# Rebuilding furnace, refractory problem, losing too much heat



## torscot (Mar 5, 2016)

Rebuilding my Italian 2KG furnace. Had a crucible break and it caused catastrophic failure. The original element and refractory was a one piece unit. Unable to locate an orig. Italian replacement. A new Hardin chamber unit fits well, works well with the thermocoupler and controls.

Relined the metal cylinder with 2600 degree ceramic wool. (1 inch thick) Everything just fit perfect.

Here's the problem. It's not holding the heat. Can't get it up to temp. It's just radiating out through the sides, Wrapped a second inch of wool around the outside, Burned all the paint off the chamber. It's black not red anymore.

Was following the directions from the company I got the replacement parts from. Called him up, It should work! I got it to 1500 degrees after two hours, and that was as far as it would go.

Any suggestions from here about how to keep the heat inside. My supplier says it's all done right, but of course it's not working. Castable refractory cement? I am now stumped

Suggestions.

I use this for pouring 1kg silver bars.


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## Palladium (Mar 5, 2016)

Just a thought and it may not be your problem, but are you sure the elements produce the same amount of heat. 2nd, check to see if they may have given you a 220v element instead of a 110v if yours is 110-120v.


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## Lou (Mar 5, 2016)

Need some photos.


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## goldsilverpro (Mar 5, 2016)

Build a gas crucible furnace. Not that hard to do.


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## butcher (Mar 5, 2016)

Besides possible problems of the elements, voltage, ohms ...

Ceramic wool is a good insulation material in some instances, but not that great for holding, reflecting, or transferring heat. 

I do not know what type of furnace you have but you may be loosing the heat through the Kwool ceramic blanket, did it originally have soft firebrick or some other type of refractory material?

Refractory materials differ widely in applications and properties, hard refractory or brick hold heat well, but can take longer to heat up, they can also transfer the heat to outside walls of the brick (by convection).
My Mistake, conduction is the proper term here of heat traveling through the brick. thanks Goran for setting me straight.

The softer fire bricks full of air pockets, reflect heat well and do not transfer heat well, they also do not hold heat as well as a more dense brick will.

There are several types of castable refractories, they are normally softer materials, and can act more like the soft fire bricks having good insulating property, depending on type the can be denser to hold heat, or more porous for insulation, they normally use a thin hard coat to protect the soft material from damage.

Refractory materials are normally picked for several reasons their insulating ability, or the ability to hold and transfer heat well, many times boilers or furnaces may use several different refractory materials in their walls taking advantage of each of the different refractory materials properties.

The environment may also be a consideration for what type of refractory material would work best, some are more acid resistant, some more alkali resistant, some would be burned in an oxygen or oxide enviroment, and temperature is also a consern of which to choose...


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## g_axelsson (Mar 5, 2016)

butcher said:


> refractory materials differ widely in applications and properties, hard refractory or brick hold heat well, but can take longer to heat up, they can also transfer the heat to outside walls of the brick (by convection).


When your refractory material transfer heat by convection you have a big problem! :lol: 
You probably meant conductance?

Göran


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## torscot (Mar 5, 2016)

Thank you butcher, I definitely have the wrong insulator / refractory to help hold the heat in the center of the furnace. I called the company I got the parts from. The first thing I thought also was the wrong voltage for the coil. It's a 110V coil. That's what I need. He also said the 2600 degree ceramic wool was the correct insulation. The element is doing it's job. Its producing heat, and lots of it. But it's just going straight out the sides.

I'll photo the unit and post up the shots tomorrow. It's back in pieces, again.......

I have started a propane fired furnace. (Too simple) I am building it to fit a 4kg crucible using an old propane tank to use for smelting from ore. (that's why I had the ceramic wool) The electric one is just not made for that kind of job. For what I spent on parts to rebuild this one I could have had that one up and running I use the electric one for my pours of silver into 1kg bars. I like it because I can control the temp so well, and get a fast accurate pour into the mold

The original element and refractory were one casting, and held the heat inside very good. After an hour of running the outside "can" was only warm

I have looked at this product. (http://www.rona.ca/en/castable-refractory-cement-3-lb-p66895005) Packaging says resists up to 2200 dregees. I didn't open the container, but a shake showed that the particle size is fairly large. If I use that in there, it gets real final and if it doesn't do the job. I just wasted a lot of time and money. Butcher hit the nail on the head. I have to find the right refractory / insulation. Any thoughts on that stuff?

I borrowed a friends furnace to complete the job I was working on. So, the pressure's off. I no longer have to get it running "yesterday".

Thanks .
Rob.


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## torscot (Mar 9, 2016)

Here are the photos of my poor old furnace. I go busy at work and this was my first chance to get these posted.

To recap. Replaced the original one piece element casting with a Hardin element. Connection good, voltage good thermocoupler good and working. Replace bottom and tp flange at the same time. Wrapped the new element with 1" 2600 degree ceramic wool compressed as compressed as I could get it. Furnace reached a max temp of 800+ degrees and would not go any further. Heat could be felt 16" away. My parts supplier (very good, reputable) says this is the correct method. He's at a loss. I wrapped a second inch of wool around the outside, and it reached 1500 degrees, scorching the nice apple red paint dull black in process. Any help here would be greatly appreciated. I know there has to be something out there to stop the heat from radiating out and drive it in. The old unit I could pick up with bare hands at full operating temp (2010 degrees).

My propane furnace is almost done, but, I want, need this unit up and running too.

HELP! (yes, I know all caps is shouting, I am shouting) There has to be a way, and I can't find it.

Rob.


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## torscot (Mar 9, 2016)

One more shot. The makers plate on the back.


