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IFBs vs Ceramic Fiber Blanket costs/value for low-fire wood kiln-build

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I posted last-year about wood-firing or propane firing for low-fire sculpture. The responses were really helpful and I've spent a good amount of time researching and understanding my options. But now comes the hard part: making actual purchases.

While I will be getting into specifics of design shortly, right now the questions I am trying to settle are more on technologies. Cost is a major concern.

I have the 21st Century Kilns book and have browsed through a few other sources (Finch, Olsen and Ian Gregory). Something keeps confusing me, though. It seems that a lot of people speak as if firebricks are going to be cheaper than other options, but my experience on pricing right now seems to be radically opposite, when comparing total insulation value.

First, for IFB's, the cheapest I can find somewhat locally would be $4 a brick for flats. A guy down the street who is moving and has tons of ceramic supplies is selling his pallet of IFB's at $2 a brick -- but I may or may not get in on that deal in time.

Keeping to a consistent metric for the insulation-aspect: most people seem to recommend a 9" wall with IFBs. That would work out to $51/sqft for surfaces with new bricks, or $25 for this special deal.

Now, for ceramic fiber blankets, my research indicates that 4" at 8# density would be roughly equivalent to 9" IFB in terms of insulation value. There are plenty of sources for very affordable fiber: ceramicfiberonline.com for example has 2300F 1" fiber at $1.48/sqft. Since I'd need 4" of that to reach the same level of insulation as the 9" IFB, that's $5.92/sqft. Even the highest price fiber blankets I can find work out to $15/sqft.

So, by my math, in my market, apples-to-apples new-to-new comparison: IFBs are 8.6x as expensive as ceramic fiber! Is there something I am missing here? How could anyone on a tight budget justify IFBs over ceramic fiber when considering insulation alone? Is the market changing compared to when these books were written or, am I getting some math wrong, or perhaps am I not finding well priced vendors?

Since with either IFB or Ceramic Blankets a mass-based castable refractory or hard brick interior layer would be ideally recommended for wood-firing, the fact that the IFBs are structural seems to not really yield any advantage at that level. And reusability is the similar on each. The only difference is durability, in which case neither are great performers relative to hard firebricks but the IFBs win -- but not so much that they will last 8.6x longer! or even 4.3x longer, especially if both are placed behind a hard layer and the firing temps are low-fire to begin with.

Now if it would be acceptable to just use a single layer of IFB's for the firing chamber, then I could see how the IFB's do double duty compared to having to create a rigid form over which to drape the blankets -- but then again, people often manage to get away with using things like old trash cans or hardware cloth to support ceramic blanket structures.

Also I've heard the argument on this forum that ceramic fiber blankets are too toxic, but I fail to see how that's of relevance in an outdoor-built kiln where the blankets are sandwiched between an inner and outer layer -- and if there is still relevance than it appears to me that, at least for low fire, there are safe blankets: ceramicfiberonline.com has a "biofibre" that is supposedly nontoxic and rated to 2012F instead of 2300F (I'd only be firing up to 1850F anyway).

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I'm pretty sure IFBs will last at least 4.3x longer than fiber in a wood kiln.  Plus they are easy to maintain.  Need to replace a brick? Replace it.  Need to replace piece of fiber? I don't envy it.  I used a gregory style flat pack fiber kiln for a year, it worked fine for a year, but the 2300 2 inch fiber was not in great shape, and that was using nice clean propane.  It's not structural at all, it needs to be pinned and supported.  It also tears very easily and mine got less durable with use.  My last firing a layer peeled off the wall and went into a bowl I was firing, that was a bummer, it was a section about 6 inches by 8 inches and was dislodged from the turbulence from the burner.

Another aspect is that IFB will hold heat longer than insulation.  The insulation isn't great at holding heat.  Handy if you want a kiln to be opened the next morning, but not so great if you want the look of a nice slowly cooled kiln.

Those would be my two concerns.  I would think hard about this because once you invest in the fiber kiln and find you don't like it, you've now wasted time and money when you could have done it better the first time.

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I appreciate your first-hand experience -- just the sort of advice I'm looking for.

My understanding was that the both IFB and fiber are best used as an outside insulation to a mass-based structure for more reasonable cool-down performance and more durable structure. In this case, not seeing how the fiber would degrade much (no direct flame/ash contact, maximum temperature exposure well under its rating, etc).

