Heat resistance of cordierite kiln shelves

[foxtatic]

I thought I was out of questions and ready to start buying materials to build a DSR3 core, but cordierite came into question over in Forsyths brilliant "Why Refractories Fail..." thread: donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

It was pointed out that cordierite starts to degrade quickly above its working temperature of 2350F is 1260ºC. I searched this thread and noticed you never actually use the word cordierite. You refer to your shelf as kiln shelf and I simply assumed cordierite.

Which made me wonder if you were indeed using that specific material?
And also wondered if you felt the DSR3 could ever exceed those temps.  

[peterberg]

I don't know what specific material it is, I bought it as ceramic kiln shelf. I am unable to measure the internal temperature of the top box, let alone the afterburner tube. Theoretical speaking, the top temperature for a wood fire that isn't blown by a fan would be 1200 ºC. The vendor claims it's maximum working temperature is 1280ºC.

As anything to go by, both experimental incarnations of the DSR3 do contain this kiln shelf material. It looks and feels like new to me, while most materials I tried in similar circumstances got degraded or cracked. Several pieces are used in multiple parts of the first development model, even narrow strips about 1.5" wide. As long as those weren't dropped on the floor or something it stayed as a whole.
In my opinion, this material is superior to all other materials I've tried, including ceramic fibre board, vermiculite board, insulating firebricks, refractory castable and firebricks and slabs.
There's a reason why this very same material is used in the newer UL-listed Liberator rocket stove, don't you think?

[martyn]

I found this snippet on line …
Here at The Ceramic Shop, we have a wide variety of kiln shelves in stock and ready to ship. Pressed Cordierite (high-refractory) shelves are the standard shelf that comes with most American-made kilns. These are also typically the most economical. They are relatively durable, they can be fired repeatedly to cone 10, and they can be used in either electric or gas kilns. There is now a lighter-weight version of these alumina shelves available - called Corelite shelves, and Hollow Core Shelves these are the shelves shown above with the hollow centers. Although they are a bit more expensive than the standard Cordierite shelves, they are just as strong, but are much lighter than the standard shelves, which makes them a breeze to load, especially in top loading kilns. We also carry silicon carbide shelves. These shelves are ideal for high temperature firing in gas kilns, and especially for use in atmospheric firings. We do not recommend them for use in your electric kiln, because these shelves can conduct electricity.

[Forsythe]

Yeah, cone 10 would be in the 1250-1275ishºC range

Some shelves are pure cordierite, some are a blend of cordierite and mullite, and some are pure mullite. The latter two typically have a little more durability and longevity to maintain their structure at slightly higher temps and resist crystallographic destruction from glaze drips... and the chemistry of ceramic glazes is very similar to wood ash. Some forms of pottery even use wood ash as the glaze itself.

Speaking of wood-fired kilns and ash glazes... the natural-draft "anagama" style kilns used in some traditional asian pottery can get as high as 1500-1515ºC — without any forced air fan. (granted that they're run for many hours and constantly stoked with 2" diameter sticks much like a J-tube rocketstove is.)

I think that 1200ºC theoretical max is for an *open* wood fire — one which isn't having air rapidly blown into it by a fan — but that 1200ºC max would also not include an *enclosed* wood fire (insulated by refractory) and having air rapidly sucked into it by a chimney stack effect à la rocketstove or anagama kiln...

Edit: That site Martyn cited mentions silicon carbide shelves — and most modern silicon carbide shelves are nitride-bonded silicon carbide, (which is stronger than plain silicon carbide, but they can have issues in wood firing at very high temps and/or any level of moisture exposure.) They're also stupid-money expensive.

[peterberg]

Nov 7, 2022 15:35:29 GMT -8 Forsythe said:

I think that 1200ºC theoretical max is for an *open* wood fire — one which isn't having air rapidly blown into it by a fan — but that 1200ºC max would also not include an *enclosed* wood fire (insulated by refractory) and having air rapidly sucked into it by a chimney stack effect à la rocketstove or anagama kiln...

During the 2012/2013 season I borrowed a digital thermometer equipped with two thermocouplers that could withstand 1360ºC. I inserted one coupler in the back of the riser, 4" above the bottom and about 1" inside the riser.
And ran the batchrocket (the one in my workshop) for hours on end, just to see how high the temp could get in there. It turned out that a level of 1000ºC (1830ºF) was easy to reach, with just one full load of hard wood species. Even 1100ºC (2010ºF) turned out to be doable, albeit with a little more effort. Highest level I ever measured at that spot turned out to be 1172ºC (2140ºF). The thing had to be run with 4 batches back-to-back in order to reach this level, coincidentally very close to the mentioned theoretical highest temp.

