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Cone 6 Firing Schedule- Nerds


glazenerd

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Nerd's Cone 6 firing schedule.

 

Modern firing schedules have been increasingly based on maturing glaze; which in turn means the clay body is not. The biggest indicators that the body is not mature is absorption, vitrification, and pin holes. The COE data given by the makers is based on firing unglazed clay at a much slower rate of climb. In comparison by application: most potters fire bisq at accelerated climbs to their peak hold: which does not allow for heat work to fully penetrate the body. When pin holes do appear, the current fix is to do an extended hold at peak to cure them. If you notice however, pin holes appear in glaze that is already mature: they are just surface blemishes. The extended hold does not further vitrify the glaze, it just gives an extended period for the clay to off-gas.

 

Stoneware is the biggest offender, however it is not uncommon for porcelain to have the same issues. Potters are trained in 200-325 mesh; but it is common for stoneware clay to have 20-80 mesh particles of feldspar floating around. In addition, glaze is directly exposed to the kiln atmosphere, the interior of the clay body is not: so it takes longer for heat to reach it. Fast ramping up to the peak hold just further complicates the issue of heat work. It also requires more time for large particles of natural feldspar to off-gas. You have probably also noticed that stoneware bodies with high iron content also seem to be more susceptible to pinholes. It is not the iron, it is because iron rich clays tend to have larger particles; which also include feldspar minerals.

 

On the technical side: meta-kaolin begins to form spinel at 2050F. Without going on into other techno talk: it is the temperature that clay bodies form the glassy matrix required for vitrification; or in the case of stoneware: densification. Silica and feldspar is actually expanding up to this temperature; and the porosity of the body begins to close up after it. Fast ramping past it to 2190F ( common peak used) means you are interfering/hampering the development of the clay. This results in a more porous body; which equates to more absorption. It can also further crazing issues: because an immature and mature body have differing COE values; and contract differently. Finally, this same temperature range is when the vast amount of off-gassing occurs because heat is reaching the center of the body. 

 

The fix:

 

Use your current firing schedule up to 2050F - no need to change any of that.

130F an hour from 2050 to 2190F with long peak hold

or- 130F an hour from 2050 to 2230F with very short hold.

--natural cool.-- unless color development is desired.

 

I have not had pin holes in years using this schedule. Several potters on the forum have switched to it with great results; and their pin hole issues are gone as well. It is rare for glaze to cause pinholes: it is usually older recipes that have vastly disproportionate amounts of feldspars, or use minerals that are known to cause pin holing. High sulfides with cause them (not sulfates).

 

Nerd

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Question: How long is a long peak hold vs how long is a short peak hold?

Hi Tara:

Kiln size is what really governs peak hold times. In smaller kilns 2-4 CF, a 10-15 minutes hold at 2190F is plenty. 4-8CF 15-20 minutes; and in 8-12CF 20-30 minutes. 

When firing to 2230 F (what I use). in my 2 CF kiln is 3 minutes.  4-8CF 5-10 minutes, and in 8-12CF 10-15 minutes. You have to factor the time difference from climbing from 2050 to 2190, and 2050 to 2230F.

 

The slow climb (130F an hour) does the lion share of the work; the hold time is to simply equalize the heat in the kiln. The 2190F, with extended hold has been in use for cone 6 firings for some time. If you have been using this cycle; start with cutting your current hold at peak in half. Use a few cones until you get it dialed in for your kiln.

 

Nerd

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I've taken the liberty of Anglicising your firing schedule Nerd.

 

Use your current firing schedule up to 1120°C - no need to change any of that.

 

50°C an hour from 1120°C to 1200°C with long peak hold: or,

 

50°C an hour from 1120°C to 1220°C with very short hold.

 

Natural cool. -- unless colour development is desired.

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Thanks Nerd, I'll give that a whirl. I've got a particular combo that pinholes. Doing other projects at the moment but I want to get back to that one to solve it.

 

A couple of questions:

 

Do you think it makes any difference whether you are glazing bisque or single-firing? It seems to me both are in the same situation in the sense that both are entering the vitrification zone for the first time. Bisque had a head start it sintering etc during its first firing but does that make a lot of difference to the processes going on as you approach maturity?

