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Old 02-09-2021, 06:26 PM
Travis Frink Travis Frink is offline
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Furnace castable questions

I am helping my friend/mentor rebuild his furnace. We poured the inner layer/structure/chamber yesterday. It went pretty well (I hope). I Had a few questions regarding when to remove forms and how long to let it dry- and how fast- before bringing it up to temp.

How long do you wait til removing forms for large castings? This is about 500kg of 1600℃ Castable. Inner and outer forms are styrofoam. Pretty much uniform 10cm castable thickness for the whole form. Overall size (l x w x h) is 90 x 80 x 90cm. The company that made the burner system (30 years ago?) and provided the basic form for this casting said we can take the forms off the next day. It seems a little fast to me and others involved with this build but most of my experience is with concrete and lime plaster and I know itís a completely different material.

How long do you let your castable dry before bringing it up to temperature. I remember seeing or hearing that putting a lightbulb (100w incandescent?) inside the furnace can keep it warm and help it dry quicker. We are in the dry season. Temperatures are low but freezing is not a worry. Once again, the furnace makerís suggestion that it can be brought up to temp starting about a week after the pour seems too fast.

The previous version of this furnace melted very nice glass for 29 years. We are rebuilding it b cause cracks in the casting seemed to have expanded to the point where we were worried the whole ceiling would collapse. It was also a self build. The owner says he doesnít expect to last as long as this furnace but we want to do it right. Iím happy for the opportunity to help someone who has helped me along and to learn through doing. The decisions are not all mine to make but Iím trying to understand the process better for now and the future.

Iíll try to figure out how to upload some photos (from my iPhone) of the forms this evening when I have more time and patience to figure out how.
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Old 02-09-2021, 07:27 PM
Eben Horton Eben Horton is online now
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The slower you do all of these things the better. Especially dry out.
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Old 02-10-2021, 01:46 AM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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The manufacturer of the castable can provide you with clear instructions to all your questions
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Old 02-10-2021, 08:36 AM
Art Freas Art Freas is offline
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In general, cement based products get strong by curing, not drying. Make sure you keep them damp (per manufacturers instructions) and somewhat warm as long as possible. The strength is created by forming hydrated crystals in the material and these need water. Once it has cured dry it out slow to get rid of free water. Once this is done you need to keep in mind that there is still water in the material in the hydrated crystals. And some of these have a lot of water. So when you heat the furnace up it sheds a lot of water as the crystals dehydrate, heat too fast and this water will do bad things. Here is a link to information on hydrates https://www.britannica.com/science/hydrate
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Old 02-10-2021, 09:24 AM
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Pete VanderLaan Pete VanderLaan is offline
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Nowhere does Travis suggest this is cement based. The material that makes the castable harden is calcium aluminate. I would first put the light bulb in the furnace and see what the temperature does. Three days is more than adequate if the castable is fairly new. Old castables are unreliable since the aluminate has already shot its wad. You can add fresh aluminate to the mix but it's not that easy to find places that sell it.

If after the three days, it feels good and warm throughout, pull the forms. I'm presuming that this thing has burners so you could put a very small candle flame in through the port. Holding a mirror up to the exhaust will give you a good notion of how wet it still is since the mirror will fog.

1100 lbs of castable is a lot of material. If you can get the furnace interior up to about 175 F after a week, then take it up to 200F and wait another week. Keep watching the mirror. Keep it under 212F.
Patience is a virtue.
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Old 02-10-2021, 09:30 AM
Travis Frink Travis Frink is offline
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Thanks. Iíve done a whatís starting to feel like more than my fair share of concrete work and understand about curing vs drying and about not wanting to heat cast parts up past 100c until after they have been dried thoroughly.

The instructions I read on the castable is that itís ďok to pull the forms and start heating it on a slow schedule to 600c ď (with more minute details of course) but ďitís better to let larger castings dry naturallyĒ. No mention of what a larger casting is and I think most of their customers are casting things like industrial furnaces, incinerators, power plants and so on. Their advice really seems to go against what I know and have been learning about anything made with cement, lime, high temp castables or pottery. We are on a national holiday tomorrow so the castable maker isnít working.

