Here is a copy of an email I sent to Legend Mining. I'll update after I've heard back from them. Greetings, I am a hobby caster and have been buying budget graphite crucibles from Legend for 6 or 7 years. I am not a really frequent caster- I might do a dozen heats per year using a waste oil furnace- but I have reached the end of the service life of a couple of my #10s and would like to make some observations and ask some questions. My melts are mostly aluminum but I have also been melting bronze for a couple of years and would like to start cast iron. I don't mix different metals in the same crucible. I use # 7s, #10s, and #18s, but the 10 is my most frequently used. Firstly I recognize that crucibles have a service life and buying an item that has "budget" in the name has obvious implications which I think is a reasonable starting point for the conversation. Both of the crucibles I've decommissioned may have been ready to go well before I actually discarded them. One (aluminum) was seriously delaminated, very light and no ring, so I threw it out. It didn't occur to me to break it to examine the cross section, I just threw it away. The other (bronze) cracked and leaked in the furnace with a full load of molten bronze in it. There was no hazardous event and I have since repaired the furnace, so there's no problem there. However on examination of the cross section I noted that the graphite was completely depleted from the top down about 2 inches to the point of failure all the way around the crucible. It gradually increased moving toward the bottom of the crucible. It should be noted in this cracking event that it was during the 3rd consecutive heat that day. This crucible had a total of about 15 heats over a total of maybe 10 sessions over 2 years. All of my budget graphites follow the same course eventually, some quicker than others depending on age and the metal being melted (heat intensity). They become crazed on the outside, uniformly on the entire crucible. Not tiny cracking but what looks more like a mozaic of 1/4 to 1/2" surfaces. Inside of the aluminum crucibles look fine. You can't tell with the bronze ones because they get coated with slag. So the question is this: is this the normal expected behavior or is there something I've been doing wrong with my crucibles? Perhaps too oxidizing?
Petee - I am finding this thread, along with a comment about "treating aluminum like a fair maiden" very interesting (cant remember who recently said that or which thread). If I sidetrack this away from your original intent, I'm sorry. I bought a Budget #10 from Legend in the fall of '17, as I was planning my furnace. I have only 6 ... maybe 7 aluminum melts on it in that entire time (probably in 5 sessions). One thing with this crucible from the first "seasoning" firing ... the outside has been pinkish, which I was not expecting nor had I seen similar in anyone else's pictures. The pink color is starting to mellow a little into a light tan pink. But if I compare that to your aluminum crucible and Gipettos (not purchased from Legend, but appears that it is the same crucible) ... my color is very different (see attachment). Also, if I remember correctly, my budget crucible also had a glaze on the outside (like they show in the pics on their site) ... but your new crucible does not appear to have that. I'm not sure what that means ... just something I noticed. Now, all but my last two firings were done using strictly siphoning through the Delevan nozzle. Those first firings were filled with fuel delivery issues and I have no doubt I was running lean. The last couple of melts I have pressurized my tank and fuel supply issues are eliminated. When my session is done, I place the empty back into the furnace and let it all cool slowly. So all that being said ... there are several questions that are running around in my noggin. Why do I see such a color difference? Or is it my imagination? Or I just don't have enough melts on it yet? For an aluminum melt, what impact does running a higher fuel flow have vs running a lower fuel flow on the crucible life ... lets say at the same A:F ratio? Here is where the "fair maiden" comment (from the other thread) comes in, I currently have my furnace set up for nearly 3gph of diesel (similar 11ish inch bore that many of us have). Should I be cutting my fuel flow rate back for melting aluminum? Thoughts on the current state of my crucible? I have a new salamander and and unknown mfg graphite sitting on a shelf ... so no big deal to retire my current one ... if needed. Again, sorry if I derailed the thread as it wasn't the intent. I think its interesting discussion about crucible life ... especially since we are talking the same brand.
I would not expect a reply of any substance. I bet the guy that gets that email wont have a clue what you are talking about. Legend is just a wholesale distributor. My morgan crucibles have never been or turned that weird shade of pink. The come black and pretty much stay that way. Maybe they just lack the right amount of graphite?