 

I presume a similar methodology should be applied to high-fire stoneware too (^9/10)?

 

Joe

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Ty Ayjay- much appreciated.

Babs- some stoneware bodies that utilize kaolinitic ball clays; in conjunction with higher SAS values will work just fine in that range. They are the exception however, not the rule.

 

Joe: your assumptions are correct. Once a single fire crosses the 1800F / 1000C mark; both methods are at the same point in the cycle. A bisq piece is simply fused together, and by no means remotely vitrified. All the Material Science books/studies that I have read indicate that 2050F / 1120C is the actual beginning point of vitrification. The reference to metakaolin beginning to form spinel at 2050F, in laymen terms simply means: aluminosilicate conversion to glass (glassy matrix). At 2190F (1180C), whatever flux that was present is spent: meaning its ability to further melt is over. That is why 2190F has become the stopping point for the minimum peak for a cone 6 firing. However, the peak production period for mullite (fancy word for glassy matrix) is 1200-1240C: which is why I fire to 2230F/1220C. If I were doing functional wares; I would certainly fire to 2230F in lieu of 2190F.  2190F with a long hold is feasible with porcelain because of the flux molarity, but it falls short for stoneware: which has 15-20% less flux than porcelain.

 

Arguments could be made about this cycle when firing to cone 10. The additional heat work certainly increases the overall density of a clay body. However, the primary range of clay to a glassy matrix remains at 2050 to 2190F. Knowing that, for me personally I would incorporate it into a cone 10 schedule.

 

Only one caution: if you are firing pieces that have layered glazes to produce a run. The 130F / 50C final ramp greatly increases the heat work done: so some adjustments may be be necessary for these types of pieces.

 

Nerd

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>Fast ramping past it to 2190F ( common peak used) means you are interfering/hampering the development of the clay. This results in a more porous body; which equates to more absorption

 

I wonder if slowing the ramp might help those in the UK who struggle with absorption on commercial clays that have been marketed with unusually wide temperature ranges? [personally I've ended up firing these at ^9 and get low absorption but some may not be willing to go so high].

 

Joe

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Nerd, I have always fired by the . . . seat of my pants. The glaze loads are fired using a short water smoke of one hr, to make certain all glaze is dry. Then a 70% to red orange heat, then full up to yellow, backing down to 90% til ^5 goes down, backing to 80% for ^6. This usually allows for ^6 45" to 1 hr later. Then I do a cool down at 30% til at red orange. Most of this control happens with the kiln temp color, and even though not rocket science perfect, pretty dang accurate as long as the cones are there. I have had problems in the the distant past with pinholing, color immaturity, and lack of proper crystallization of glazes from too fast a move past ^5 and ^6. So made appropriate adjustments that have worked pretty well.

 

 

best,

Pres

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Joel:

Interesting correlation. My conclusions were based mostly on studying the issue, and looking at results on the glaze surface, but also snapping them and viewing the interior up close (400x).

scaling

Like this one; where the clay looks like swiss cheese microscopically: IE gas pockets / porous

My final conclusion was based on: Endothermic sublimation and thermodynamic equilibrium. I have a Nerd friend who graduated with honors in Trig/Calculus. I can set the values; and he can run the math.

 

I fully expect to see this schedule drop even lower in the next few years. I am seeing larger and larger pieces of feldspar minerals in raw clay materials. A friend uses Soldate; and is reporting increasing problems with pin holes and in his case: craters. I have some recent mineral data updates and have talked to several mines in the Midwest and CA: they also report increasing amounts of feldspar contaminants. So again, I expect to see this problem increase as they dig through more contaminated areas.

Nerd

 

My test kiln 3850 watts per CF and my primary crystalline kiln 2200 watts CF (super insulated): i can run through high end ramps much faster than 60C. Remember also: specifically called this a cone 6 schedule. On a cone 10: in a standard kiln: that sounds right though.

