Crucible melting

[gregmueller]

I recently had a blower go out on a bronze casting furnace mid way through a melt. I now have a silicon carbide #70 crucible half filled with frozen bronze.
Do I dare put the crucible back into the repaired furnace and remelt the "ice cube of bronze" will if be too much for the crucible resulting in cracks shatters--etc?? Or is it no big deal??
Any one have the joy of this problem before??

Thanks

Greg Mueller
Bowling Green OH

[fritchie]

I recently had a blower go out on a bronze casting furnace mid way through a melt. I now have ........ Or is it no big deal??
Any one have the joy of this problem before??

Thanks

Greg Mueller
Bowling Green OH

Greg - Nothing like after-the-fact- wisdom, but my founder has had similar problem a couple of times, and he keeps several iron “ingot molds” on hand for just such occasions, plus if he has extra metal at the end of a pour. These are made of 3 flat iron sheets each, about 4 - 5 inches wide and 12 -15 inches long. A troughlike shape is made from the three sheets, with a flat bottom and sloping sides. Smaller iron rectangles close each end.

As far as your current problem, I’d be very leery of just trying to remelt. Bronze shrinks about 2% linearly on solidifying, and expands accordingly before melting. I’m not sure of the crucible diameter, but if it’s 10 inches, say, that’s a crosswise expansion of about 0.2 inches, probably quite a strain on the crucible.

Sorry I don’t have a good suggestion. Is it feasible to cut across most of the bronze with a torch? Hopefully, someone with more experience will have a better perspective.

[sculptorsam]

Is it feasible to cut across most of the bronze with a torch? Hopefully, someone with more experience will have a better perspective.

I know very little about the subject at hand, but to expand on your torch idea... Would it be possible to hand-heat, or even melt, a fair portion from the top, thus expanding the bronze upwards? instead of down and outwards if it were heated from the bottom? Makes me think of the problem of a bowl of wax left to cool. If you reheat it evenly over the bottom, the pressure will build under the solid top layer until it explodes. Anyway, just a thought.

Good to see you over here at the forum, Greg. Hopefully somebody can help you out.

Sam

[jwebb]

I tried to answer this yesterday, but that post seems to have gone astray. This happens quite often in the commercial foundry where I work. But we don't use Silicon Carbide crucibles; they are ceramic, and melting is by induction rather than gas fire. What we do is tilt the crucible to about a 45, and heat it very slowly until it starts to melt around the edges. Then you can crank it up. I'm not sure if this works because the metal expands more linearally than radially, or because the crucible expands with it. But it works. I haven't had this experience with the Silicon Carbide crucible thugh, and don't know how forgiving they are. But, given how expensive they are, and the cost of the alloy, I would try it. If you can pop the metal out of the crucible in one chunk though, and get it cut up into smaller chunks with a bandsaw or (?) I'd do that instead. As Fritchie says, most founders keep a couple of ingot or "pig" molds handy, so they can "pig out" the metal when something goes wrong. The ones I'm familiar with are cast heavy steel, with a negative open shape like a skinny loaf of bread , which will form new ingots for re-use. I hope this helps. Let us know how it works out.

[fritchie]

I tried to answer this yesterday, but that post seems to have gone astray. This happens quite often in the commercial foundry where I work. But we don't use Silicon Carbide crucibles; they are ceramic, and melting is

I think Russ cancelled his duplicate post, jwebb. He put up two by accident. I answered one post and you did the other at just about the same time. I’m surprised yours wasn’t shifted; I don’t know the mechanism.

On Sam’s idea of heating from the top, I suspect that won’t work, but if nothing else turns up, it may be worth a try. I don’t think the bronze would shift until it essentially begins to melt, and by that time , all expansion would have taken place.

And, although the silicon carbide crucible will expand also, that expands much less than bronze, nearly zero.

[sculptorsam]

I should have just kept going with my wax analogy. To reheat the wax, place the bowl half on the hot plate, half off. You effectively heat one side and allow the heated wax to expand/bubble up one side. As jwebb suggests, if you can heat it at an angle slowly, it would create a similar effect.

Anyway, I should really stay out of this and let the experts discuss amongst themselves...

Oh, and it was me that deleted the other thread. As jwebb had already duplicated his post over here, I didn't move it. Sorry for any confusion.

[Araich]

Could you drill some holes into the bronze in an effort to decrease the expansion problem?

[fritchie]

Probably two good approaches here, but the proof is in the pudding. I thought about drilling holes also, but didn’t post because I thought the bronze would be too thick for most drills. Couldn’t hurt, though; go as far as you can, and maybe in an X pattern across the center. That would give the bronze more space to expand.

