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Seeking More Technical Understanding Of Firing To A Cone


Dick White

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This is an esoteric question that I have long wondered about but not found an answer in the usual technical texts, but maybe some of the experts here can help further my "education" about things that probably don't really matter except to us dweebs and nerds. The issue is what exactly does a pyrometric cone equivalent mean to a glaze vs. a clay body. I understand that pyrometric cones are a measure of heatwork, which is a combination rate of temperature rise at the end of the firing and absolute temperature. In kitchen cooking terms, it is about temperature penetration so the interior of the roast or cake is fully cooked. A faster temperature rise requires a higher ending temperature, and vice versa. The ceramic needs to be baked all the way through. So, here is my conundrum. What about the glaze?

 

We talk about some recipe is a cone 6 glaze. The usual Orton cone table shows cone 6 is anywhere from 2165F @ 27/hr to 2269F @ 270/hr. That's a hundred degree range. It matters because of the ramp rates -  the thickness and constitution of the clay body needs the heatwork to mature. A faster ramp still needs to penetrate, which takes time and as a consequence of the fast ramp, will go to a higher final temperature. But what about the thin layer of glaze on the surface of the body? Does it need the same sort of penetrating heatwork to "mature?" Or are we worried only about the melting point to reach sufficient fluidity to bond with the ceramic substrate without excessive fluidity that runs off? It seems to me that the latter is what matters at the glaze level, given that it is just a thin surface layer.

 

Going back the the temperature component of the Orton table, cone 10 can be achieved at 2284F @ 27/hr. This is a mere 15 degrees above a fast cone 6, and, indeed, a fast cone 6 is over 30 degrees hotter than a slow cone 9. What is this going to mean for glaze recipes that are nominally rated by cones? Can a nominally cone 6 glaze be used indiscriminately in a slow cone 10 firing? Or a nominally cone 10 recipe in a fast cone 6 firing? Or will a cone 6 glaze be barely melted and underfired in a slow true cone 6 firing and the same glaze running across the shelf in fast true cone 6 firing? Or am I missing something?

 

In case you are wondering if maybe I've gone off the deep end, yes, of all the things I have lost in my life, I miss my mind the most. What has piqued my interest again is some of the recent discussion of clay body maturity and the propensity of the commercial clay industry to present their wares as suitable over seemingly absurdly wide ranges, e.g., cone 6-10, coupled with the common reliance on programmed controllers to produce an expected result without an actual witness cone for reference. People push the buttons that say "cone 6" and think that's what will actually happen. The program may end at 2232F as measured by the thermocouple (let's make an a priori assumption here that the thermocouple is accurate, purely to simplify the argument) based on a programmed 108/hr ramp. But did the kiln actually perform at that rate? It probably didn't - under-powered, worn elements, tight load, whatever. Thus 2232F could have meant anywhere from cone 7-9 depending on how long the kiln slogged on before topping out. If that is what is happening, maybe that bogus cone 6-10 clay body is not as underfired as we thought? But what about the glaze? Maybe that to-die-for cone 10 glaze I've been pining for is not so far out of range? Just wondering what others think.

 

Cheers

dw

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Dick:

 

I will be answering part of your questions shortly. I am waiting for my new toy to get here: then you will have 20x to 1600x magnified pictures that I can use to illustrate what is happening in a clay body.

 

The big big issue in clay is flux levels: usually added in % of recipe: which has been the trade standard: but seriously wrong. (and yes I will prove that). I will be posting some lengthy, and rather boring test results and opines: unless of course you are a NERD!! ..then you will enjoy them.

 

In the meantime: I can tell you that clay matures from the bottom up. That means that the part of the piece that is contact with the shelf: matures the fastest. From there, the melt begins in the center of the body and the fluidity of the flux moves outward to the face. I will be posting some pictures to illustrate that. Potassium and sodium are both gases at cone 5-10: and that gas works from the center of the body outward. As it builds, fluidity builds. LT (magnolia mud) and I have had PM conversations about the effects of pressure, and the build of potassium and sodium in a gaseous state. A kiln per se does not provide any pressures: but the interior to the exterior of a clay body does provide small amounts. >>>. it gets complicated.. hopefully I can explain and illustrate it enough so that others can make sense of it.  My words do fall short often.....