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Joel:

 

I think stoneware and porcelain need to be grouped separately: meaning the chemistry of porcelain is different on the basis of flux molarity and uniformity of mesh/particle sizes. Stoneware runs 2.75 - 3.10% alkali molar, and porcelain runs 3.50 -4.25 alkali molar in typical cone 6 bodies. Just this difference alone explains maturity differences between them in a cone 6 firing. Mesh (particle size) being the other primary problem child; porcelain runs a consistent 200-400 mesh area, and stoneware has a blended body ranging from 20 mesh up to 400 mesh. (averages used). Here are 2 examples: both fired to 2230F (1220C), using the schedule outlined.

6StoneLR

Stoneware- with 20-40 mesh particles still plainly visible.

ClayGlaze interface

Porcelain - Nerd blend- similar to Southern Ice.
 
While firing both at the same time is not a problem: the example just shows how much extra heat work is done with this firing schedule. However, it also shows why stoneware burbs gas so much during a standard firing: there are large particles of feldspar minerals as well. The schedule gives the time needed for heat to reach the center of the body.  The 130F (50C) so far has proven to be the most effective; although one potter did drop down to 125F. An even slower ramp will certainly do no harm; the only issue would be layered glazes: they may very well run way more than anticipated.
 
The other issue with stoneware and porcelain gets into a more specific technical/chemistry issue: thermal conductivity.
Sodium is 140 W/mK.   Potassium is 100 W/mk.
 
Heat transfers slower across lower numbers, and faster across higher numbers (simple terms)
The other issue is mass: heat absorbed by 20 mesh, or heat absorbed by 400 mesh. (obviously absorption time is different).
 
Porcelain uses sodium primarily; which absorbs heat quicker, and into a much finer mesh.
Stoneware uses potassium primarily: which absorbs heat slower, and into much larger mesh.
The application of these would be: heat work/transfer/absorption into large potash feldspar meshes towards the center of the clay body. Which makes this firing cycle much more critical for stoneware bodies; but porcelain would also have some added benefits.
 
Nerd
 
Note: I am aware that glaze/clay text books cite 800C (melt.) However, rate transfer, mass, and conductivity are not factored into the actual application.800C will fuse, but not fully incorporate into a melt that converts aluminosilicate (clay) into spinel.
 
 
 

 

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Pres Posted 16 January 2017 - 04:26 PM
"...I have always fired by the . . . seat of my pants. .. to red orange heat, then full up to yellow, backing down to 90% til ^5 goes down, backing to 80% for ^6. ... Then I do a cool down at 30% til at red orange. Most of this control happens with the kiln temp color, ...."

Judging temperature by the "color" of the ware is probably more accurate than by measurement of a thermocouple, because the color of the pot is direct information on the temperature of the pot where as the reading of the thermocouple is temperature of the thermocouple which is only near one or two pots in your kiln. 

 

Reading the color require a lot of experience, and good visual access to the hot pots.  Pres, I envy both your kiln setup that allows such visual access and your skill.  Go with what works for you.
 
LT

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Pres Posted 16 January 2017 - 04:26 PM

 

Judging temperature by the "color" of the ware is probably more accurate than by measurement of a thermocouple, because the color of the pot is direct information on the temperature of the pot where as the reading of the thermocouple is temperature of the thermocouple which is only near one or two pots in your kiln. 

 

 

 

Because the heating elements are in the walls, and because most studio electric kilns are relatively small in the big picture of kilns, if you pack them relatively evenly the temperature at one side of a kiln shelf will be very consistent with the temperature on the other side of the shelf. Top to bottom is a different matter, of course.

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I pack my glaze ware relatively tight. That is there is maybe a 1/4" spacing between bodies of pots, I also try to have as little horizontal space as possible til the next shelf. As others have said, matching height to stilt is best way to go, allowing for tighter packing. When firing, my bottom is always ahead of the middle and top on switches, and I do check peeps at all levels. At the same time googles are most helpful, but need to be adjusted to. So at times I just look at the color, without the goggles, but not looking directly into the kiln. That is most disastrous. You have to learn how to run things, and how to protect yourself. Color charts are available, but not always directly accurate. At the same time relying on the cone pack for the end stage is very important, also comparing top and bottom color to cone pack peep color to make certain all is within range. If you have been watching all of this as going up, then top to bottom will be very closely aligned.

