# Crucible size and surounding space size for furnace ?



## Majorstrain (Apr 1, 2009)

Hi Guys

I'm converting and old 9kg (20lb) gas bottle into a furnace.
I think I will use a #6 crucible size (4 1/2 - 5 in diameter) all be it home made from steel. 
If this is the right size then how much gap do I leave between the crucible and the refractory sides for efficient heating and circular hot air flow?
This is for a gas fired furnace.

Cheers
Phil


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## Mike N (Apr 1, 2009)

My furnace came with a 6" dia. steel crucible. It has a little over 1" clearance per side inside the furnace. I made up a 5" steel crucuble, I like it better & seems to melt my aluminum faster than the big one does.


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## Majorstrain (Apr 4, 2009)

Thanks Mike,
That's given me some dimensions to work with.
Cheers
Phil


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## Majorstrain (Apr 5, 2009)

> Hi Phil
> A good general rule is 2.5" to 3" of clearance between the crucible and furnace wall and 3" below and above the crucible
> to achieve good combustion if you are going to melt cast iron or bronze , you can get away with allot less clearance for aluminium.


Thanks Rob, excellent info, :bow: although it does bring up more questions in my mind. 

I have read that refractory makes a pretty poor insulator, so I was going to line the inside wall of the old gas bottle with the white wool type insulating material that is about an inch thick (not wool but a high temp material) then cast the refractory on the inside of that. The refractory is the commercial stuff. 

As a rule of thumb what would the minimum thickness needed to be to withstand melts for bronze of iron?
Although I do only intend to do aluminum at this stage, the info would be good for a larger furnace later as well.

The question comes about because if I use the correct combustion gap this will deem the final crucible size.

ps. The gas bottle has a 12 inch diameter and is 11 inches high (although 1 1/2 inches of that will be the lid) . I was planing to put a 3 inch exhaust hole in the top. No thoughts on the burner yet.

Phil


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## Majorstrain (Apr 5, 2009)

> who's furnace design are you building?
> Rob



Thanks again Rob, excellent info.

I don't have a furnace design to follow as such, I have just spent time looking at different designs on Google and picking out different bits I like, dimensions are a bit thin on the ground though. The rest is in my head (that dangerous ).
The closest to what I'm making is Tel's mini furnace.
http://www.homemodelenginemachinist.com/index.php?topic=4318.0

No guarantee that the furnace will work out, but I'm in it for the experience and had the gas bottle laying around.
If it does work, I'll post some drawings in the file section.
Phil


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## Tin Falcon (Apr 5, 2009)

Rob: You may want to consider a copy of _Gas Burners for Forges and Kilns_ by Micheal Porter. This has instructions for making a foundry from a 20lb propane bottle as well as how to make burners Also the US Navy foundry manual is considered a classic and recommend reading for foundry work. 
Building a gas foundry is not exacly rocket science but it is possible to have an unwanted launch if not properly executed work safe. 
Tin


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## Kermit (Apr 5, 2009)

WOW. The weekend isn't even over yet.

Good looking work for such a fast job. Very nice


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## RobWilson (Apr 5, 2009)

Thanks Kermit 
it may take the rest of tonight to clean up the oil i spilt all over the yard ???


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## Metal Mickey (Apr 5, 2009)

RobWilson  said:
			
		

> Hi Phil
> My furnace has a wall thickness of 2 3/4" and have had no insulation problems ,some manufacturers recommend 3" to 4" this may be to sell more of there product . I use ganister for my lining witch is a fire clay ,silica mix it is allot cheaper (£2.80 a bag 4 bags to make the furnace i am currently using) than castable but is a pain to use when making lids.
> 
> Rob



Hello Rob, can you tell me your supplier of both ganister and silica mix.....many thanks Mike (MM)


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## Metal Mickey (Apr 5, 2009)

Many thanks Rob. I'll check them out. MM


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## Majorstrain (Apr 6, 2009)

Thanks Tin and all,
I'll chase up those books and do a bit more reading.
Phil


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## Slangbela (Apr 6, 2009)

Hi Rob,
I saw from your mention of NE Foundry Supplies that you are in N E England and found that you are in Whitley Bay. I am only a few miles away from you in South Shields. I built an electric furnace with fireclay, grog and sand. Like your Ganister, the lid was not easy. I eventually reinforced the wet Fireclay with strands of fiberglass. This increased the wet strength of the refractory. The fibers may have melted in hot places bit I have not seen any signs of it. I use welded and lined S/S pipe as crucibles.
  I use it mainly for casting aluminium, have made an enlarged Gingery shaper, and I am now making a Mill. I have been casting on my own now for several years, Self taught and from what I have picked up on the web. I am a retired Engineer. What is Ganister?
It would be good to contact someone locally to swap ideas. If you are interested E mail me off line.

Colin


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