 

Nerd

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Thanks, Nerd, sounds like your new toy will be fun to play with. Your theory on clay body maturation rates is comparable to what I've heard about glazes, that they melt from the inside out, which can affect things like the timing of when reduction should start and other esoterica. What I'm wondering about here is whether the nomenclature of a "cone" rating is even appropriate for glazes. I can see how it matters for a clay body (and your upcoming work will be very interesting and useful in that regard) but it seems like the maturity of a glaze would be primarily related to a certain range of absolute temperature, not heatwork. That could then be related to the heatwork of a particular cone at a known (and well controlled) ramp rate, but otherwise calling a particular glaze recipe "cone 6" (or whatever) is misleading?

 

dw

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1st off Dick back in the day and that day for me was in 1974 in glaze calc class we learned about cones and glazes and clay bodies .

Since that day I have put down a few cones and melted a few glazes sort of speak working full time since as a potter.

One you get the clay body down -That is find one thats works for you in your temp reange (does not matter what that range is) for me its cone 11 soft but for you it may be a midrange body since thats what many now use.

I fire my glazes to what cone they need not the body-I already have the body down what I need is the correct melt that whats important for me.

Now you already know about the time temp/time relationship -that is a fast fire give very different results than a slow fire on the glazes . The cones measure this heat work on the glazes as well as the body. But what I need to know is the glazes so thats what cones do for me. 

Since I use gas the ramp is my hand turning up the burners

For you computer control freaks it button pushing but the effects on glaze is the final outcome-Cone are what this based on and the computer has taken that away until the thing that controlles that  wears out -then a cone would still work.

I have an digital pyrometer and an oxy probe -all good tools that make me less reliant on cones but they still are the base of glaze control for me. For you it may be a ramp rate or a final temp on a readout but for me I like to really know whats going on inside and nothing will work better than a cone.At least so far .

That said I never use cones for bisque I just use my eye and a few modern readouts-the eye overrides all.

Back to your glaze cone misleading theory - its a starting point really as fine tuning is always needed with glazes and cones.

I call certain glazes soft cone 10 or hard cone 11 for example but thats only for potters working in my range.

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DW said - but otherwise calling a particular glaze recipe "cone 6" (or whatever) is misleading?

My short answer is: yes it is.  My 1.75 CF test kiln could ramp 1000F an hour if I would let it. So I could run it up to 2232F in just a touch over 2 hours (no hold): and do doubt the glaze would still be raw after it cooled. Just as it is common for ^6 burners to hit 2175-80F, with extended holds to produce a low ^6 melt. I have picked cone 6 glazes apart, did some simple data tables trying to quantify what constitutes a ^6.

 

Glazes are analyzed by Seger Unity, Si/Al ratios, mol %, and formula limits. Some use acid index to further determine the correct melt. However, so far I have yet to see any detailed formulation based on alkai % and total flux %. After all, no flux, means no melt: and/or % of flux determines the cone at which it begins to melt. ( please note "begins") This forum is littered with threads" need help with a ^6 glaze" etc. etc. When you review the recipe: at times my first thought is: and you will continue to have problems because there is only enough flux % to (fully) melt at cone 10. Or the reverse: my glaze is running off my pots-help? Of course it is, it has enough flux for ^5, and you are firing to cone 10. So unless there is info out there about flux level % per cone: I see a gaping hole in formulation.

 

I use type S, far more accurate and reliable in my opinion. My little test kiln has almost 1200 firings, just put the second TC in there not long ago. Read a thread in another forum by another member of this forum on type K. He had 3 of them, and switched out all three to compare them in a cone fire. He noted a 300F difference between the three: that is substantial. So regardless of the type of TC in your programmable kiln: you have to calibrate them. Absent a pyrometer, the only way you can accurately calibrate a TC is with cones.