I'm looking for new crucible photos, found this one after some not so hot firings in my homemade refractory furnace which later began to melt into foamy green glass from the perlite melting. This supposedly came from Qingdao Tennry Carbon Co. who have been in business since 2006. They make carbon arc furnace electrodes and other graphite products, their current website shows no sign of this style of crucible at all. This is the same crucible before it had all the graphite burned out and began to craze on the surface. Edit: The original finish was a coating of graphite.
No derailment here. All of the crucibles I've ever gotten from them have been covered with what I presume to be graphite. Its gray in color, has a bit of a greasy feel although not really greasy, and makes your hands dirty. Definitely not what I'd consider a glaze in the normal sense. Nothing to worry about with the color changes as far as I can tell. It's probably a predictable change in the material as a result of heat. If two crucibles went through identical treatment their color changes would probably be the same. I've never tried to contact them before but I have it on good authority that they're knowledgeable and responsive, so we'll see.
10 or so melts then failure or evidence of cracking is BS. I've fired one of mine way more than that and she still looks brand new. Let's see what they say, just be prepared for the finger pointing right back at you! It's never the products fault.
I baked my crucible (in my home oven) to 250°F for half hour and every 15 minutes I raise the temperature 50°F stopping at 450°F (after 2 1/2 hours). I let it soak at 450°F for half hour then turned off the oven. Next day (12 hours later) I took it out. A few days later I set it into the furnace empty for about 20 minutes. I got it glowing a dull red then just shut down the furnace and left it in. This is what I considered conditioning. It was slightly glazed on the inside and highly glazed on the outside. The glaze was still there after the oven, but no longer there on the outside after the conditioning. (My crucible was glazed on the inside and out.) The crucible took on a dull gray. After seven aluminum melts, I have found the inside upper 20% portion of the crucible is losing its dark graphite look and turning to greyish clay colour. I have been scraping my sidewalls only during melts and after melts when I put the crucible back into the furnace, with a stainless steel spoon. At room temperature I can just peel out everything in the bottom. Of the seven melts, I only took one of them to the brim. The rest of them I only filled 80% full. Maybe this is where the 20% inside top failure is occurring. Here is interesting videos to me.
Those videos were very interesting. Did you notice that the failures were mostly from mechanical mistreatment with a few chemical related. No mention of thermal shock failures. I still use a six year old A6 Salamander that has seen upward of 250 aluminium duty melts. The first 100 or so I had some direct flame impingement from not having a tall enough plinth. It is loosing the glaze now at a faster rate so it might be time to retire it. My normal routine is to charge the pot, start the propane burner on the lowest pressure that will sustain a flame while I setup the pouring area (three to five minutes), then ramp up the burner. After the pour, I inverter the crucible on a cast iron grate to let is cool. I've treated the A10 Salamander the same way with no problems. (That was my 'fair maiden" comment.)
Very interesting videos Matt. Your Google-fu is much stronger than mine. I'm beginning to think that outside condition of the crucible surface is what marks the mechanical decline. I'm also starting to suspect oxidation as the primary enemy. These type crucibles, specifically whatever the formulation and coating of Legends's Budget Graphite Crucible, just don't have the glaze properties to handle the punishment I'm dealing out over any considerable period. And once that glaze is breached its just a matter of time, particularly under the same continued conditions. And although I'm not exactly utilizing scientific methods to tune my burner I don't think I'm doing anything extraordinary. I'm thinking that the amount of air I'm running to acheive the temps I'm looking for is exacerbating the problem. That may speak to the capacity of my furnace both in terms of size and efficiency (uninsulated) but that's another rabbit hole which I'm not going to go down today. Ultimately I think I'm just pounding the crap out of cheap crucibles. Meanwhile I have their Salamander Super A waiting in the wings. On the furnace clean-up front I did a burn yesterday and got a majority of it off the walls, primarily by just letting it run down the sides. None of my designs for easy bottom cleanup worked very well so I've got a shit-mess down there. It's colder down there and the metal doesn't move so well, and even when the burner is off there's no way to effectively mechanically scrape it while it's hot. Access for hand chiseling is out so it looks like I may have to go pneumatic (shudder). Fortunately my walls are an inch thick so I'm at least hopeful that the bore will hold up. We'll see. Pete
Pete, When I was using my furnace as a bulk scrapper (no crucible) I placed a 4 x 4 x 1/2" steel plate with lug nuts welded on as legs to keep the drain hole open. It gets cherry red and radiates the heat downward. You might try that to clean out the bottom of your furnace.