I was hoping to get a sense for what other people have done with similar size castings for studio glass furnaces. Although the products are different, sometimes the experience of similar users is as valid as the engineers who are used to using the products for completely different applications. At least thatís what I experience here where people often only deal with the same scenarios all the time so they canít/wonít comment on anything else. Plus, if thereís a problem, theyíre afraid of being held responsible.

The whole casting had really heated itself up today (24hrs after casting) from the chemical reaction apparently. Planning to pull the wooden supports/reinforcements off and make forms for dampers (used to route flue heat to annealer or up and outside) and gathering port tomorrow. May wait til another few days to pull the actual styrofoam forms off the furnace casting.
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Old 02-10-2021, 09:32 AM
Art Freas Art Freas is offline
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Calcium aluminate is considered to be a cement.

https://en.wikipedia.org/wiki/Calcium_aluminates
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Old 02-10-2021, 01:11 PM
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While concrete take 28 days to fully cure and needs water added during that time, furnace castables don't. It is appropriate to keep moisture in the castable while it is setting. Furnace castings don't really gain strength to the best of my knowledge in longer terms. They do retain the chemically bound water until it is driven off. This must be done slowly to prevent steam expansion cracking the casting.

Concrete is very different than a furnace casting. Both indeed contain calcium compounds. Calcium aluminate is indeed used in mortars as well as fireplace mortars but the application is somewhat different.

I have made up what we used to call Block mix which essentially was a 1/3 cement , 1/3 vermiculite and 1/3 tenn ball clay, plus a mess of water. We used to mix this on a tarp on the studio floor and we used it as an exterior insulating mix that was quite trowelable. That was handy since it could be vacuumed easily. It was not intended to be a hotface casting at any time. It was about six inches thick.
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Old 02-10-2021, 02:40 PM
Art Freas Art Freas is offline
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Castables need less but they need it. The water allows the dehydrated predecessor to form hydrated crystals. then when you heat the water is expelled from the hydrate but the crystal lattice remains. Otherwise the bags would cure without opening at the same rate as the castable with water added.
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Old 02-10-2021, 05:39 PM
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I tend to advise people on what is going to work in their applications based on fifty years experience. Your casting experience is done in less than three days in my experience and often less. This amount is a lot of goop, presumably fresh.


I'm not trying to score any points with Travis. We go back a long ways. The bags? Indeed they will cure. A lot of us know that. Avoid it .
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Old 02-10-2021, 05:46 PM
Eben Horton Eben Horton is online now
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Bags sure will set up on their own. Here in humid Rhode island, I will wrap any left over bags that are open or not opened in shipping plastic to make them air tight. Otherwise it will go bad.
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Old 02-10-2021, 06:21 PM
Art Freas Art Freas is offline
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Quote:
Originally Posted by Eben Horton View Post
Bags sure will set up on their own. Here in humid Rhode island, I will wrap any left over bags that are open or not opened in shipping plastic to make them air tight. Otherwise it will go bad.
That is the humidity adding water. These materials are hygroscopic until you use them. In a perfect water free environment they would stay good for a very long time.
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Old 02-10-2021, 06:46 PM
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Quote:
Originally Posted by Art Freas View Post
That is the humidity adding water. These materials are hygroscopic until you use them. In a perfect water free environment they would stay good for a very long time.
***
That's correct. I move mine to airtight containers as soon as they come in. So will type 1 +2 cement, so will potters plaster.