 

Nerd

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I just want to say that Nerd is providing all the answers to questions I had in glaze chem class that drove my instructor (who taught like it was a cooking class) absolutely batcrap crazy! (His answer was usually "go test it and see.") I am definitely a nerd, and enjoying this immensley!

 

For Dick, I think I agree with Mark's approach for most* studio pottery: the melt is all. Temperature and time, heat work, etc are all ways of expressing roughly how to get to the desired end "meltyness" with a certain amount of predictability. Cones measure finished melt more accurately than pyrometers because they show the actual, rather than the theoretical that is a numbers chart.

 

The first kiln I learned to fire was a tiger torch raku kiln, and we did that by eyeball. How hot was it? Couldn't tell you. How much time did it take? Roughly 20 minutes, but it depended on how dense the piece was, and which glaze was on the piece. We used raku glazes that supposedly melted at the same time and temperature, but clearly they were each a bit different, because the copper sand type ones melted sooner than the fat white crackle glaze. This was judged by looking into the kiln, and seeing if the glaze had finished bubbling up and had smoothed out enough yet. You want a properly melted end product, and what that means for each clay and glaze combination can be different. Generally speaking though, we have a group of glazes that we know from experience melt in the same range, so they're refered to as cone six, or cone ten, or cone 3 glazes, because they all melt "pretty close" to each other, not because they're actually melt exactly the same.

 

I'll actually argue that programmable electric kilns are a specialized piece of equipment that are lousy to learn on, because they don't give you a clear understanding of what's happening inside. If a programmable electric kiln is your only experience of firing, I think it hinders people's understanding of the whole process. They're a fantastic tool to help knowledgable technicians achieve more and more specific results, but you have to be able to calibrate them properly, and understand why that matters for the programs to be helpful and used to their best capacity. Cone sitters are genius in their simplicity as a physical shutoff, and should be required, backed up with the visual cones.

 

*I say most studio pottery, because most glazes are more forgiving than crystalline ones in the firing. Close is good enough in more than just horseshoes and hand grenades, except when it isn't. Crystalline glazes, glass slumping and other forms of warm glass, all REQUIRE more precision. And I think that the more you know about your materials and your equipment and how it all works together, the further you can push your work.

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In my experience, cones and heat work do matter when it comes to glazes, but only once you get the melting started. I have done a few tests with firing 2 cones low and holding to achieve the desired cone (cone 4 with a hold to get to 6, cone 6 with a hold to get to 8). Several of my glazes did not melt much at all, several looked a bit underfired, and several looked the same. Those that looked fine were the ones that I know have a wider firing range than the others. I know this from the times my kiln has put up and error code and shut down early. I know those that were not melted have a much narrower firing range for the same reason. In general, my glazes that are higher in frits tend to be fine when holding to achieve cone, whereas those with little to no frit need to hit a higher temperature before the hold time has an affect. So yes, heat work matters, but only once the glaze is melting.

 

It would be interesting to do a really, really long hold to see if the glazes would eventually melt due to heat work. I once helped with an anagama firing where we held at 1850F for 5 days, just burning wood to build up ash. At the end of 5 days cone 5 went down, still at 1850F. It would have been interesting to have some mid range glazes in there to see if they would melt.

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Neil, this is exactly along the lines I was thinking. We know in the history of ancient oriental ceramics that their kilns were fired long and low. The glazes appear similar to effects we get only in modern fast high fire kilns, but did they get their melt at the lower temperature because of extended heat work or because of what we would consider a mid-fire glaze recipe?

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Dick:

 

Perhaps some historical research will answer the question about 13-16th century porcelain production: and cone firing (temps). Meissen porcelain was first introduced in Europe after Marco Polo had bought back the earliest pieces from China.

