View Full Version : How hot will the outside of a kikn get?

Tom Clifton
02-06-2012, 06:15 PM
More of an academic exercise. Empirical testing says it it is hot, don't touch it...

Glass notes has a nice section about layered furnace construction and the combined efficiencies of (per inch) hard brick, soft brick, comig up with a total number and popping out a btu/hour loss number per square unit of furnace surface area. The intent of the exercize is to show that adding a layer of frax to the outside of the furnace will save you xx% operating cost

Enter the inquiring mind...

My assumpton is that there is a law of dimishing return on insulation. Assuming the thermal gradient through a uniform insulation layer is logrythmic. - if you plot the temp defference every millimeter through a layer of insulation it would be a log curve. Gets a bit hinky when you traverse through layers of FireBrick, IFB and Frax, but all in all it should be log. (I think).

The presumption here is that you have to square the thickness of the insulation to double the insulating valur - or is that inverse??

Back to the temperature gradient. Furnace design is based on a number of give/take factors. The cost of operation, the cost of materiels, durability and ease of use. I suppose you could make a spherical oven 100 meters in diameter with no openings and melt glass for a nickel. Of course, you couldn't get close to it, nor could you get the glass out. Never mind that you spent the national debt buying the materiels.

Why all this blathering??? On the assumtion that given the surface area on the hot side, the surface are on the outside, the thermal efficiecy of the insulation of the furnace in loss per btu/hr/m^2 (or something) I'd believe you can calculate the temperature of the outside of a furnace in a 100 degree ambient temperature when the inside is 2200degrees.

The whole point here is that glass insulation is much less expensive than frax, and is there some point where it makes sense to use cheap insulation as you would never get your money back buying the expensive stuff..

Now - the last question/.statement. Isn't this better than discussing the merits of 'organic art'?

Richard Huntrods
02-06-2012, 11:36 PM
Just wrap a big copper coil around the thing and be done with it. ;-)

Hugh Jenkins
02-07-2012, 01:45 AM
Tom, lots of good thoughts here. One of the things that happens when you change materials is a barrier to heat transfer. If the new material is a better insulator than the previous layer, a new "hot face" is created at the interface. So there is not a nice smooth geometric reduction in temperature from the inside to the outside. The best high temperature insulators like frax do not work any better than much less expensive mineral or glass insulators at their appropriate temperatures. Layering works out to be much better and much cheaper than a lots more fiberfrax.

What I consider to be an urban glass myth is the idea that you can build so large with very thick walls, that you lose more heat from the larger surface area than you save with more insulation. It just is not true unless you have a perfect heat transmitting material. Even hard brick does not compute to give this result.

So plan a wall with the best material for the calculated heat at each layer and you will get great results. One very underused material is microcell insulation. It is effective in the 1500 to 1800 range, and nothing comes close, but it runs $20 a square foot for 1/2" thick, does not mould like frax blanket and does not like moisture. Great for dry flat wall construction.

I like a wall made of 2" castolite, 4" of frax, 2" of mineral blanket or board, and 2" of fiberglass batting. I surface with insulating cement and you can put your hand on it anywhere.

Ben David
02-07-2012, 05:00 AM
What I consider to be an urban glass myth is the idea that you can build so large with very thick walls, that you lose more heat from the larger surface area than you save with more insulation. It just is not true unless you have a perfect heat transmitting material. Even hard brick does not compute to give this result.

This idea comes from design rules for insulating steam pipes.
There IS a concept of "critical thickness" such that convection losses from the larger surface cancel the insulation value of thicker insulation.

But when you scale up to the temperature differential of a furnace or glory...?

Google "critical radius insulation" - here's one good page:


And here's a link to a sample calculation very similar to a glass furnace scenario:


Tom Clifton
02-08-2012, 12:41 PM
Google "critical radius insulation" - here's one good page:

Good information. First knee jerk reaction was "what a crock" but then the realization of how a heat sink works....

Also - a very concise answer to my question provided by Henry Halem in his Glass Notes webpage

This is so much fun I may have to build a very small pilot wire furnace...

Richard Huntrods
02-08-2012, 12:49 PM
The Mark Lauckner wire furnace, esp. the 40lb unit is probably the most efficiently insulated design at this time. Even cooking at 2250F, the 'cold face' of the furnace can be touched with the bare hand. At blowing temp (2100) it's comfortable to put your hand on the side, and at 1850) it's warm.

Mark uses a combination of firebrick, then fibrefrax, then rock wool insulation (roxul, same as you can buy for the home) contained in expanded metal mesh (not sheet metal).

His video explains his insulation strategy.



Hugh Jenkins
02-17-2012, 06:17 PM
I have been trying to think out what it is that does or does not apply between the heat sink publication and our application. When insulating steam pipe or other process plumbing like hot water pipes, they go through a cycle of hot and cold fairly often. So when you start hot flow, the pipe and insulating material have to be heated before or as the heated steam or water can be delivered at full temperature. The absorbed heat is then lost in a cooling, non-use phase. that is where finding effective insulation with minimal heat absorption comes in and a compromise for insulation and mass might apply. Specific heat (absorbed) and transmitted heat (conducted) are different beasts.

When we (I) heat up a furnace, I don't much care about a few hours extra heating time if the eventual surface temp is kept at its lowest. This is a steady state situation that is different than periodic heating. But, glory holes are periodic heat cycle units and it would seem that they would be more similar to the steam pipe analogy. We generally agree that some mass makes them work better, but the absorbed heat is basically thrown away every time the GH is turned off.

Long use periods for a GH are cheaper by the hour than short ones. Initial heat up can be longer with brick but performance can be better over the long haul. If you want a fast heat GH and less absorbed heat loss then go with all frax. But, I will put my brick and board insulated GH up against frax after heating up any time.

Pete VanderLaan
02-17-2012, 06:45 PM
...and to continue the conclusion. I prefer the mass from brick because when I open the big door, I don't experience a sudden heat loss and I get to finish my work the way I wanted to, not the way I was forced to.