I too charge before, but will start giving it a bit of a warmup. Up to now, it's been light the fire and hit the blower! whoopa
Concerning cleaning out the furnace: Is there any chance of tilting the furnace and training a weed burner into it and having that melt out the Everdur? I am guessing there is a good reason this won’t work. But I had to ask. Denis
I have a couple of bronze splatters inside my furnace on the floor... When it gets up to temp, the bronze just remelts. They just kinda scrape and scoop up off the floor. Not really hurting anything in small amounts.
I actually got several lbs out that way right away. I laid the furnace on its side and left it run wide open for about an hour as shown in my first post. That was after it had been running for at least 2 hrs before the event. But, yaknow, I'm finding that "wide open" causes my furnace to run colder at the bottom so it may be worth another try laying it back down again at a lower burn rate. It's clear to me now after running the furnace with no crucible in it yesterday what the dynamics of excessive air look like. I could clearly see that my target in the bore was black when I was running at higher air volume but reddened up when I turned fuel and air down. Unfortunately that's also the exact area where there's a big'ol chunk of metal hanging on tight. You've prompted me to give it another try. I'll use my oil burner as I doubt a weed burner would bring enough to the party. Right on man. Whether it's aluminum or bronze, and I assume iron, small stuff needs to be loaded into the crucible first with the lid open, not loaded through the lid of a running furnace. It's surprising how large of a piece will get blown off to the side or even right back out of the exhaust hole, tongs or not. Or loading over-length pieces that hang over the rim. It makes for a stupid mess. Ask me how I know. Pete
Pete, I am thinking that a weed burner just might have enough heat to get the job done. It gets up to around 3000° on a decent burner. And your bronze melts a thousand degrees lower than that. It isn’t so much the amount of heat that it would produce but I think that as you pointed out the direction of flame is very important and you could direct it where you want it a lot better. I think if you directed the flame somewhat tangentially to the side wall and induced a swirl you might very well be able to press that flame right down into the bottom of the furnace where it needs to be. I have a feeling that your furnace will end up in a lot better shape that way than if you try to chip it out. And if the process fails after a half hour or 45 minutes of burning you haven’t lost very much. You may need to set the tank in a larger container of water to prevent freeze up of the propane tank. Denis
I have all of the equipment to do it as you suggested. You seem pretty confident about it so I'll give it a try.
The flu I have, I have been getting castings made for me for over 20 years by foundries. The addiction is I like burning shiiit. Pushing my education and giving back to students at the same time is part of the the addiction. The course I teach covers the advancements and introduction into the Iron Age. The videos that I provided were pushed from the manufacture to my educational facility. In most situations it is a lost art with automation and safety restraints. The course I teach has about 10 hours of foundry education. Sometimes, I'm considered the nutty professor that actually will show a video of me pouring metal at home with good safety. Like so many other trades, I just like to keep this one alive!
I am not THAT confident. But, I use a decent burner to "set" Satanite in my furnace. I have been surprised at how much heat it generates. I can get the Satanite on wool up to bright red in only a few minutes. Obviously, your furnace has more mass and the bronze has some additional mass. I sure would be tempted to give it a try. Most likely I would try to mount the burner on a holder of some sort that would direct the flame in the desired orientation and expect it to take 15 to 30 mins to start seeing metal drip. I have found that there is some fiddling with the position so that exhausting gas from the burner does not interfere with the air (oxygen) supply of the burner. That will all become apparent as you try this. Good luck! Hopefully you can record parts of your adventure on video. Denis