On the heating and melting issue, it’s probably matter of heat conductivity of bronze versus wax, and the rate and place of applying heat. If you can tilt the crucible on its side and heat with a strong flame from the top, enough to melt bronze at that point so it can flow out, I think you could do this. I suspect if you just apply heat more or less evenly but slowly, expansion will kill you.

Let us know the result, and good luck! Any more suggestions, continue to post.

[ExNihiloStudio]

I've never experienced this problem myself, but everything I've read about the subject warns against leaving any unused molten metal in the bottom of the crucible because it will expand and damage the crucible when remelted. Since the crucible is half full I'd bet that it will break if you try to melt it normally. Worst case scenario would be for the metal to expand and crack the crucible without you noticing, it melts without leaking, and then when you grab the crucible with tongs the whole thing crumbles and spills molten metal all over the place. I'm assuming that the crucible is wider in the center which would prevent you from knocking out the solid metal. I know crucibles are not cheap but considering that you could damage your furnace again or seriously injure yourself you may have to destroy the crucible to salvage the metal and purchase a replacement crucible.

Metal expands with heat long before it melts so I don't know how a tilt would help. I've read about putting tight-fitting machined parts together by heating one part in a kitchen oven and cooling the other in the kitchen freezer to expand and shrink the parts to make re-assembly easy. That's only a temperature range of 30 deg. F - 400 deg. F, not even close to the 2,000 deg. F or so you'll have to get that bronze up to.

Drilling holes makes the most sense, but I think the idea is to create a path of least resistance which means the holes would need to be very very close together so the expanding metal can close the gap instead of pushing against the walls of the crucible. If you can do that you'll be very close to cutting out the metal without resorting to a re-melt.

Think it through before you do anything because the worst case scenario could literally kill you.

[Araich]

It may sound obvious, but long ago I welded similar or smaller steel rod to some mixed sized drill bits so that I had a long depth set. Some are 40cm long. They work fine and given the softness of bronze it would be easy enough (if boring) to drill out a pattern.

Myself, I would drill a full circle a few cm's from the outer wall, with each hole right up against the last, and perhaps a couple in the center.

[sculptor]

then torch the center of the bronze in the upsidedown crucible with oxy-aceteline ?

gauntlets and faceshield anyone?

-----entertaining vicarious adventure-------

Greg:
let us know what happens?

rod
sculptor (http://sculpture.alturl.com)

[gregmueller]

Hello
You are all appreciated for your insight-- thanks for your time--I am leaning toward a new crucible at this point. However I am considering propping it up on fire brick at an angle with the holes drilled and ingot mold trays under neath in an investment kiln where I can control a very slow temp by the hour on a control panel. See if it sludges out of there???

Sculptors helping sculptors---its a good world to be a part of

PS Iron pour here at Bowling Green State U Fri Oct 22 with George Beasley of Georgia State Univ if you happen to be traveling through our "swing" state.

Greg Mueller
Bowling Green OH

[jwebb]

You can melt bronze quite easily with an oxy-acetylene torch. I would hang the crucible from a chain hoist at an angle, upside down, over an ingot mold if you have one, and melt all or most of the bronze out with the torch. You might contact some founders in your area. I think this happens quite often, and is not that big a deal.

[jwebb]

Maybe I'm responding only to myself on this one - as everyone else seems to have moved on. But this has been roiling around in some recess of my brain, and the following has finally emerged: You had a certain amount of molten metal in this crucible, and then removed the heat source. Both crucible and metal cooled. At some point, depending on the alloy, the "solidus" temperature of the metal was reached and it began to solidify. As it did so, it shrank. It had to, it's a universal law of nature. Sure the crucible cooled and shrank also, and it has a different coefficient of thermal expansion (and contraction) than the metal. But that metal solidified inside the crucible, without cracking it asunder. If you now apply heat to both crucible and metal, and do it slowly and as evenly as possible so that there isn't a rapid thermal shock, the metal cannot expand before and during melting any more than it shrank during and after solidifying. I don't think there is a problem here, other than taking it up to temp slowly.

[ExNihiloStudio]

The problem isn't the shrinkage, it's the expansion. When metal melts the crystaline structure of the metal changes. The crystals spread out which is why metal expands when heated. Metal turns liquid when the crystals are far enough apart from one another to allow them to move around. When the metal cools the crystals move closer together and link up against one another. At the beginning stages the alignment can be very regular but later becomes more irregular as space becomes scarce. The regularly aligned crystaline structure is very strong so engineers try to design castings to direct the cooling properties of the metal to encourage efficient alignment of the crystals. The point I'm making is that the solid block of metal in that crucible is a very compact and efficiently cast piece of metal and it is very likely to press itself against the walls of the crucible when it expands. It's more compact than when it was first charged, hence more likely to damage the crucible when it expands. It's still the same amount of matter but the properties of the metal has changed a little. I don't think there is proof positive that the metal will have enough room to expand in it's tighly confined space. My ignots molds are made from heavy pipe with flat bottoms and it takes quite a bit of pounding to knock out the bronze once it's cold.

