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Adding Refractory Insulation To Outside Of 2.5 Inch Kiln Wall


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My gas conversion kiln has a 2.5 inch wall so I'm burning a lot of extra gas because of the thin walls. I want to make an outer insulating layer that would be 3-4 inch thick to improve firing, slow cooling by reducing heat loss.

 

I have fire clay and perlite. Do I need to add mullite or a calcium aluminate cement; or can I use a different type cement in my homemade castable?

 

I have read over and over again not to use portland cement because it spalls and and sometimes explodes. Is this still true for the outer insulating layer?

 

Kast-o-lyte costs $116 for 55lb bag mullite is slightly less.

 

Any suggestions?

 

Jed

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Why not get a roll of fiber? It is more energy efficient than castable. Much easier to install.

I find it on ebay.You don't need the alumina in the castable unless you are doing soda firing. And then you need it on the inner layer. Alumina can get expensive.I have built several kilns using these recipes. they need to be at least 8 " thick if using only castable.

If using a homemade castable, the recipe (I posted it on here a long time ago) call for portland cement,

fire clay, vermiculite (avoid the type with asbestos if possible) sawdust can be used instead.

here are some recipes from Lili Krakoowski

castable recipe

 

In November 1981 CM Miska F Petersham, working at that time on Fiji Island,

had an article about A Castable Kiln.

 

He published three recipes.

 

Castable Kiln Body I

 

Alumina Hydrate 1

Coarse grog 4

Coarse Sawdust 3

Fireclay 4

Portland Cement 2

Vermiculite 2

 

All parts by volume

 

Castable Kiln Body II

Coarse Grog 2

Coarse Sawdust 2

Fireclay 2

Refractory Cement such as Sairset 1

Portland Cement 1

Vermiculite 2

 

All parts by volume. Petersham warns that the critical ingredient is the

Portland cement. Too little and the mix dries too slowly on the catenary

arch frame, too much and the castable sags at c., 10.

 

The stuff is mixed like cement.

 

Castable Insulation Body (Applied to outside of kiln:

 

Fireclay 4

Kentucky Ball Clay 2

Portland Cement 2

Sawdust 2

Vermiculite 3

 

Andrew Holden has this in his "The Self-Reliant Potter"

 

High Tempeature Insulating Bricks Fire to 1200 to 1300 degrees C.

 

China Clay 1 part

Sawdust 1 part

Mix with water. he uses homemade wooden molds

He adds that crushed old HTI bricks can be added. Shrinkage of bricks about

15%

 

He also adds taht "the potter Mike Dodd" built his kiln raw, "with a hot

face skin of unfired china clay, and sawdust bricks. He fired the bricks

when firing the kiln.

 

Outer Skin Insulating Blocks

 

Any ball clay 4.5 paarts

Coal cinders or coal ash 4.5 parts

Cement 1 part.

 

These are dried in the sun. He found wooden frames (molds) sanded, 6 x 9 x

18, and 9 x 9 x 4.5 the most convenient size.

 

His mortar is

 

Lime 10 parts

Sand 30 Parts

Cement 1 part.

 

This list for whoever asked, the other day, about making kiln building

materials.

 

Holden suggests mixing on a concrete floor or on a sheet of plastic

(assuming one does not have a mixer.) I have found that a largish

wheelbarrow works fine for concrete and should do for this. The barrow can

"go" to the building site to carry the dry materials, and then used to mix

the stuff in....

 

 

 

Lili Krakowski

 

Be of good courage

 

Marcia

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Any type of fiber material is going to insulate better than a clay/sand/etc castable. Alpine kilns has always used vermiculite (large size) with portland cement for their backup layer without issue. It's just enough cement to keep the vermiculite form settling. The easiest and safest fiber product to use is a mineral wool rigid board, like FBX Durablock. It's easy to cut and much easier to handle than fiber blanket, and while I would still wear a mask while working with it, it's not going to fill your space with awful fibers like blanket will. I used it as the backup layer on the door of my gas kiln, and it worked great. One layer would make a significant difference, but two layers would be great.