 

https://en.wikipedia.org/wiki/Meissen_porcelain

 

Both Sung/Ming Dynasty porcelain was a very simple two part mix of kaolin and pei-tun-tzu. Peter Issley, in his book "Macro Crystalline Glaze" describes pei-tun-tzu as a high potassium/alumina silicate potash/feldspar. He also states the Ming/Sung Dynasty pieces were highly translucent. I have done enough porcelain testing to know that the clay body has to have more than 60% silica, and greater than 5.00% alkali to achieve a translucent body at cone 6.  So there is a starting point in trying to define ancient firing techniques. I could see hitting 2100-2125 with several days: not sure I could see below that. Then you have to factor wall thickness of the piece: would also play a role in translucency during those times.

*Although, if pei-tun-tzu were higher in potassium content than 5.00% alkali: then lower would certainly be possible.

 

Nerd

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Dick, I've always assumed that the reason a short hold at a higher temp equals a long hold at a lower temp is that the rate at which the chemical changes occur in the glaze or clay-body increases with temperature, above some minimum thresh-hold. I don't think thickness is the critical issue. If the rate at which the reactions occur is comparable to the rate at which cones soften, then a glaze or clay-body rated at, say, cone 6 should be mature regardless of the firing speed, provided the cone 6 cone is bent.

 

Of course, this is a big 'if', and assumes the temperature of the cone is the same as the temperature of the glaze / clay-body. Also, that's not to say the glaze will look the same, since things like crystalization and smoothing over of pinholes depend on the firing profile.

The Orton website does state that

The starting premise of cone development was that a blend of ceramic materials compounded to exacting proportions should behave similarly to the ceramic ware being fired in the kiln. A great deal of work later the starting premise was confirmed...

It would be nice to know what 'similarly' and 'ceramic ware' mean in this context.

By the way, according to this presentation by Dave Finkelnburg, fritted glazes melt from the outside in, while feldspar-based glazes melt outwards.
 

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Guest JBaymore

Neil, this is exactly along the lines I was thinking. We know in the history of ancient oriental ceramics that their kilns were fired long and low. The glazes appear similar to effects we get only in modern fast high fire kilns, but did they get their melt at the lower temperature because of extended heat work or because of what we would consider a mid-fire glaze recipe?

 

Heat work.

 

I've put 14 down with never getting hotter absolute temps than 1260 C (2300 F). 

 

best,

 

................john

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Guest JBaymore

Dick:

 

Perhaps some historical research will answer the question about 13-16th century porcelain production: and cone firing (temps). Meissen porcelain was first introduced in Europe after Marco Polo had bought back the earliest pieces from China.

 

https://en.wikipedia.org/wiki/Meissen_porcelain

 

Nerd,  Can't remember if I mentioned this.... you might like the historical fiction book "The Archaneum".  https://www.amazon.com/Arcanum-Extraordinary-True-Story/dp/0446674842

 

best,

 

.............john

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With apologies for inserting pragmatic queries into a mostly theoretical discussion, going back to your #8 comment, Neil, do you recall from your testing approximately what length of hold at cone 4 achieved cone 6 (and also the hold at cone 6 to achieve cone 8?).  

 

And what effect might firing to a lower temperature and holding to achieve the heat work associated with a higher cone have on an electric kiln?  My theory is that, if done consistently, it may prolong element life but wear out relays faster (assuming they are not solid state). 

 

I have recently moved to ceramics from working with fused glass where heat work is a well developed and quantifiable concept  as there is less variability in the chemistry of the materials.  Firing as "low and slow" as possible is often preferred to maximize control and consistency of the end result.  I'm curious about the transferability of this to ceramics.    

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With apologies for inserting pragmatic queries into a mostly theoretical discussion, going back to your #8 comment, Neil, do you recall from your testing approximately what length of hold at cone 4 achieved cone 6 (and also the hold at cone 6 to achieve cone 8?).  