To conclude and based on these measurements, the 1200ºC level is the maximum that can be reached inside an enclosed space, in my opinion of course. What can be reached in an open fire I honestly don't know, but I highly doubt it would be the same as in an enclosed space.

[Forsythe]

Nov 8, 2022 2:25:14 GMT -8 peterberg said:

Nov 7, 2022 15:35:29 GMT -8 Forsythe said:

I think that 1200ºC theoretical max is for an *open* wood fire — one which isn't having air rapidly blown into it by a fan — but that 1200ºC max would also not include an *enclosed* wood fire (insulated by refractory) and having air rapidly sucked into it by a chimney stack effect à la rocketstove or anagama kiln...

During the 2012/2013 season I borrowed a digital thermometer equipped with two thermocouplers that could withstand 1360ºC. I inserted one coupler in the back of the riser, 4" above the bottom and about 1" inside the riser.
And ran the batchrocket (the one in my workshop) for hours on end, just to see how high the temp could get in there. It turned out that a level of 1000ºC (1830ºF) was easy to reach, with just one full load of hard wood species. Even 1100ºC (2010ºF) turned out to be doable, albeit with a little more effort. Highest level I ever measured at that spot turned out to be 1172ºC (2140ºF). The thing had to be run with 4 batches back-to-back in order to reach this level, coincidentally very close to the mentioned theoretical highest temp.

To conclude and based on these measurements, the 1200ºC level is the maximum that can be reached inside an enclosed space, in my opinion of course. What can be reached in an open fire I honestly don't know, but I highly doubt it would be the same as in an enclosed space.

Natural draft wood kilns easily exceed those temperatures on a regular basis. I've personally gotten a rather small natural draft wood-fired kiln up above 1350ºC without much effort. Getting that last 50ºC up to 1400ºC was a challenge, though, likely due to the thin atmosphere at high elevation and the limited height of my chimney stack. Those temps were taken through the stoke-holes with two separate IR digital thermometers capable of reading up to 1500ºC.

The only thing I can think that might be causing the lower temperatures in your particular configuration is the presence of the open bell immediately downstream of the riser, which could act as a draft limiter — buffering the stove's ability to draw as powerfully as though the entire flue channel were "enclosed" to a diameter closer to the firebox, riser, chimney, and system size.

The bell will allow for a lot more gas expansion — and later compression of those expanded gasses— more closely mimicking an open fire than a straight-shot flue channel, or a combustion chamber matched to the size of the expanding gasses as they burn, and then steadily necked-down to the size of the air intake(s), post-combustion, as an anagama-style natural draft wood-fired kiln does.

[Forsythe]

en.wikipedia.org/wiki/Anagama_kiln

The wikipedia article quotes the natural draft wood-fire kiln's upper range at 1400ºC, but (anecdotally) I've heard many reports of natural-draft wood-fired kilns pushed close to 100ºC above that (Going above 1400ºC is usually avoided unless the "style" the potter is shooting for is the slumping/partially-melted look of very heavy wood-fire and reduction effect. The top temperature I've heard of was 1515ºC, and the pieces looked like melted confectionery frosting.)

[Forsythe]

...come to think of it... I'd bet that the open bell's pressure-buffering effect is why Trev's Vortex design hasn't worked in your open-bell system without over-fueling. In those circumstances, the burn rate would be determined almost solely by the primary & secondary combustion unit shape... relatively uninfluenced (or less-influenced) by either downstream (positive) backpressure (created by a flue that's consistently system-size from the secondary combustion chamber's exit all the way to the chimney-stack) — or the higher draft (negative pressure) created by a taller stack.

Negative draft suction would be limited by the bell's tendency to expand the volume of gas within it from the chimney-stack back-end...and positive back-pressurization would be limited by the bell's tendency compress the volume of gas from the combustion unit front-end.

...I feel like I've had an epiphany. 😅

[peterberg]

Forsythe, does it occur to you that IR thermometers are not good in measuring flame temperature? These are specifically designed for measuring the temperature of surfaces. For that reason, one wouldn't be able to measure room temperature with these, for example.

I'm inclined to think this discussion belongs in the materials section, not in a development thread about a specific core.
I'll move it to new thread in Materials.

[Forsythe]

Nov 8, 2022 5:44:32 GMT -8 peterberg said:

Forsythe , does it occur to you that IR thermometers are not good in measuring flame temperature? These are specifically designed for measuring the temperature of surfaces. For that reason, one wouldn't be able to measure room temperature with these, for example.