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## FrugalRefiner (Mar 9, 2016)

torscot said:


> Wrapped the new element with 1" 2600 degree ceramic wool compressed as compressed as I could get it.


I wonder if this might be causing any problems. Can you give us a little more detail (pictures?) of what you mean? What do you mean by "wrapped the new element"? It's probably just my lack of knowledge about this type of furnace, since I'm not that familiar with their internal construction. Compressing most insulations does not help them insulate.

Dave


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## torscot (Mar 9, 2016)

Insulation and compression of insulation. I thought that insulation should never be compressed because it is the trapped air that provides the insulating qualities. Apparently it's the ceramic that provides the insulation. I tried it both ways. As I have (had) a good supply of the wool. No heat was retained what so ever by having the wool uncompressed. I was following instructions from my supplier when installing it compressed. The can is 7" deep. I stuffed 9" of wool in there.

The element unit. If you look very close at the top photo. There is an inside porcelain cylinder, approx 1/4" thick, wrapped around it is the element in a chalk like matter, approx 3/8" thick, then an outer porcelain cylinder approx 1/2". The element unit is three pieces joined together. This leaves a 1" gap between this and the outside can for insulation. 

RL


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## Palladium (Mar 9, 2016)

I wonder if you could just sit the middle part in the base and fill in around it with loose perlite beads instead of Kaowool?


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## torscot (Mar 9, 2016)

Perlite beads. Just looked them up. 
"Perlite loose fill insulation is an inert volcanic glass expanded by a special heat process. The resultant lightweight product is a white granular material, which handles and pours easily. It provides a quick, inexpensive and permanent method for efficiently insulating masonry walls. Depending on design conditions, reductions in heat transmission of 50 percent or more may be obtained when perlite loose fill is used in the hollow cores of concrete block or cavity type masonry walls."

This thing will be up near the melting point of some types of glass. 1000C I don't know on that one I'll put it on the list to check out. Thank you.

I was thinking vermiculite? But vermiculite in what matrix.


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## Barren Realms 007 (Mar 9, 2016)

Where is your element wrapped at in relation to the arrows I have drawn? 

Is the element wrapped around the inside part shown by the red arrow or is it wrapped around the outer shell shown by the blue arrow?

You said you have replaced the top ring too. Do the old ring have as large of an opening as the new one does?


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## torscot (Mar 9, 2016)

Barren Realms, Red arrow. Element is against the inside porcelain cylinder. Element is 1/4" from the chamber. The top ring opening fits snugly around the crucible, and is then covered with the insulated lid.

Rob.


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## goldsilverpro (Mar 11, 2016)

For the gas furnace, I would use castable refractory with a higher temperature capability. 2200F is too low for many melting jobs. They make castable that can be used for 2800-3000F.


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## Lou (Mar 11, 2016)

Typically an insulating castable (or refractory wool if it's not tilting) is put in at 2-3" with an inch or so of monolithic refractory hard face that can stand the fluxes.


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## butcher (Mar 11, 2016)

It is hard to tell much looking at this on a post, or the pictures,without actually seeing the furnace or its construction, and never having one of these furnaces,or taking one apart, I can only make a guess.

it looks to me from one of the pictures there is a lot of heat loss at the bottom of the furnace, the furnace also seems to have an outer refractory wall, I cannot tell from the picture if it is actually a castable refractory, or just the ceramic blanket discolored, with the ceramic k-wool blanket inside of the harder refractory material, I suspect the furnace winding around the outside of the ceramic shell.

I would check the outer refractory material, it looks like it may have been a castable refractory, is the bottom in good condition?
Or
Was there a soft fire brick sleeve originally around the coil winding, possibly even a round flat piece at the bottom?


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## torscot (Mar 15, 2016)

Well, we can close this discussion. After some diligent searching, some quick Italian lessons, and some begging and pleading. The manufacturer in Italy is sending a complete factory replacement chamber / element / thermocoupler unit to a US distributor, who will ship it to me. The price was ridiculous, but they are a high quality unit.

I built a large propane furnace from scratch using ideas from backyardmetalcasting.com for less than 1/3rd what I paid for the parts for the Italian furnace. I still have to tweek it. It's drying right now, so there might be some posts coming on that one............So now I have something to smelt with and something to melt with. Now I have to make some $$$ with these things.

I did find one excellent product with remarkable thermal resistance. 1/4" is all it would take. Aerospace grade technology. Only around $5 grand to reline the furnace!

Thanks for the suggestions guys. I just decided to go with factory replacement parts. So, if you're rebuilding one of these things. Don't cheap out! It cost me good in the long run.

Any one want to buy a good almost unused 2 kilo Hardin furnace liner, base, top and thermocoupler?

Thanks

RL.


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## saadat68 (Mar 31, 2018)

Hi
In some videos in youtube they use a mixture of plaster and play sand and perlite for refractory:
I have 2 question: 
What is plaster? Here we have lime that use in building (wall) or sometimes in sculpture. Is it plaster?

Can we use green sand instead of play sand? 

https://images.persianblog.ir/517398_cAdSZo5f.jpg
https://encrypted-tbn0.gstatic.com/...Ll6VRYrSeWJ52V3KG-Uu_MgQkWDyhlF3lKFwFu7oGZaEt

Thanks


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## Owltech (Mar 31, 2018)

1" Kaowool performs MUCH better than 2" Kaowool compressed down to 1"


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## snoman701 (Mar 31, 2018)

Depends on the type of plaster, but yes, it can be just calcined lime. Or it can be gypsum. 

I wouldn't use any homemade refractory mixes for anything that you need to be reliable.


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