doesn't burn in the presence of crucible it is wasted. Flame exiting the furnace normally is a sign if insufficient air for proper combustion or insufficient space inside the furnace to burn the fuel. Art B
Welcome. How long ago were you using your old furnace? Were you ever on our old forum, called Alloy Avenue? There are a bunch of great furnace building threads on here where members have documented how their furnaces were put together. You might take some time to read through a few of them to get some ideas and see if anything has changed since you were into foundry before. One thing that might not have existed when you were melting stuff before is cheap furnaces available from ie. Amazon/Vevor etc. IMO if you go the cheap and easy route you get a cheaply built furnace and miss out on an opportunity to learn how to use one well first through researching a superior diy build, but they do seem to be a pretty popular way to jump in. However most of them you still will want to build safer crucible tools than what comes with them, and the ceramic fiber lined units only come with rigidizer for the kaowool, no actual refractory. A lot of them have their tuyeres angled too much (IMO) toward the crucible itself rather than coming into the furnace at a tangent, based on pictures I've seen. I doubt the crucibles they come with are top notch either. Definitely an option, but maybe not quite as plug and play as their marketing wants you to believe. I've got 2 main crucible furnaces that I use. The low mass kaowool and satanite propane furnace for one-and-done nonferrous melts, and the heavier duty oil fired one with an inch of dense castable refractory protecting the fiber blanket insulation aka thermal mass for back to back melts and iron casting, or I suppose anything that needs a bigger crucible than an A12. I like them both a lot, either of them can melt anything up to and including cast iron. They each have different applications that they are well suited to though. Links to the build threads are in my signature below. Most hobbyists will rarely if ever be doing back to back to back melts on the same day to be able to take advantage of the thermal mass of a preheated heavy duty furnace that is going to take a while to heat up from cold, so if that sounds like you I would recommend the kaowool and satanite approach. Satanite is a refractory mortar than you can just mix up and paint onto the kaowool and fire immediately, then add another coat and fire again, repeat until you're happy with it. No forms to build to cast the refractory or slowly ramped up day-long heats to cure it. It's cheaper and much easier to build than a hot face made of castable refractory. You might build it up as thick as say 1/4" (I bet mine's more like 1/8"), much thinner than a cast hot face would normally be, so unlike a build with lots of heavy refractory to soak up heat, the low mass furnace can get heated up to glowing hot within just a minute or two when starting from cold. The thin satanite coating will crack, but it's very easy to patch, just mix up a little more and smear it on. I used 5# of it to coat the ceramic fiber blanket in my propane furnace that fits up to an A12 crucible, but it's good to have some extra on hand. A heavier duty build might be more fun though, if you were looking forward to working with the castable refractory forms etc. I'll never regret building my oil furnace, even if it doesn't get used nearly as often use as the propane furnace. Good luck! Jeff
Thanks for the post Jeff, this is my first time on any forums for casting. Was probably about 10 years ago since I did a melt. I'm a carpenter by trade and like building my own stuff so i think ill go that route. I think I will start with a kaowoll/satanite furnace to play around and get back into it and see if i want to continue in the hobby and possibly build a bigger furnace. I assume amazon would be the best source to purchase crucibles, kaowoll, satanite, and other accessories? For your smaller furnace is it fueled with propane and a blower?
Hello Tops, Thanks for responding. Attached is what I have left over from years ago, I haven’t put any thought into crucible sizes. I have this propane line I made, is that considered a blower or would you have to hook something like a vacuum in reverse up to it?