Well, sure, of course. But does it occur to you, Peter, that the temperature of a surface within a combustion zone would not be hotter than the flame temperature from which it is receiving its heat?

If we're going with the premise that an IR thermometer is "not good" to measure flame temperature because it only registers surface temps... that would necessarily mean that the flame temperature was even hotter than the 1350ºC registered by the IR thermometers.

(The temperature attained by the material exposed to the flame would be the more germane of the two in either case, regardless — whether it was a ceramic being fired to a target temperature, or a firebox structural material nearing its thermal tolerance threshold.)

I'm inclined to think this discussion belongs in the materials section, not in a development thread about a specific core.
I'll move it to new thread in Materials.

Edit: okay. I thought a key topic at hand was how the downstream mass’s flue channel arrangement would have an intimate, inextricable relationship to the performance of a specific core — specifically the vortex aspects that had or hadn’t worked in the DSR3. Seemed like a fit here, but if that’s “materials” just let me know where it ends up.

[peterberg]

IR thermometers aren't good for flame temperatures because they are going haywire in the sense that they measure a far too high or a far too low temperature, not clear what triggers what. They don't measure the wall temp through the flames but are confused by the flames instead. Through glass has the same effect, is the glass measured or what? Tried both possibilities, the results were downright silly, nothing to rely on in my opinion.

The original question was: can a cordierite kiln shelf withstand the heat in a DSR3?
I'm inclined to say it can, according to the vendor I bought mine, it's cordierite. I've established years ago that the flame temperature in my batchrocket wasn't higher than 1200 ºC, yes? Which follows that the wall temperature was much lower, as you mentioned.
Answer to the question: yes it can.

[Forsythe]

Nov 8, 2022 8:04:44 GMT -8 peterberg said:

IR thermometers aren't good for flame temperatures because they are going haywire in the sense that they measure a far too high or a far too low temperature, not clear what triggers what. They don't measure the wall temp through the flames but are confused by the flames instead. Through glass has the same effect, is the glass measured or what? Tried both possibilities, the results were downright silly, nothing to rely on in my opinion.

Right, That's why you aim the sensor at the hot face of the object undergoing the heat work.

With a sensible approach to the temperature-taking, the readings I took were steadily consistent with themselves, and consistent with each other, to within 2 degrees of each other's readings at their widest deviation. Certainly not what one could reasonably consider silly or haywire or unreliable. IR thermometers are widely relied-upon tools within multiple heat-intensive industries for spot-reading temperatures where it actually matters — on the materials being subjected to those very high temperatures.

But, of course, you don't have to take my word for it on how hot a natural-draft wood fire can actually get. Just consider the nearly 1000 year history of porcelain-firing in China, using nothing but natural-draft wood-firing kilns to fire kaolin clays with potassium feldspar to 1350-1400ºC. Or consider that Erica Wisener (co-author of the book on RMH building) has repeatedly stated that large-diameter J-tubes can get as high as 1600ºC. Are we supposed to throw all that mountain of knowledge out the window because Peter's unique batch-box-in-a-bell only gets to 1180ºC, and he has an unreasonable mistrust of IR thermometers and literally everyone else's temperature readings taken with them because he can't use them as they're not intended to function by aiming them through actively burning flames?

Does this matter in the context of a stove built-to-exacting-replication of Peter's DSR3's temperature? Probably not.

Does it matter if you've been misinformed regarding the highest achievable temperature attainable by a natural-draft wood fire, and build something based on a DSR3 design, but using a different downstream mass with different flue-channel configuration — or a larger firebox? It absolutely does.

If you falsely believe that a natural-draft wood fire is only capable of reaching 1200ºC, then you're apt to select materials that may not handle higher temperatures without decomposition and/or structural failure. (after all, those materials are more-than-likely cheaper than their higher-rated counterparts.)

And that can be a serious health & safety issue when your semi-experimental build blows past the 1200ºC+ temperature you've been mislead to believe was the highest possible temperature of a natural-draft wood fire.

[peterberg]

Your opinion, not mine. You are entitled to it, so am I.

By the way, regarding what I said about the original question which started this discussion?
Do you have experience with this cordierite material, not only an opinion based on what you think?

[Forsythe]

Nov 8, 2022 11:50:21 GMT -8 peterberg said:

Your opinion, not mine. You are entitled to it, so am I.

By the way, regarding what I said about the original question which started this discussion?
Do you have experience with this cordierite material, not only an opinion based on what you think?

Temperatures aren’t opinions, Peter. And these are well-documented temperatures.

[martyn]

There is this thread ….
donkey32.proboards.com/thread/3789/got-2372-degree-thermocouple-ideas