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Old 01-11-2011, 01:40 PM
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Default Castable vs firebrick...which is tougher

I was under the impression that firebrick would be tougher than castable, due to it's controlled curing during it's manufacture. That because of the controlled curing and perhaps the aggregate inside, that it would last longer, and perhaps due better with the thermal shock and expansion. Can some elaborate on this?
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Old 01-11-2011, 03:44 PM
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Default Re: Castable vs firebrick...which is tougher

Castable is essentially the same in content as firebrick. The aggregate used is fired,crushed, firebrick. The difference is that in our application the castable is not fired to a high temp. The material remains pretty stable until it reaches 500C. Between 500-1000 C lots of chemical changes take place and over 1000 it again is pretty stable. If you have a castable oven do not try to fire it really hot in an effort to attain the magic 573 C where quartz inversion takes place. The uneven heat rise is simply likely to cause stress and cracking, better to keep the temp below 500C
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Old 01-12-2011, 08:57 AM
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Default Re: Castable vs firebrick...which is tougher

Castable is concrete. My main objection to it is that it's as ugly as homemade sin, but it can make a workable oven.

You're correct to think it's not as strong as firebrick. Firebrick is fired to a high temperature, and is more like a ceramic block. That said, all firebrick structures are held together with a refractory concrete matrix of some sort, so the structural argument may not be entirely correct.
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Old 01-12-2011, 12:57 PM
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Default Re: Castable vs firebrick...which is tougher

In Australia castable refractory has largely replaced firebrick in industrial applications. The main reason , I think, is its ease of application. It is gunned (sprayed) on. When it needs replacing it is smashed up with hammers and a new lot gunned on again. There is no need to have specially shaped bricks using this method.
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Old 01-12-2011, 01:37 PM
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Default Re: Castable vs firebrick...which is tougher

Reasons = cheaper, faster, more profitable.
Tougher, better? what was bricks and steel, is now castable and graphite(not cheap, yet).
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Old 01-12-2011, 02:34 PM
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Default Re: Castable vs firebrick...which is tougher

Firebrick all have a higher density and heat storage capacity as compared to the majority of refractory castables. Materials designed for gun/spray installation are all primarily maintenance materials and generally do not have the structural integrity of a properly build brick structure. I trained a large number of folks on Allentown gunnite setups when I worked for AP Green. Use gunning materials for patching up a furnace, not building an oven.

IF one is set on using a castable, inquire for a high-frequency cement vibrator (rent it). IF one is available, check into "flux" castables - very high densities and strengths can be achieved, the closest approaching that of brick while using a castable. HOWEVER great caution is required during the initial heat up - it is very easy to build up steam pressure within the material and have it explode! (Not a desireable result). Also the water content added is critical with this family of castable refractories!!! The material looks pretty dry but start vibrating -> it flows. Stop the vibrator and if the vibrator is still in the material you can not pull it out.

High strength castables should be fine for our ovens - just do not expect them to store quite as much heat. If you only plan on one fire a week not a big deal. If you want pizza one night, back bread the next, and slow cook the following, use high heat or super duty firebrick. I also prefer one layer of blanket on the dome followed up by mineral wool (2 inches is nice). That reduces the heat flow from within the dome to the outside.

Regards,
Sam

Last edited by altamont; 01-12-2011 at 02:39 PM.
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Old 01-12-2011, 04:42 PM
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Default Re: Castable vs firebrick...which is tougher

Sam,

I thought ceramic blanket was mineral wool! Pleaseadvise.

John
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Old 01-12-2011, 08:29 PM
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Default Re: Castable vs firebrick...which is tougher

Mineral wool is a vague term that means different things in different places. It can mean anything from the correct sort of refractory insulation blanket, to domestic insulation that has organic binders and is completely inappropriate for oven insulation.
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Old 01-13-2011, 05:51 AM
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Default Re: Castable vs firebrick...which is tougher

DMUN is correct (and I thank you).

When I refer to "Mineral Wool" I am thinking of a material used here in the USA as abackup insulation; available most commonly as a 'board' but also loose and (rarely) in blanket form. It is not considered a refractory material.

Ceramic Fiber, on the other hand, is considered a refractory material and - in industrial furnaces as well as pottery kilns - often used as a hot face lining material able to withstand higher temperatures (but not physical or chemical abuse/attack), greatly reduce heat loss (compared to brick) and store darn little heat (useless to us as a replacement for brick for our hot face / domes). It is fireproof, has excellent insulating properties, and, as mentioned withstand high temperatures without degrading - perfect as our first layer of insulation around our domes. I mention "first layer" because, once you progress outward from there temperatures are lower yet and there are other materials that insulate better at those lower temperatures (why I recommended "mineral wool" such as Insblock-19). Get out even further someone could eventually even get to temperatures suitable for fiberglass insulation. But most home-owners (and even restaurants) probably would not bother with that - just continue with bulk "mineral wool)...

Insulation offers a great many factors and preferences, alone with ease of obtaining, ability to work with it, so the discussion could go on and arguments arise which I do not advocate. Being an engineer I tend to go on until my audience is sound asleep.

So, wake up, jump into the fun, and build an oven!!
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Old 01-13-2011, 01:07 PM
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Default Re: Castable vs firebrick...which is tougher

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