Do you feel that IFB by itself would be appropriate for a wood-fired kiln? Would single-course caternary be structurally sound enough? If so, I could conceivably justify the cost by using it, otherwise I'm simply not sure if I can afford to spend so much. If it needs to be double-course, then I can use cheaper used firedbricks or refractory and do fiber on the outside.

My direction, if I chose fiber, would be something like this kiln by Joe Finch: http://potteryandpaintings.co.uk/kilns/downloads/ClayKiln.pdf -- the fact that normal mineral wool is fine up to 1300F or so, I would tend to do less fiber than he used and more mineral wool, since the cold-side of even 2" of fiber will be likely well below 1300F if I'm only firing my sculptures to 1850F. By my calculations this sort of kiln would be one of the cheapest possible while still highly insulative and durable.

My other concern is budget as regards exploration -- even if a fiber kiln lasted only 50 firings it appears to me that it would be 1/8th the cost and represent a lot of exploration and learning for low investment, putting me in a situation where I could more confidently know what my needs were and understand the relative value of different options.


Note this is for unglazed, low-fire sculpture to a maximum of Cone 04 (and more like Cone 08 is fine).

Overall most every source for kiln building besides primitive kilns (which is really what I would build if I weren't so energy conscious) seems to be trying to either:

  1. high-fire (thus high temp demands on all components and more extreme expansion/contraction), or
  2. raku-fire (high btu, very fast, thermal shock to deal with, easy access to get work out).

Meanwhile I'm looking for a rather slow, safe low-temp bisque for thick work. I would feel all the engineering would change pretty dramatically if this was accounted for. I'm not actually sure how much fuel would be saved or better results would be achieved using low or no insulation compared to standard recommendations (9" IFB) not when I fire Cone 06 with a slow ramp and slow cool down. It could be that all this fuss about insulation in my particular context amounts to 1/10th of a cord or something much smaller than the amount of embodied energy and cost in all this high tech IFB/fiber/etc.

Personally, at low fire temps,  I don't see why I couldn't just use a simple mix of fireclay/sawdust/etc as an insulated cob of sort. It would seem to be that it would be easy to just have a mix that begins more refractory/massy for structure/durability and moved to a higher insulation exterior (vermiculite/sawdust/paper) and then a weatherproofing exterior layer. Some expansion cracking issues to design around, but seems doable. I'd rather not reinvent a wheel though if I see no signs of others using the same approach perhaps there's a reason.

I'm hoping to get something up quickly and cheaply and learn as I go.

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For wood fire box areas you NEED hard brick .

Most folks who build wood fired kilns use 100% hard brick due to the fact that they last.

In the  areas aways from the wood you could use IFB like K26 or K28. Youi will find in heavy use or higher heat areas K23 will spall over time.

You can use fiber to back up any of these materials for insulation. If the wood heat source is distant fiber can work as well.

In all the years here (in this forum and elsewhere ) I have seen folks try lots of things that are not standard either to save money or they have a better idea.The outcomes are predictable. 

Heck I'm one of them-I build a salt kiln with hard brick ,soft brick and fiber roof. I learned where the limits are. I did not try this to save money (except for fuel costs)

I did learn a bunch and after building that kiln and firing it 10 times  and thats after building 10 kilns so thats saying something.

Now the other factor is how much one will be using the kilns they build-once a year -every week or something more or less. You can dig a pit and fire clay but theres not a  huge market for sale items from that pit.

Whats the intended long term plan for firing?? Many factors to consider.

If you have time and are resourcefull  used hard brick can  be had cheap.I just saw a few pallets on facebook marketplace last month near me for 1.50$-to 2$ a brick depending on how many you bought .

Edited by Mark C.

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I don't think I would want castable for the interior of a kiln. It really doesn't form up as dense and integrated as hard brick. It would surely tend to crumble and disintegrate and fall on the kiln contents, imo.

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2 hours ago, Rae Reich said:

I don't think I would want castable for the interior of a kiln. It really doesn't form up as dense and integrated as hard brick. It would surely tend to crumble and disintegrate and fall on the kiln contents, imo.

Home-made castable is definitely a no-no, however commercial refractory castable is very hard and dense and durable. I've used it and seen it used on many salt/soda and wood kilns. In grad school we used it for the throat arch (back of the firebox) of our train kiln, and we've used a certain type for lining the fireboxes of the soda kiln at Lillstreet Art Center in Chicago for the last two builds, to keep the soda puddles from eating through the bricks. You just have to get the right type for your application. It's quite expensive compared to bricks, but it can save you in labor.

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