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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 offline
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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 online now
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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 online now
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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 online now
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Calcium aluminate is considered to be a cement.

https://en.wikipedia.org/wiki/Calcium_aluminates
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