Repairing Refractory Cracks

Could be you fired the furnace a little too soon, although minor cracks tend to be common in my experience. Unless the cracks are such that the refractory can fall out, I'd try to ignore them. If not, you may be able to fill them to some degree with a wash of some kind, the kind used to line ladles. It won't hide them completely, and you'll still see irregularities on the surface, but it might restrict escaping heat to some degree, assuming your cracks are through and that's happening.

1/4" stainless shell? How did you come by such a nice housing?

Harold

Sodium silicate.

If they are contained inside a shell and are only hairline I would not worry about them..........sounds like maybe it was heated up a bit too quick or perhaps a bit too long at a time without sufficient cool down between initial heats and steam may have played a part in producing the cracks, easy to do if your not extremly carefull. Visit my website:

expressed are those of my wife, I had no input whatsoever. Remove "nospam" from email addy.

Hi Terry:

Sounds like you're rolling right along. I believe that cracks are a fact of life in these furnaces. While disturbing at first, after you make 20 or 30 melts, you won't even notice them anymore! Did you see any cracks in mine? I know they're there but I don't pay any attention. I remember how unsettling it was to have the paint on mine scorched for the first time after spending so much time building it.

I've read this somewhere. Take some uncured refractory and sift it through a fine screen, like a tea strainer. Make a paste of the resulting powder and work it into the cracks with a squeegee. Haven't tried this, maybe it works.

By the way, where did you get that refractory?

-- Gary Brady Austin, TX

To a very large extent they are a fact of life, and cosmetic. You can try patching them, but the nature of that beast is that they crack again, and it's not worth fussing about. Rather than wasting too much time, energy, or money on them, just put aside enough money in the "recast the lid" fund that you can do that without blinking when they get beyond cosmetic, which will also eventually happen.

Buy a bottle of kiln wash from any ceramics dealer if the cracks are larger try some kaowool and kilnwash

Gary, Nice to hear from you. The furnace shell itself has only a few minor spider lines at the base which don't concern me. But with the lid I'm more concerned about because I may not have properly cured it. I got the refractory from Able, a place I found in Houston that makes the stuff. They are on the web:

. In fact they also have the fiber blanket there at very reasonable prices. It worked out to about $1.00 per sq ft even for the 8 lb density. The 3000 degree refractory was $28.00/75 lb bag. We just drove down and picked it up. They have a number of insulating refractories also at $25/ 40 lbs. Unfortunately when I cured the lid, I kept it under a wet towel for several days before begining the drying process. I spoke to the tech guy down there before I did the furnace shell itself, and he said not to do that but to start heating it one day after the curing cycle. The stuff is really hard after a 24 hour cure. This I did for the furnace but not for the lid. I think I am going to finish vitrifying the furnace with this lid and then re-do the lid. I want to make my exhaust hole a little bigger anyway. Right now it is 3-1/2" and I think I want to go to

4-1/2". The furnace/burner seems to be working well otherwise but at this diameter I don't think I will be able to feed metal in from the top. I've also read that some who have melted aluminum OK could not do brass until they removed the top completely because their exhaust hole was too small. I didn't notice any cracks in your furnace. Did you have them?

Hi Terry,

There is a product out that is Fiber Frax in a tube. It is a ceramic high temp. refractory but in a paste form much like a heavy tooth paste. You can fill the cracks with it using a putty knife.

Randy Hansen SC Glass Tech. Scam Diego, Comi-fornia .

Ok...*checks*

How much is that per cubic foot?

Oooo, you did wrong. You'll never go back after using an insulating product...

The LWI-26 (Plibrico brand) that I usually use is 80lbs/cu. ft. 2" is enough to melt bronze in half an hour in my #6 size furnace. I could go faster if I had more fire...

I'll admit that I've never tried a hard rock type refractory, but me, my propane tank and my back don't intend to any time soon anyway!!

My stuff says a similar process, fire after 24 hours. I don't see any harm in keeping it moist or drying slower or waiting a few days before firing, it'll just be set harder (more of the cement having reacted).

RED FLAG!!! FYI, castable is *already* solid, and composed entirely of refractory materials anyway. it might sinter a little, but vitrify it will not. At least for a few thousand more degrees! Vitrification applies to clay products.