What's your point?
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Old 02-10-2021, 08:18 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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Respect that steam. I cast a door for a furnace made of 2” angle iron one foot square, with maybe a 1,5 mm plate laid in facing out. I cast the door and dried the whole thing in the annealer first right under 100 C 24 h then maybe 200 c for a day. It ran sideways on two 2” angle iron rails with an air piston pulling it and a counter weight closing the door.
I mounted it on the (tank) furnace, and lit the gas low- the furnace was in a quite small room- 440 sq ft or so, and after the furnace reached about 300 C there was a tremendous explosion in the room. In the micro seconds you experience this, you think gas explosion, but no it was the castable- it blew all the castable into the furnace and blew out the flame, the door stayed in the rails but buckled the angle iron rails outwards by the recoil-2” iron...had it come off the rails it could have killed somebody. As it was it just scared the hell out of me.
I can only agree to the above suggestions. I think a 100W lightbulb is a vastly underrated effect fo 500 kg of castable, Id put in a 2000W fan and monitor the temp with a controller.

Last edited by Michael Ahlefeldt-Laurvig; 02-10-2021 at 09:33 PM.
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Old 02-10-2021, 08:29 PM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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As anybody knows thats knocked down an old furnace... the castable sure cures under repeated heat cycles - the hotter the top temp the harder castable
Well it probably sinters

Last edited by Michael Ahlefeldt-Laurvig; 02-10-2021 at 08:35 PM. Reason: Last comment
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Old 02-11-2021, 06:26 AM
Travis Frink Travis Frink is offline
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We pulled the forms off the casting and it looks like it’ll work. Unbeknownst to me, another member of the team phoned the maker and confirmed it was ok to pull the forms and get it drying out. It sounds like we’ll be waiting on the drying process and even trying to speed it up. I’m not the one making the final decisions though.

Michael, thank you for that story. I was worried about moisture but didn’t know it could be that dramatic. I would’ve needed a new pair of pants! And, I had to put a little extra effort into swinging the 5.5 kg hammer when breaking up the old furnace. Just the weight wasn’t getting the job done. That surprised me cuz I’ve broken up a lot of concrete with a lot less effort using that hammer.

Last edited by Travis Frink; 02-11-2021 at 06:53 AM. Reason: Can’t type with my opposable thumbs
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Old 02-11-2021, 06:49 AM
Travis Frink Travis Frink is offline
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Pete,

The castable was all fresh. Mixed in an open top “mortar mixer” (sorry, don’t know the proper name in English). My mixer has been “well used” over the years and doesn’t scrape the sides and bottom too well with this dry of a mix so I got to work all that into the mix with a shovel and scraper while the mixer was running. Reminds of the good old days before OSHA made everybody play safe. Don’t try this at home kids!!! Didn’t lose any body parts but I haven’t been this sore in a LONG time.

I recall you mentioning that clay-cement-vermiculite mix before. How effective was it as insulation? Do you think adding some sawdust would create more air pockets and increase the effectiveness of the insulation? I have a big pile of “red clay” that used to be walls and low doorways from my house and the old barn I took down. This clay is supposed to do quite well with heat and is what was/is used for the long wood fired “climbing kilns) that the local potters used to fire their work. Some of those kilns are kept at 12-1300+ for days to a week while firing and are used for years and years.

I really miss the beauty of old New Mexican clay and earth architecture. I think a furnace made with a little of that feel would be nicer to stand in front of all day than what I work with now. Also, after using earth traditional earth walls and homemade clay-lime plaster on my house, I think I am comfortable working with a trowel- and I seem to have forgotten how badly my elbows and shoulders hurt after doing all of that plastering!

Last edited by Travis Frink; 02-11-2021 at 07:00 AM. Reason: Memory lane
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Old 02-11-2021, 07:06 AM
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Castables, for the most part, don't sinter, they melt. Most 90% or higher alumina castables can be sintered since they contain so little calcium. They are more china clay and alumina. When I look at a castable, I have long ago been advised to D-rate it 15%, so I do. Calcium does lower the melting temperature after all.

