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Ceramic fiber downdraft


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I've been operating my home designed and built 36 cf ceramic fiber downdraft for 20+ years under the premise that extra mass at the lower part of the kiln would make the kiln fire better.  Primarily, I'm talking furniture.  I have 1 inch mullite shelves on half inch posts over the kiln floor (2x k26 soft bricks) and then one set of 4 18x18 3/4 " shelves.  The rest of the shelves are silicon carbide and advancer.

What I'm thinking about is how necessary are the heavy shelves low in the kiln?  Frankly, I'm tired of messing with kiln wash and would like to add 2 more sets of advancers.  I guess the only way to know for sure is to try it, but theoretically speaking what do you think?  

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I am thinking they are energy wasters actually. My guess is you have them there to store heat and slow the cool down, but Lots of fiber means less loss and really slow cool down if you choose to insulate as such and a thermally efficient kiln without all the mass to heat up. You can control the firing speed with power so no worries about firing too quickly. Firing shelves never did anything for me really, they always look basically the same when they are done. Of course something firm to set your wares on probably means IFB but heavy shelves, I don’t see it unless the kiln fires super unevenly....... maybe.

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On 2/8/2021 at 8:27 PM, CactusPots said:

what do you think?  

I almost bought Advancers but they talked me out of them for my electric conversion gas kiln, due to a possibility of shelf post sandwiches retaining heat, while the rest of the shelf cools, which can lead to cracks.

I reckon if your burners are close to the bottom shelves, you may find it impossible to heat a shelf evenly slow enough.

If it ain't broke...eh..

I would change knowing I may have a couple crappy firings with the new setup.

Sorce

 

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On 2/8/2021 at 8:27 PM, CactusPots said:

What I'm thinking about is how necessary are the heavy shelves low in the kiln?  Frankly, I'm tired of messing with kiln wash and would like to add 2 more sets of advancers.  I guess the only way to know for sure is to try it, but theoretically speaking what do you think?  


Take a look at Hank Murrow's fiber kiln: 

Hank has been in the fiber kiln business for a long long time; a go-to-source of insight on fiber kilns; an expert on both firing and building fiber kilns. 
His website: pages: http://murrow.biz/hank/   
http://murrow.biz/hank/kiln-and-tools.htm
and his sideshow photos of his kiln: http://murrow.biz/hank/hankweb-all/page1-2.htm photos 100 t/m 115. 

LT 

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9 hours ago, Sorcery said:

I almost bought Advancers but they talked me out of them for my electric conversion gas kiln, due to a possibility of shelf post sandwiches retaining heat, while the rest of the shelf cools, which can lead to cracks.

I reckon if your burners are close to the bottom shelves, you may find it impossible to heat a shelf evenly slow enough.

If it ain't broke...eh..

I would change knowing I may have a couple crappy firings with the new setup.

Sorce

 

I have a cut shelf for my damper.  It has to be ridiculously uneven as the inside is getting direct blast and the outside 6-10 inches is exposed.   Not the advancers, but the cheaper silicon carbide shelves.  Have not seen a problem with this setup.  I have a dozen or so firings on the only set of advancers that I have and I only use them on the top where the cooling is slowest.  They are still perfect.

So here's another question:  are the advancers more sensitive to thermal differences than the standard silicon carbide?  Except for the fact the SC shelves warp, they are just as good for glazes not sticking as the advancers.  Yeah, the advancers are lighter also, but the warping is the big factor.

I've heard that "if it aint broke" thing before, but it doesn't apply to me.  I'm a diagnosed tweaker.  Not the druggie kind.

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Glazes can and do stick to the Advancers, however the glaze doesn't eat into them like regular silicon carbide.

HERE is a link to information about how Advancers do not hold up to uneven heating as well as traditional silicon carbide or cordierite shelves. They recommend firing Advancers no faster than 275F/hr below 1000F in electric kilns, and avoiding direct flame contact in gas and wood kilns at low temps, as well as avoiding cool air coming into the kiln. Yet more reasons I'm not interested in them. I regularly fire at at 400-450F/hr below 1000F, and regularly open my kiln at 300-350F when cooling. If I have to add an hour my firing schedule to accommodate the shelves, am I really going to save much on energy costs because they're thinner? I'd say cordierite is probably the most forgiving in terms of tolerating uneven temperatures, because they are much more porous than silicon carbide. Although I've seen regular silicon carbide used for damper shelves, too.

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I fire two gas kilns with advancers. One is an up draft with 6 burners blasting flames on the shelves edges with no effects.

This kiln is really uneven heating

The other is a large downdraft car kiln with bag walls-

The thing is they need to be dry (never damp) unless you are slow bisquing them.I do this after washing them as my porcealin sticks to them.

I keep them off concrete and on wood. They do not like say the damper pulled and be close to that cold air  sucking in (I never pull my damper to cool)

Just some common sense  really  in using them. They do not like pointy uneven shimms in a large stack of weight either. That is say pointy chucks of shims to add to the top of a post under a load.

I use abouit 60 of them for deacdes now and the space they save with wares has made them free many times over and all other shekves take up more space. They are today as flat as when I bought them -

They are so superior I would never switch back. In a typical load of 35 cubic feet I save over a foot of ware space every fire-glaze and bisque in every stack (three stacks in kiln of shelves)

I can sell them always for at least 2/3 of new at any time as well no matter how much I use them. They never warp ever. 

If you like to hose your shelves with water and Quik cool your kiln these are not a good fit.

They pay for them selves pretty quickly-I bought mnost of mine in the 90s-now they are all free. I did about 4 years ago buy 10 used ones (actually never used ) for $110 each -never used them yet. They are 12x24sThey are spares

Edited by Mark C.
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1 hour ago, Mark C. said:

They pay for them selves pretty quickly-I

My experience with the knockoffs has been very similar. They carry the same thermal shock warning but I have fired them in excess of 550 degrees an hour early in the firing when I had the power to do so.  Later during reduction, just haven’t had a kiln with enough power to crack 300 degrees per hour. Really no issues and the kilns fire up to 300 degrees different from top to bottom but flip during reduction and finish relatively even after red heat. Heavy SiC or Mullite for instance end up weighing nearly as much as the wares on them so in effect old shelves make the kiln fire with almost double the load, period. Mass is everything it’s that simple. Old heavy shelves will consume the energy just like your wares but come out looking the same after firing them.

The advancers light weight reduce this by at least half, or more often like 3/4 so they do save significant energy in many instances and can even give back some spare fire power to an old  kiln, whether electric or gas.

As far as gaining height, it does add more space, but likely not a whole bunch in many firings but that feature is certainly a benefit as well.

No glaze damage,  less foot damage for the occasional drip are all a reality as glaze just doesn’t stick. For some fluxed porcelain, kiln wash to prevent plucking. Never, ever, ever fire them moist, it is a serious issue so whatever shelf holder you create, keep it off the floor and clean. I would never return to old shelves for all  the above reasons.

Energy is still cheap but they do save significant energy for sure, 30-40% mass reduction is just huge in every firing.

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