(Yeah, I've been reading up on ceramics engineering...)

That's about what I've got on mine - 8" dia., 9" tall inside dimensions. How big is yours? I'd expect a good #10 furnace for a 4 1/2 to 5" vent hole.

Bah, your scrap sucks. I never have anything bigger than 2" across, and that includes the copper wire bundles I melted a few weeks ago (WOW, pure copper is fantastic stuff... a shame it dissolves oxygen like a bitch!).

Well okay, I've got some big pieces of aluminum, but I'll break those down some day.

I think everyone gets cracks... just a matter of going several times faster than recommended. I mean, the burnout schedule for my LWI-26 states 50°F per hour until 1500°F (presumably to eliminate any chemically combined water), after which 100°F per hour to operating temperature. After the first burnout, any heating or cooling should be done at 200°F/hr maximum.

Now consider.....that's ten hours to heat the furnace "properly" just to do ONE bronze pour!

Mine's at temperature, melting in 15 minutes...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

See also "furnace cement" in the fireplace aisle at the local hardware store. Usually $8 per 64oz, gray paste. Don't leave it open or expose to CO2 gasses...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

My id is 10-1/2" and 14" high. There is 2" of refractory blanket on the outside and 2" of the hard castable (3000 degree). The shell is 1/4 inch stainless steel (scrapyard food service tank). I ran some more high heats today and also finally got some cracks in the furnace body itself. I'm getting used to the fact that these cracks are probably just a fact of life and probably what one expects when you run such a mass over such a wide temperature range. Has anyone successfully cast an fired furnace that hasn't cracked? In about 20 minutes the inside got up to bright orange-yellow heat. I don't think it will get any hotter than that though. I calculated my burner to be running at 250K BTU/hr. The outside of the furnace hit 140F in that 20 minutes. Right now, 6 hours later the air in the interior is at

900F, and the metal shell temperature is at 120F. The shell temperature peaked one hour after shutting down at 170F. I don't see how I could have babied the thing anymore during the cure and heating than I did. Using a kitchen range heater element and variac, I spent 8 days bringing the heat up slowly to eventually reach 800 degrees interior temperature before firing the furnace with the burner. And then I did short 5 minute low heats and slowly built up. If I were to do the whole thing over again I don't know what I would change. The insulating castables that Able had in stock didn't go up to the 3000 degree that I thought I should have for the hot face. I did seriously consider just using the fiber blanket alone as you suggested some time ago with an ITC 100 coating, but I found too many conflicting viewpoints on whether this was a good idea. I couldn't come to a conclusion about the long term robustness of the coating, and the I started to get concerned about a short temperature temperature time constant. I found a post in a casting group by 'Frugal Machinst' for whom I have respect and who had built two furnaces - one out of just hard refractory and another with a mix of insulating and hard refractory. He concluded that the hard castable alone worked better. The hard castable furnace heated faster and the outside temperature was lower after multiple runs. Of course there could have been some other variables that created his result. For instance, from the photos on his website, it wasn't clear that the burner entries were identical although they could have been. Maybe it is not only the thermal resistance that affects efficiency but also the thermal capacitance. And maybe thermal capacitance isn't all bad. Maybe the reflected IR in the interior of the furnace comes into play and the whole thermal mass doesn't have to be heated to raise the interior temperature. And then maybe the thermal capacitance becomes a benefit in increasing the time it takes the heat to propogate to the exterior and be lost. It's pretty complex. Has anyone really built and used a gas fired furnace solely out of Kaowool? How does it work? How many heats have have you done? How fast does does it heat up? I finally decided to split the difference and put equal thicknesses of both just to get going.

"Tim Williams" wrote in message news: snipped-for-privacy@corp.supernews.com...

Pretty much...

Well, my medium sized reverb doesn't seem to have any cracks, but they're probably hidden by the chunky, well used surface...

Oh, with so much refractory (what, 200lbs?) it'll get a lot hotter in an hour.

Not suprising, the kaowool is doing its job.

Tells you something about the characteristics - the kaowool insulates very well, holding in the heat contained in the hard refractory.