I would not add sawdust to a block mix, it is flammable after all. Vermiculite, which you can obtain pretty easily from a greenhouse supply is the go to material for block mixes. That ratio I quoted was in weight, not volume. It is really cheap to make your own and has lasting strength. We found just throwing all of the parts on a tarp on the floor and then lifting corners of the tarp back and forth mixed it quite well. We shoveled it onto the crown. Our exterior insulation was about 5-6 inches thick on top of a mizzou (60%) castable at the time. Great insulator!
Steam is a killer. When water converts to steam it has a massive expansion, something like 270 to 1 . Fire departments love it because it absorbs BTU's so well. Just open a door, swing your preconnect around once on fog , close the door and move on.
On a casting? It's the fastest way to destroy a good casting I know of. The mirror and looking for fog is your friend.

I miss New Mexico too.
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Old 02-11-2021, 09:41 AM
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"Steam is a killer. When water converts to steam it has a massive expansion, something like 270 to 1 . Fire departments love it because it absorbs BTU's so well. Just open a door, swing your preconnect around once on fog , close the door and move on."

Amazing how well that worked, we did that a lot. Our first run district only had hydrants in about 70%. We did a lot of tanker relays.
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Old 02-11-2021, 10:31 AM
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Quote:
Originally Posted by Michael Ahlefeldt-Laurvig View Post
I can only agree to the above suggestions. I think a 100W lightbulb is a vastly underrated effect fo 500 kg of castable, Id put in a 2000W fan and monitor the temp with a controller.
*****
I start really slow. It's touchy feely for me on this. I run 100 watts and it's remarkable how fast the casting warms. The moisture in the casting transfers heat really well. I wait until I feel it on the outside, not just the inside. 100 watts for at least three days feels right but the proof is in the feel. Once I'm done with the light bulb, I go to a venturi on a candle setting with the door wide open. Gradually, I close the door and keep feeling the outside of the crown.

I think it was Kenny Pieper who taught me about the mirror and the fogging. That was years ago.
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Old 02-11-2021, 11:36 AM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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Yes but a domestic heating fan has a high temperature safety fuse that triggers around 50C so there is no risk of overheating the castable (boil temp)
One might speculate that burning a small propane flame in the furnace might actually be inducing moisture to the materials since water is a byproduct of gas combustion- at least while the materials are below boil temperature
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Old 02-11-2021, 11:42 AM
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well, it's certainly true that water is a by product of combustion but I doubt it is additive in the overall approach.
Experience tells me to go slow when driving off water. I can't recall where I've ever even seen a 2000W heater. 1200, 1500 yes but they're ungainly. I put whatever i'm heating with inside the furnace.

Both approaches would work.
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Old 02-11-2021, 11:55 AM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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We have 220V here, almost all little heaters are 2000w. They typically weigh 10lbs maybe, but 1000W would do the job anyway
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Old 02-12-2021, 10:48 AM
Travis Frink Travis Frink is offline
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Quote:
Originally Posted by Pete VanderLaan View Post

I would not add sawdust to a block mix, it is flammable after all. Vermiculite, which you can obtain pretty easily from a greenhouse supply is the go to material for block mixes. That ratio I quoted was in weight, not volume. It is really cheap to make your own and has lasting strength. We found just throwing all of the parts on a tarp on the floor and then lifting corners of the tarp back and forth mixed it quite well. We shoveled it onto the crown. Our exterior insulation was about 5-6 inches thick on top of a mizzou (60%) castable at the time. Great insulator!

...

On a casting? It's the fastest way to destroy a good casting I know of. The mirror and looking for fog is your friend.

I miss New Mexico too.
Thatís a lot of vermiculite if itís all you weight. No wonder itís such a good insulator. Did you mix it dry? Was there an ideal consistency? Soft enough to spread but less water is better?

The clay I have and used for making and repairing walls usually needed a few hours to days after mixing before it was thoroughly ďwetĒ and ready to use.

For the ďmirror and fogĒ check, do you place the mirror inside the warmed furnace and look for it to fog up (casting still damp) or not (casting hopefully dried out)? Or do you place it just outside the door/gathering port? I searched here and googled it but didnít find a good answer.
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Old 02-12-2021, 11:27 AM
Michael Ahlefeldt-Laurvig Michael Ahlefeldt-Laurvig is offline
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Travis if you put it in the furnace it wont condense , will it?
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