 

. . . . . .the rest is up to the kiln gods. . .  

 

 

best,

Pres

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This is actually the best news I've heard in a while (or at least since last week when I learned so much from Nerd). Those of us with big old kilns - emphasis on old - should stop wringing our hands that our kilns can't push 270F/hr into 2269F for a "fast" glaze firing. That mine can't even do 100F/hr (need to change the elements, but that's a different story...) is actually better for the ware. And it is also useful information that I should slow down the gas kiln at school. Some of the students have been whimpering about some of their glazes pinholing, and now it looks like it is not the glaze at all, but the body. Keep talking Nerd, I'm listening even if it doesn't always look like it...

 

dw

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Dick:

I have been going through a 244 page collection of technical papers from 1932 up to 1984 discussing clay issues. Most are from the American and British Ceramic Societies: interesting topics. A forum member sent me the link two weeks ago. In 1938, they were discussing (SAS) specific area surface; as the primary basis for formulating clay bodies. They also discussed water absorption (WOPL) in determining the plasticity of clay. (1936). Using soda and potassium ash to control drying, or to electrostaticaly repel clay particles: which we call deflocculation today..

 

In 1932-1938 there was an ongoing discussion and experimentation surrounding the most effective way to "close up" the porosity of a clay body: which we call vitrification. Several articles talked about maximizing the effects of heat at the upper range of feldspar "deposition": meaning its most effective range. Some citing the need for "extended" heat work in the upper range of firing.

 

So in essence, I am just rediscovering what was already discovered long ago: but I already suspected that. In the 60-70's, the focus shifted from the scientific realm into the studio potter realm. Slowly over time what was proven scientifically, was replaced by what was most efficient. The studio potter movement has been the driving force in the industry for over 50 years: perhaps it is time to put back some of the scientific that was sacrificed for efficiency. Many of the problems, and many of the variables are solely caused by our demand for the clay to do: what it simply is unable to do. By adding one hour to the firing schedule: many of the problems we deal with vanish. Pin holes, craters, absorption, and is some cases: crazing. We created some of the problems ourselves. Up until the time of the modern kiln; firings took 36 to 96 hours; now we want the same results in 8. Then we wonder why those pieces lasted 100-300-500 years, and ours have problems in the first week.

 

Nerd

 

Before and after results would be nice..

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Any of my fellow Nerds, seen the below discussed specifically for clay?

Links would be much appreciated.

 

Nerd

Yes,

 

Wikipedia is not a good source for learning thermo.

 

Gibbs free energy is a fundamental aspect of physics and chemistry. 

 

I will dig out some references for you later today.  

 

LT

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Did you ever get "Materials Science and Engineering: An Introduction 6th Edition"? The used cost for this older edition is... ~$10 on Amazon.

I am not sure how it will help you, but I still think it will. I wasn't into pottery when I was taking this class but I recall the book being pretty good.

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LT:

 

The basic principles and applications of Gibbs I understand. Was hoping you or others might have a link to a paper on a study I read almost a year ago out of a University in Germany? They were specifically testing/analyzing heat work done on kaolinitic samples. I recall the rate of climb at 30C. Apparently I did not save the link.

 

Matthew, I recall you recommending that book about six months ago in another thread: wrote in down.  Been reading this one- similar, but potter specific

Nerd

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I feel a fair amount of pottery technical knowledge is rather... inbred?

Of course, most people who get mechanical engineering degrees don't end up in pottery. If I was currently taking mechanical engineering materials science classes, I am sure I would make or find pottery-specific problems to solve. But today I don't really wish to revisit it and just guess.

 

Incidentally, my ∆6 firing schedule which I arrived at independently finishes

150°C/hr to 1100

100°C/hr to 1220, 5 minute hold

down80°C/hr to 960 (ish, I think, I don't really remember) and maybe a 30 minute hold. I haven't been using as many iron-based glazes recently so the cooling part does very little.

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