I'm also guessing where the castable ends and the kaowool begins, was only beginning to heat up. After several hours at temperature, the kaowool will be doing some work, and due to the relative conductivity of the castable, said interface will be about 1600°F at least (for say, 2300°F inside).

Your furnace would make a skookum pottery kiln...

Good heatup to 800, but how long did it take to get up to what temp?

Bah. On full power, my new crucible furnace gets only 2300°F at the end of a bronze melt (half hour). It gets a good 2700 around the tuyere though. Going to try cast iron some time soon, should get a bit closer to tops.

Not really any need for 3000°F rated refractory if you're not melting steel, remember...

Bah. It's a commercial product, and you're even using it for its intended purpose. If nothing else, you can reapply more later.

He also has a lot of it, and fuel too.

Well, what's your refractory weigh per cubic foot? Did you get a data sheet quoting conductivity?

Sorry, but duh...

You can run up 2000°F across an inch of the stuff, and it'll be hot, but it'll suck down the watts like NUTS as the stuff behind heats up.

That's why I get "only" 2300°F after half an hour. It'd go through the roof if I let it, but my metal is fully heated by then.

On the other hand, your furnace could be fed just one kilowatt and get a good glow inside after a number of hours. That's what would make it a good kiln, the combination of high thermal mass and insulation outside of it.

The idea here is then that the refractory only comes into play (i.e., gets a reasonable temperature across it) after a long period. That's why your outer temperature peaked an *hour* after shutting off - the initial 'wave' of heat propogated through it in that time.

See link above, except oil fueled as well. E-mail the guy if you want...

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Terry Mayhugh wrote: [thoughts about factors in furnace efficiency, etc]

Remember one of the most basic characteristics of home furnaces: most of the heat goes out the vent!! Don't worry about insulation around the refractory - it's most useful to keep the glowing-hot refractory a safer distance from your skin. You could have 2 feet of insulation and a cool-to-the-touch shell and your efficiency would be only negligibly better.

All you really need to care about is how fast your charge is melted. It's going to be really inefficient, period! (For gas-fired, electric is another thing.)

Bob

Tim, Answers to your questions below:

Refractory is about 140 lbs per cubic foot.

Refractory thermal conductivity (K) is about 8.38 BTU/sq ft/hr/In/deg F at

2000F

When I ramped up the temperature during the preliminary heat cycles I ramped up roughly 50F per 12 hours and then finally spent 24 hours at 800F.

As soon as it quits raining around here I'm going to ramp the burner up to full power and run for an hour to see if I get any further cracks. I don't have a pyrometer yet and so I won't be able to tell the interior temperature except for approximating it from the color of the interior. I plan to measure the temp rises vs time with thermocouples behind the refractory and at the exterior shell. When I get a pyrometer built I can also measure the interior temperature. My current thermocouples will only go up to about

1200F or so. Maybe, for kicks, I can come up with a math model of what I've ended up with.

Back to the cracks. After thinking about it more, I'm now wondering whether I might do more damage by trying to repair the ones I already have. After reviewing the log I kept of the curing cycle, I don't think these happened from improper curing or too rapid heat application. They may just be related to the thermal expansion of the material. The data sheet lists the expansion coeff as a function of the temperature:

220F -0.1% 1500F -0.1% 2000F -0.3% 2250F -0.3% 2550F +0.2% 2732F +0.2% 2910F +.6%

I probably got to the -.3% point during the initial firings with the burner when the cracks showed up. And the sudden transition between plus and minus expansion may be what created the ones in the furnace itself during my last high heat. The cracks appear to close up when the furnace is at high heat. The inner circumference of my furnace wall is about 31 inches. At 2000F I guess I would expect it to shrink by about .094". I now have three vertical cracks which are about .015" wide each and this accounts for half of the change. If the refractory isn't flexible enough to withstand this amount expansion and contraction during real use then it broke as it needed to break. If I successfully fill the cracks with an equally rigid material then I may force additional cracks to show up. What do you think? What I don't know is whether the particular material I used is especially bad in this regard or if it typical.

< When I get a pyrometer built I can also measure ,,,>

When you get a pyrometer built, I want to look at it!

temperature

Terry Mayhugh wrote:...

"... /In/ ..." ??? The conductivity goes *up* as the lining gets thicker?? Surely, that can't be right. Bob