The worst case scenario I outlined below still applies and I personally would not run the risk. It's cheaper, safer, and delivers better piece of mind to just replace the crucible. I posed the same question to a founders' webboard and got the same answer - destroy the crucible to salvage the metal and replace the crucible with a new one. You can't nickle and dime this because it's too risky and the last thing you need one more thing to go wrong in a complex process that requires a lot of a lot of things to go right.

[jwebb]

The expansion is the reverse of the contraction or volumetric shrinkage that occurs during solidification. Both occur crystal by crystal. If the metal shrank and solidified in that vessel, it cannot expand more than it shrank. I've worked in a very large Investment Casting facility for 30 yrs, and this occurs quite regularly. Throw it away if you want. It's no skin off my nose.

[gregmueller]

This information has been incredible---I never thought I would get such in depth responses thanks
Right now I have the crucible tilting in a kiln bringing the temp up 50 degrees an hour --will let you know what the out come is

Greg

[fritchie]

The expansion is the reverse of the contraction or volumetric shrinkage that occurs during solidification. Both occur crystal by crystal. If the metal shrank and solidified in that vessel, it cannot expand more than it shrank. I've worked in a very large Investment Casting facility for 30 yrs, and this occurs quite regularly. Throw it away if you want. It's no skin off my nose.

jwebb - What you’re missing here is that the bronze went from liquid to solid as it shrank, and bits were free to move around. What had been, to exaggerate, a tall, thin shape of liquid metal slowly changed into a short, fat shape of liquid metal and then of solid metal, under the effect of gravity. As the metal is solid now, it can’t reverse the process and let some metal near the crucible wall move back up the top, the reverse of what happened before.

The solid will expand as a whole, subject to the source and rate of heating. And that short, fat mass of metal will exert quite a pressure on the side walls of the crucible, and less on the bottom. Pretty much everything said earlier is correct.

(And, the thread had gone sort of dead. Let us know what happens, Greg.)

[jwebb]

Must be nice to be omniscient, so you can tell others what they are seeing and what they're missing. Gravity acts on the metal whether it's liquid or solid, and has very little to do with it. And the coefficient of expansion (and contraction) operates volumetrically, in all directions, not just linearally. The metal solidifies first on the surface, in this case, because it's in the open atomosphere, and a disc-shaped skin around the top will be the only solid metal in contact with the crucible walls. From the top down it will take on an inverted somewhat conical shape, with a good deal of space around it. The majority of the shrinkage will occur as "pipe" or "center-line" shrinkage, which will be open at the surface. (The same thing occurs in an ice cube. You can look right down into it). IF it's heated evenly and slowly the crucible should easily withstand the pressure from that surface skin expanding. Melting will begin at the bottom, and if the crucible is tilted at about a 45 degree angle, the molten metal will come up to the crucible lip and melt away that disc-shaped skin. This is something I have watched, many times. I'm not just pontificating about it. Granted, this crucible material is different, but the principles are the same. If it doesn't work in this manner, I'll be the first to apologize. But all you will have lost is the crucible, which you are going to lose anyway, if you throw it out full of bronze. I sincerely hope this works for you.

However...This site has become a real drag. Instead of talk about sculpture, it's mostly a clique where a favored few pat each other on the back and exercize their huge egos by lecturing everybody else, on their own favorite subjects. The last time I tried to puncture a windbag or two, I was threatened with expulsion from the site. Well, don't bother. I don't want to be part of it any more. Adios, amigos. I have better things to do.

Joe

[ironman]

Hey Joe, Sorry to hear you're leaving this site, giving in to the windbags, whatever! You're a valued contributor and I for one will miss your insights and advice. Maybe you just need to take a break for a while, give it a rest and then come back.
Omniscience & pontification be damned!
Have a great day,
Jeff

[sculptorsam]

I'm with Jeff, Joe. Hope you keep coming back. I'll try to keep my pontification to a minimum.

Sam

[JAZ]

Me too. (Ditto Jeff's and Sam's views about your participation and the pleasure of reading different views)/ Especially about this subject, which I know absolutely zero about. Joe, what this site needs is a good variety of voices with different experience. And everyone needs to have a bit of patience with people who may have different views. One thing that makes this world interesting is all of the different perspectives everyone has. You, and Fritchie, and Sam and Jeff and everyone. We all have individual experience and views. That's a good thing.
JAZ
JAZ

[Araich]

Joe, I sincerely hope that you stay, your posts have made for some of the best reading around here...