Amazon is one place. I see you're also in Canada. I am in Ontario, Ottawa area. There is a foundry supplier in Milton ON called Foundry Supply Source that caries good quality crucibles and all sorts of refractories etc. Bit of a trek for me from here, but the place is awesome and they are very hobbyist friendly (call or email ahead to order and set up a time, and be prepared to spend some extra time there getting shown lots of other cool stuff). I do not know if they keep satanite on hand though. It's available to order from IIRC 'maritime knife supply' on this side of the border. Maybe other places too? You might have a pottery supply business close to you that can set you up with your ceramic fiber blanket insulation and some other necessities, but maybe it's cheaper to order online these days? Not sure. Recently a friend told me there's a place in Montreal that sells quality crucibles for cheaper than Foundry Supply Source too. If that's closer for you, I can try and find more details about it, I might try them too next time I buy new crucibles. I do use a blower for my burners. I use basically the same burner for both oil and propane fired heats. When I run oil, the propane is only needed to preheat the furnace until it is glowing hot inside, then I can gradually swich over to a needle valve regulated gravity feed oil drip injection while backing off the propane until the furnace it running on pure waste motor or veg oil, diesel, jet fuel or whatever it might be. It needs a blower either way. For propane that means a hair dryer; for waste oil/diesel it's a shop vac or leaf blower. The burner in your pic looks naturally aspirated (no blower needed to run it). Forced air vs naturally aspirated, that comes down to individual preference really. Yours should work fine from what I can see. Some people do seem to have better luck hitting higher temperatures with a forced air burner though (or by adding some forced air to their normally naturally aspirated burner). Looks like a hair dryer or other small blower could be added to your burner pretty easily if needed. Jeff
Turns out I'm only an hour away from Maritime Knife Supply, never even knew it was there! And they have everything I need!
Indeed, when that happens, reduce gas usage or increase air. And a forced air furnace requires less space for combustion because of the leaner (= more air) mixture.
Hottest flame vs getting a charge of good quality metal up to a high pouring temperature, is it always the same thing? A lean tuning might be used to prevent some hydrogen contamination, whereas a rich tuning might help prevent some oxidation of the melt. Is some fire above the vent hole in the lid always necessarily wasted? I'm no rocket surgeon, but G3j talks about it here: Food for thought. I'd love to get Art's take on all that since I think he might actually be a rocket surgeon. I can say for sure that my oil fired furnace won't melt iron unless there's a miniature sun sitting up there above the lid. (Not quite half a meter tall in my case though) Anyhow, OP was wanting to melt copper. IMO 2" of ceramic fiber blanket behind a 1/8-1/4" thick hot face of satanite should get there without issue, as long as the burner is running somewhere pretty close to neutral. @brandon henneberry good luck and do let us know how it goes with that Canadian vendor, I wound up getting a 20# box shipped up from the US a few years ago before I found out about them. I don't know that they were in business yet at the time. I'm in no rush to do any patching right now, but I am all out of satanite (sold the rest to a friend building a firebox for a steam engine boiler) and I know I will need more eventually... Jeff
The Flame out the top can be caused by a couple of things. With oil the need to have sufficient space for the flame to complete combustion with-in the furnace can be critical to the heat transfer, If that space is two small you may need to carry some fuel out of the furnace in order to get the highest flame temp. They way to find out how much fuel /space the furnace can work with, slowly increase both the fuel and air until it starts to exit the furnace. Then add air and see if the flame pulls back into the furnace, that is what will happen if you have insufficient air, then add fuel until flame starts exciting the furnace. when adding either air or fuel causes the flame to excite the furnace the fuel air ration matches the furnace. As the melt heats up less heat is removed from the flame, this in effect reduces the size of the combustion /fuel burn area, and you will get flame exiting the furnace. This happening at the very end of a melt caps the crucible with full flame temp 6" of flame exiting at the very end of the cycle should be the most that is needed. Also until the oil has completely vaporized the flame is cold due to raising the oil to ignition temp. Also any excess air beyond what is needed for combustion cools down the flame temp, because of raising the ambient temperature air to the flame temp. Art B
Well, how I tune the furnace is by a thermocouple (alumina sheathed) poked through the furnace wall just below the top of the crucible and opposite the burner input. This temperature is usually hotter than the metal inside the crucible, but I tune this temp by playing with gas or air inlet. A few weeks ago I could not get cast iron molten, the aforementioned temp did not get hotter than 1380 C, despite ramping up gas. When I lowered the propane input from 45 to 30 psi without changing air input, the temp increased to 1500 C in 10 minutes and the iron melted.
Sorry I missed the previous reply. The 20# shell looks like a good start. If you had 1" kawool and 1/2" of a harder refractory, the diameter would reduce by 3" and allowing at least 1" a side for combustion and then tongs, you'd be looking at 6-7" max diameter crucible. You'd need to do a similar calculation with the floor and plinth and lid. My floor is soft firebrick with a refractory hot face, lid is Kaowool and hot face. Mine was designed a little bigger around and taller to account for liquid fuel combustion.