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Guest JBaymore

Jed,  remember that you'll have to unjacket that existing kiln lining if you add some backup insulation.

 

The cold face temperature of any refractory layer will increase as the heat loss off of its surface is diminshed.  The fact that the 2 1/2" electric kiln wall was exposed to the ambient air (through the stainless jacket) is what allows the manufacturer to USE that stainless steel in that application.  The temperature of the cold face is such in that case that the stainless will tolerate it for an extended number of firings (expected life of the kiln).

 

But if you place ANY kind of insulation over that outer jacket of stainless metal, now the temperaure that the stainless is reaching duiring a firing goes UP.  Depending on how effective that new layer of insulation is, that will determine how much that interface temperature (what used to be called the cold face) will increase.  It will not take much added insulation to cause the stainless to deteriorate a lot and eventually rot out completely.  And at the moment, the structural integrity of the inner brick lining is dependent on the inward pushing tension (compression) being created by that tensioned metal jacket.

 

Ceramic fiber is not "magic"  becasue of its insulating properties... iwhich is not too much different than IFB per inch.  It is mainly becasue of its low thermal mass per unit volume when compared to all other refractories.  You store less heat eneergy in the refractories when heating up the unit.  So if you go the fiber blanket route, do not compress the fiber too much.  That increases the density per cubic foot... and decreases not only the insulating qualities of the dead air spaces in the fiber... but also gives you more material (mass) to heat up in the firings.

 

Home made castable is always an option, but do not expect TOO much out of it compared to industrially supplied materials.  There is a reason that the commercial stuff is more costly. (BTW....... that price you listed for kasto-o-lite sounded quite high. Check around.)

 

The Portland versus Calcium Aluminate cement for the hydraulic binder in a home-made castable question is based totally on to what temperature the HOT face of the castable will be subject.  This temperature can be calculated for any given set of wall cross section and firing profiles ..... but if you are not comfortable with calculus.... it can't ;) .  So you have to use experiential knowledge based on observed results. 

 

As long as the Portland is a SMALL component of the castable mixture....... it can be used in a material that is subject to stoneware temperatures.  But it will not last as long in that service as the calcium aluminate as a binder.  You do not need much of a % of Portalnd to get significant binding/hardening.

 

best,

 

......................john

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WOW,,, Thank you very much.  I am always amazed at the wealth of knowledge that you give.  Marcia, it took you a lot of time to write down the recipes....Thank you.

 

John, I will be removing the stainless steel wrap and use the castable to stabilize the IFB interior.. I will be duct taping the IFB once I take off the stainless sheeting.  The duct tape should hold them in p[lace until the outer castable goes on.   I think I have enough Kaowool for one layer sandwiched between the IFB and the castable.

 

 

Jed

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Heres my suggestion

take off your stainless sheet metal.

wrap with two layers of 1 inch thick fiber(get it on e-bay or wherever you can)

use the stainless sheeting to rewrap fiber-you can clamp it with long connected stainless hose clamps sold at any hardware store-just add a few two footers or three footer until they are long enough to go around-forget the duct tape idea-tape will fail.

Cover the few inches of fiber showing betwwwen the stailess wraps with some found sheet metal.

This will work better than any homemade castable and will all be low cost.

Mark

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I would NOT use duct tape. It will stink when it burns up in the first firing. If you are set on using a castable. Brace the inside of the kiln walls. Build an outer shell the size or thickness of the castable you plan to use, and pack it with the castable. Make sure to use some type of insert for the peepholes and burner ports..whatever your design is. I still think fiber or fiber board as Neil suggests is far superior for insulating and less mass.

As John says the steel jacket MUST be removed. It you use fiber, you could reuse the steel and add to the hose clamp ties.

Somewhere in my pyrotechnic data the info says 1" of 8# fiber density is equal to 5 courses of hard brick or one or two of insulation brick. Mass and density still conduct heat.

Mark must have been posting at the same time I was. What he says.

 

Marcia

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