[sculptor]

say it ain't so Joe

a little background...
Long ago and far away...I went to southern illinois university to study with bucky fuller-----much to my chagrin, i discovered that a seemingly endless number of minor courses taught by his acolytes( a 2 year journey) was required before i could take a real bucky class; fortunately, most rules are like single faced fences and the records were 1/2 paper and 1/2 housed in a punch card computer---so, i hit the stacks, and took all the prerequisites in 2 terms--by the time the university records cought up with the overlapping of prerequisites and sicced a dean on me, it was a fait accompli, I was studying with bucky, and had a good grade average--so, i feigned ignorance and was allowed to continue----bucky said it was "a good example of creative problem solving...albiet a tad unorthodoxed"....bottom line.....the next term bucky got pissed at the reagents, and left carbondale-----my young ego felt that he had walked out on me----so, i got pissed, drunk, high, and dropped out----took me a long time to realize that even old men have fragile egos and forget my dissapointment and remember the good times of us walking through the campus and his accepting my endless questions and helping me to find better questions....not a whole helluva lot of answers, just better questions....

Must be nice to be ...Instead of talk about sculpture,... few pat each other on the back and exercize their huge ... by lecturing everybody else, on their own favorite subjects. The last time I tried to puncture a windbag or two, I was threatened with expulsion from the site. Well, don't bother. I don't want to be part of it any more. Adios, amigos. I have better things to do.

Joe

so, Joe, (borrowing your phrase from an earlier conversation) "who the hell are you".......to walk out on me? get yer ass back in here and siddown

rod
sculptor (http://sculpture.alturl.com)

ps everyone needs an occasional "pat on the back"
pps----finishing greenhouse,cutting center drum for a 6 step 180º spiral stair--set saw at 15º and have a total 1º-2º over on 6 faces-12 cuts--small tweak-minutia, and closing in on the end...still wanna finish that and relay a small roof before taking a copy of mermaid to wyoming before the snow dumps on the bighorn mountains(po-folks like me tend to sleep in the truck on long trips)these folks been waiting long enough...early frost... moving plants indoors ...garden covered in tarps...chickens laying 7 a day now...sharing with neighbors...ordered 2 new husqvarnas for commission to carve a mushroom onna 15 ft pine----lotsa resin fouled the jonsereds.....how to clean pine resin off the saw?...fuel is getting damned expensive..dead heat on a merry-go-round.c'est le vie?

[bobcoon]

O.K., so with or without Joe...What happened to the crucible!!!!!!!!!!!!
My money goes on the notion it popped!

Bob

[obseq]

Joe,

Stick around! Between the differing perspectives on this thread alone, I've been trying to soak up every bit of information on a subject I know little about.

Like JAZ said, we need some opposing viewpoints.

[fritchie]

jwebb - Let me add my request that you rejoin the forum. You have been a valuable contributor, and I hate to see you leave. I can’t say I regret my last post because I still consider it accurate.

You may be right that this slug of metal had a hollow core from shrinkage, because it cooled and contracted from the outside in, but that doesn’t change the argument about expansion and danger to the crucible. A ring or band of metal will expand pretty much the same way as if it were solid - it simply can’t expand into the hollow inner space.

As someone posted, the proof is in the test and method of melting. The crucible either did or didn’t break.

[WBK]

I hate to see you leave as well. I'm one of the silent group that gets a heck of a lot from just reading here. Your posts have been most informative, your experience is very valuable to those of us just learning.

So what happened to that darned Crucible anyway? Did I miss something?

[gregmueller]

i am plopping down $160.00 for a new crucible--ain't worth the stress

Thanks all--i appreciate the insights

Greg Mueller

[ironman]

ALL OF THIS SUSPENSE AND ONE LOST MEMBER (JWEBB) FOR THE PRINCELY SUM OF $160.00, YOU'VE GOT TO BE KIDDING ME!!!!!!!!!!!!!!!!!!!!!
HAVE A NICE DAY,
Jeff

[sculptor]

ALL OF THIS SUSPENSE AND ONE LOST MEMBER (JWEBB) FOR THE PRINCELY SUM OF $160.00, YOU'VE GOT TO BE KIDDING ME!!!!!!!!!!!!!!!!!!!!!
HAVE A NICE DAY,
Jeff

AND
We still dunno if'n the damned thing actually broke
jeeeeeeeeeeeeeez(and lots of deleted expletives)
if that's the bottom line(sucks) the game wasn't worth the candle

rod
sculptor (http://www.sculpture.net/community/attachment.php?attachmentid=997)

[WBK]

Human behavior never stops amazinging me. It's complicated, but that's what makes it so fascinating. It was never about the $160.00 or the crucible. It never is.

...Hope JWebb comes back.

Bill