 

And what effect might firing to a lower temperature and holding to achieve the heat work associated with a higher cone have on an electric kiln?  My theory is that, if done consistently, it may prolong element life but wear out relays faster (assuming they are not solid state). 

 

I have recently moved to ceramics from working with fused glass where heat work is a well developed and quantifiable concept  as there is less variability in the chemistry of the materials.  Firing as "low and slow" as possible is often preferred to maximize control and consistency of the end result.  I'm curious about the transferability of this to ceramics.    

 

6 to 8 was a 40 minute hold. 4 to 6 was about twice that long. In my unscientific tests, the 6 to 8 hold time increased element life by at least 20% versus firing all the way up to 8. There didn't seem to be any noticeable affect on relay life, but the relays on my big kiln last forever since they're housed in a freestanding box away from the kiln. They just don't get very hot compared to kiln mounted control boxes. That said, the relays on my small kiln last a really long time, too.

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Cones are unique little creatures which, strictly speaking, have meaning only to themselves and other cones. They are simply milage markers along the heat work highway. That doesn't sound like much - until we acknowledge that we are hurtling down that highway virtually blindfolded. Then, suddenly seeing those milage markers is huge.

 

I would take Orton's statements about relationships between their cones and our ceramic ware with a grain of salt. Unless all your glazes have the exact same chemistry, mass, particle size and shape as an Orton cone, their behaviour and eventual appearance is only distantly related to whatever cone you routinely fire to.

 

Rather, a cone properly down tells us very little, specifically only that we have passed that particular point where that cone melts. However, the beauty of the cone of course is that it does this every time, again and again, with high consistency. No matter what we do to the kiln, or what we think we see inside it, that little cone is a pillar of truth in a sea of uncertainty.

 

From this perspective, cones don't have anything to do with either glazes OR clay bodies. We could just as well be firing pet rocks - the cones would still work the same. However, as Diesel was getting at, glazes and clays that seem to work well (in the eye of the beholder of course) around milage marker 10 come to be called cone 10 glazes and bodies.

 

They could well still have desirable properties when only fired to, say, cone 8, but are not necessarily at their optimum. If optimum = best melt, then every glaze must have some point along the heatwork highway where they are completely melted, off-gassed, smooth, etc. but have not yet started to run off the pot onto the kiln shelf. That point depends on so many different variables (glaze chemistry, clay chemistry, pot thickness, place in kiln, etc. etc) that it will differ for every pot in every firing.

 

We could make our own cones. Assuming one has a consistent supply of high quality materials, one could make pyrometric cones to the same recipe and chemistry formula as, say, our favourite base glaze. These tailor-made cones would probably enable you to fire your base glaze close to optimum every time. But as Mark pointed out, a soft Orton cone 11 is plenty close enough for him and his glazes, so making ones own cones is probably overkill.

 

So it is pretty much a certainty that none of our pots are ever fired to their optimum. There are no absolutes, it is all approximate. But most of the time, that is good enough - as long as we think it is. Seen in this light, there is scope to fire all sorts of clays, glazes and other ceramic materials over wide ranges, as long as we are honest with ourselves and others about their degree of functionality.

 

And the little cones will just keep marking the miles...

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I love the mile marker analogy.

And you're right, even the chemical composition of Orton cones is an arbitrary method of measuring the "doneness" of a given ceramic piece, and the doneness is a lot more subjective than the most technical amongst us wish to admit sometimes, even in highly technical wares. I was speaking to another artist who had taken a residency at Medalta, and she had been speaking to Tony Hanson. The firebox conversation held around the soda kiln that night had drifted towards consistency, and the ability to reproduce exact results in a soda kiln, and how that could be achieved with sufficient testing. Specifically, how Tony felt that the crazing in the soda drips were a flaw that could be eliminated by employing chemistry, and really studying the interface between the clay surface and its reaction with the sodium, and how that particular chemical reaction changes the coefficient of expansion to create a crazed glaze once the buildup is thick enough.

The artist rather thought that Tony was missing the point of a soda firing in general.

 

Yes, it's possible, but in what arena is it necessary, or even desirable? Nerd has answered that for himself, that he needs a certain level of technical precision in order to understand, and be able to create the exact work he wants.

 

And speaking of atmospheric firings, to complicate matters can we address the fact that wood ash and bicarbonate (and sodium chloride too) all flux out Orton cones, but we still use them, ostensibly because the airborne fluxes are working on the pots at the same time? They're still a mile marker, but a slightly less accurate one at that point if you're considering clay body maturity and you're looking to fast fire instead of the low and slow that allows for more thorough heat penetration. But they're still good enough for the purpose.

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I think functional ware allows for much less slippage on this point.  If the piece just sits on a shelf it can have a million problems as a ceramic object, but if it looks good....

 

Functional ware HAS to perform.  Underfired clay bodies - even if "the manufacturer said it was OK" - are porous, weak, unsafe, etc. etc. 

 

A "cone 6 glaze" is only shorthand for "a bunch of people have fired this glaze until cone 6 was down and have asserted that this amount of heatwork was sufficient for the glaze surface to meet their expectations and needs enough that they would probably fire this glaze to the same cone again next time."  It is no guarantee of anything, and often not even a good guide in your specific situation.  This is why I have big problems with the whole concept of glaze limits, which are built around cones...  But that is another story.

 

The maker and seller is the last line of defense.  Functional potters have a responsibility to make sure that their clays and glazes are not only properly formulated to not craze, resist heat shock, cutlery marking, dishwasher cycles, etc.. but also that they are FIRED properly to deliver these properties.  That requires MUCH closer control over materials and processes than making decorative ware, and a knowledge of when these materials and processes have failed.  Ultimately, lives are literally at stake. If you really care, you will periodically get your wares lab tested to prove up their functionality in the relevant tasks. 

 

This is hard work which takes almost monastic dedication to achieve.  Many other fun and interesting experimentations in ceramics land must be forgone to keep on the righteous path.  And they must have the fortitude to accept losses and discard pieces which do not achieve the necessary standard, sometimes whole batches of them.  Indeed, professional functional potters must be the holy men of our endeavour.

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OK the small gas kiln has been firing for 9.5 hours and the digital pyro says its 2398 degrees but cone 10 is still only 1/3 down and moving very slowly as it should at this point for good glaze melting-I'm waiting until cone 11 is 1/2 way the pyro will say 2400 and something at that point

The Oxygen meter says .58 so the reduction is just where I want it. 

Looks like I'm up for awhile -I just lit the big kiln for an early start on tomorrows glaze fire.

I could have held the kiln on l;ow for weeks and maybe gotten to the same spot but I only want to be firing for about 10-12 hours.

The moon is out thru the fog

cones are what I as Curt says measure my miles and I'm driving a lot every week.

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The more I read this thread, and think about the numerous other threads: and how cones are used: the more I realize a shift in application. The shift/ or trend I am seeing is the use of cones to predict the melt of the glaze: instead of the maturity of the clay body. I also see firing schedules centered around the melting of the glaze, rather than the maturity of the clay. (Not all, but an application trend) There also seems to be a trend to fit the clay to the glaze, instead of fitting the glaze to the clay. By which I mean, some search out clays that make their glazes look good (I get that), but create problems because they are mismatched. Glazes should be adjusted to the optimum firing cycle and range of the clay: not vice versa. Glazes have a broader range of variations of temperature: clays do not: despite universal claims of a broad firing range. Cone 6 temperatures are much more unforgiving than cone 10 on several levels: just as cone 10 firings have issues of their own. A broad stroke of general applications for firing clay bodies will come back to bite you later. Bending a cone 6 perfectly might mean the glaze is done, but that does mot mean the clay is.

 

Nerd

 

Logic: the time it takes for heat to move through a 1/4 to 1/2" thick slab of clay is not the same amount of time heat takes to move through a 1/16th inch layer of glaze.

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